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Rhododendron: The Species / Classification

Family

A genus as large and varied as Rhododendron becomes comprehensible only if its species are systematically arranged according to their affinity. The classification currently used in Britain was devised by Sir Isaac Bayley Balfour of the Royal Botanic Garden, Edinburgh, and further developed by other botanists there after his death in 1922. When work on the genus began there early this century the most authoritative classification was the one that Maximowicz had provided in his Rhododendreae Asiae Orientalis (1870), in which the genus had been divided into seven sections. But, as Balfour pointed out in discussing Maximowicz’s section Osmothamnus, ‘In the years that have passed since Maximowicz wrote upon the East Asiatic Rhododendrons, China has supplied us with more Rhododendrons than were then known from the whole world.’ Taxonomic units below the rank of Maximowicz’s sections were needed if the new material was to be rendered intelligible, and so the practice grew up in the herbarium of grouping together species according to their apparent affinity, each group being termed a ‘series’ and given the name of the oldest or best-known member of the group. This was usually either a Himalayan species, e.g., the Thomsonii series, or a Yunnan species discovered earlier by one of the French missionaries and described late in the 19th century by Franchet, e.g., the Taliense series. It was essentially an informal and tentative scheme, which can be seen evolving in Balfour’s discussions of the new species (more than three hundred of them) that he described between 1916 and 1922 in Notes from the Royal Botanic Garden, Edinburgh.

The Balfourian method was never given formal expression in Sir Isaac’s lifetime, and might have retained its empirical character were it not that gardeners growing the new rhododendrons from China and bor­dering parts were in urgent need for some guide through the hundreds of names they had to contend with. For the benefit of members of the Rhododendron Society a tentative classified list of species was compiled by Edinburgh botanists and circulated in 1924 (Rhod. Soc. Notes, Vol. II, pp. 215-27). It was, presumably for the sake of completeness, extended to cover the whole genus (with the exception of the Malayan, Indonesian, and New Guinea species). The fourth edition of the list, much amended, appeared in 1928 and is the basis of the arrangement adopted in Species of Rhododendron, published in 1930. This book is a remarkable monument to the collaboration that then existed between botanists and amateur growers. Published by the exclusive Rhododendron Society under the editorship of J. B. Stevenson of Tower Court, it was the work of three leading botanists: Dr Rehder of the Arnold Arboretum, who described the azaleas;

H. F. Tagg of the Royal Botanic Garden, Edinburgh, who treated the elepidote rhododendrons; and Dr J. Hutchinson of Kew, who was responsible for the lepidote rhododendrons and most of the azaleastrums. Single-page descriptions were given of each species and for the first time all the series were provided with diagnoses and with keys to their constituent species. The book was a fine achievement and has probably been as much used by botanists as by gardeners. In the forty-five years since it was published many new species have been described and brought into cultivation, and various amendments have been made to the classification originally adopted. An abbreviated edition is published by the Royal Horticultural Society as Part I of The Rhododendron Handbook, revised every five years or so.

It is an obvious defect of the British system of classification that the series are of unequal taxonomic status. Those originally constituted by Balfour were for the most part equivalent to subdivisions of Maximowicz’s sections, and the term ‘series’ was not inappropriate for them. But some series are certainly entitled to rank as sections, e.g., s. Anthopogon, s. Ovatum, and s. Stamineum, while the azaleas, which in Species of Rhododendron are grouped in a single series, represent two whole sections of Maximowicz’s classification, each of which should probably rank as a subgenus. However, the botanists who instituted the series classification, and those who have since sought to improve it, have not had as their aim to provide a systematic arrangement of the whole genus, much of which lies outside their sphere of interest. A classification of conventional form was published in 1949 by Dr H. Sleumer (see Bibliography), in which the genus is divided into subgenera, sections, and subsections. It is, to a large extent, based on Species of Rhododendron and the earliest of the revisions by Cowan and Davidian (further mentioned below). Where appropriate, the British series (or, in some instances, subseries) are given the rank of subsections, and are given names according to the rules of botanical nomenclature; thus the Thomsonii series and the Heliolepis series become respectively the subsections Thomsonia and Heliolepida. Some series, such as those mentioned at the beginning of this paragraph, receive higher rank. Sleumer’s system has the merit of providing botanists with subgeneric taxa of the usual form, but the author himself claimed no finality for it and indeed anticipated that it would need modification in the light of later research.

In the following synopsis no attempt has been made to present a botanical classification of the genus, but the series, instead of being treated in alphabetical order, have been grouped under four headings: Lepidote Rhododendrons; Elepidote or Hymenanthes Rhododendrons; Azaleas; and Azaleastrums. This arrangement, it is hoped, will make for a better understanding of the genus.

Since 1946, revisions of some of the series of Rhododendron have appeared in the Rhododendron (and Camellia) Year Book, published by the Royal Horticultural Society. The work was begun by Dr J. M. Cowan and H. H. Davidian of the Royal Botanic Garden, Edinburgh, and continued by the latter after Dr Cowan’s death in 1960. References to these important revisions, which have been of great assistance in preparing this new edition, are given below in the appropriate place.

Lepidote Rhododendrons

The lepidote species are characterised by the presence of scales on various parts of the plant, most obviously on the undersides of the leaves. These scales, really glandular hairs of complex structure, consist of a very short stalk surmounted by a multicellular head, which is usually surrounded by a rim made up of radially arranged cells, the whole scale, seen from above under low magnification, having the appearance of a shield with a central boss or umbo. The edge of the shield may be entire, crenated, or sometimes (as in s. Anthopogon) deeply incised. Rarely, as in s. Trichocladum, the rim is absent. These elaborate glandular hairs take the place of the simple glandular hairs found in other types of rhododendron, but never among the lepidote species. In addition to scales, the lepidote species also frequently bear eglandular hairs, but these are always simple; the branched hairs so frequent among the Hymenanthes rhododendrons are unknown among the lepidotes. The Edgeworthii series, which is unusual among the lepidotes in having a dense indumentum, is no exception to this rule, since the covering is made up of simple hairs.

It has been found that the presence of scales is, at least in the species examined, almost perfectly correlated with another character, namely, that the leaves are convolute in the bud, i.e., folded forward, whereas in all other groups of Rhododendron they are revolute, the margins being folded back so as to conceal most of the undersurface (J. Sinclair, Notes Roy. Bot. Gard. Edin., Vol. 19 (1937), pp. 267-71).

As in most rhododendrons (except the Azaleastrums) the flowers are produced from terminal buds, but three series have axillary inflorescences, i.e., the flower clusters are borne in the axils of the uppermost leaves and truly terminal buds are not formed (or at least not as a general rule). Some other lepidote species (e.g., R. yunnanense and R. lutescens) also have axillary inflorescences, but in these a terminal bud is formed and usually produces flowers, so the axillary clusters are really supplementary (but in R. lutescens, on some shoots, all the inflorescences are axillary and the terminal bud vegetative).

In the species treated here, the corolla is mostly narrowly to widely funnel-shaped (verging on rotate in some species), tubular or campanulate, and the limb is occasionally zygomorphic (especially in the Triflorum series). Other shapes are found among the Malesian lepidotes and when these are taken into account the corolla-shape is more diverse among the scaly-leaved rhododendrons than in any other group.

In most of the hardier species the seeds are plain, i.e., without appendages. In the section Vireya (the Malesian lepidotes and their few Sino-Himalayan allies) the seeds are tailed at each end, and similar but shorter appendages occur among epiphytic species, e.g., in R. edgeworthii and some members of the Maddenii series.

Polyploidy, rare elsewhere in the genus, is common among the lepidotes. In two series, s. Cinnabarinum and s. Heliolepis, no diploids have been recorded; and in two others, s. Lapponicum and s. Triflorum, polyploids outnumbered diploids among the plants examined (Janaki Ammal et al., R.Y.B. 1950, pp. 94-6).

The lepidote rhododendrons are the most numerous group in the genus, with over 500 species at present recognised (though over half of these belong to the mainly Malesian section Vireya). They also have the widest ecological and geographical range. All epiphytic species are lepidote, but so too are virtually all the alpines; the northernmost rhododendron, R. lapponicum, and the southernmost, the Australian R. lochae (not treated here), are both lepidote. They are sparingly represented in North America and western Eurasia, but in Sino-Himalaya they are very numerous and are the only type of rhododendron found in the Malesian region, apart from the exceptions mentioned on page 542.

In Sleumer’s classification the lepidote rhododendrons are split into four subgenera. The series with (?) constantly axillary inflorescences are grouped in two subgenera – Rhodorastrum (Maxim.) C. B. Clarke and Pseudorhodorastrum Sleumer – the former comprising the Dauricum series, the latter the Scabrifolium and Virgatum series. It is debatable, however, whether this character should be given such taxonomic weight. All the other lepidotes are grouped in a single subgenus with the exception of the Trichocladum series, which is given subgeneric rank as subgen. Pseudazalea Sleumer, another controversial decision.

There is much to be said for keeping all the lepidote rhododendrons in a single subgenus, at least for the time being. The subgenus would take the name subgen. Rhododendron, of which the principal synonyms are: Rhod. sect. Lepipherum G. Don; Osmothamnus DC.; Rhod. sect. Osmothamnus (DC.) Maxim.; Rhod. sect. Eurhododendron Maxim., in part; Rhod. subgen. Eurhododendron Series b C.B. Clarke; Rhod. subgen. Lepidorhodium Koehne.

s. Anthopogon

Revision: R.Y.B. 1947 (2), pp. 55-86.

Dwarf evergreen shrubs with small aromatic leaves; scales lacerate (with a jagged edge). Flowers in terminal clusters, very shortly stalked. Corolla tubular with a spreading limb, less than 1 in. long, white, rose, or pale yellow. Stamens commonly five, occasionally up to ten, included in the tube. Ovary five-celled; style very short. Its distribution is: borders between Pakistan and Afghanistan (R. collettianum); Himalaya; thence east and north to Kansu; Siberia. A very distinct group of 14 species, easily recognised in flower by the concealed stamens and style. Its position in Sleumer’s classification is: subgenus Rhododendron section Pogonanthum.

s. Boothii

Revision: R.Y.B. 1948 (3), pp. 60-79.

Evergreen shrubs, often epiphytic, varying in habit and size of leaf. Scales on underside of leaf uniform in colour. Inflorescence terminal, with up to ten flowers. Corolla usually campanulate or tubular-campanulate, white, rose, pink or yellow, five-lobed, up to 134 in. long. Stamens ten. Style stout and bent downwards (except in ss. Tephropeplum). The whole series is confined to the wetter parts of the Sino-Himalayan region, from Bhutan through the Assam Himalaya and S.E. Tibet to upper Burma and N.W. Yunnan.

ss. Boothii. – Dwarf or medium-sized shrubs, often epiphytic. Leaves 1 to 5 in. long, glaucous beneath. Inflorescence with three to ten flowers. Corolla campanulate, yellow. Five species, the four in cultivation all uncommon and rather tender.

ss. Megeratum. – Dwarf shrubs. Leaves up to 2 in. long, hairy at the margin and often on the upper surface; undersurface glaucous. Inflorescence one- or two-flowered. Corolla campanulate to rotate, yellow or white. Two species, both fairly hardy but early flowering. See R. leucaspis.

ss. Tephropeplum. – Shrubs up to 10 ft high. Leaves up to 5 in. long, the undersides glaucous and usually densely scaly. Corolla tubular-campanulate, white, pink, purplish, yellow, or cream. Style slender, straight. Four species, the commonest being the variable R. tephropeplum.

s. Camelliiflorum

Represented in cultivation only by the type-species (q.v.).

s. Campylogynum

Revision: R.C.Y.B. 1954 (8), pp. 78-84.

This series has now been reduced to a single species, R. campylogynum (q.v.), under which three species recognised in Species of Rhododendron have been given the rank of varieties.

s. Camtschaticum see Therorhodion, Vol. IV.

s. Carolinianum

Small or medium-sized evergreen shrubs with leaves mostly 2 to 334 in. long. Inflorescence terminal. Corolla funnel-shaped or funnel-campanulate, about 112 in. wide, light rosy purple or pink, sometimes white, scaly outside. Stamens ten. A group of three closely related species, allied to the Heliolepis series, natives of the eastern USA.

s. Cinnabarinum

Tall or medium-sized evergreen shrubs. Leaves mostly 112 to 3 in. long, often glaucous when young, densely scaly beneath. Inflorescence terminal (also axillary in R. keysii and the closely related R. igneum). Corolla usually tubular or tubular-campanulate, but campanulate in R. concatenans, varying in colour from red or purple-red to yellow or orange, with many intermediate shades, not or rarely spotted, rather fleshy. This is essentially a Himalayan series, extending as far east as the Mishmi Hills. Of the five species, two – R. concatenans and R. xanthocodon – are closely related to R. cinnabarinum and will probably be sunk in it before long. The only other cultivated member is R. keysii. All these are hexaploid, i.e., have three times the normal number of chromosomes in their body cells. The interesting R. tamaense, at present unplaced, shows some kinship with the Cinnabarinum series but is also near to R. oreotrephes of the Triflorum series.

s. Dauricum

This group of two or three species has been given the rank of section (sect. Rhodorastrum) because of its rather distinct inflorescence. The flowers are produced singly (rarely in twos) from clustered axillary buds and the young growths spring from below the flower-bearing part of the shoot. For further details see R. dauricum and R. mucronulatum. The series is confined to N.E. Asia (including Japan). In Sleumer’s classification it ranks as a subgenus.

s. Edgeworthii

Revision: R.C.Y.B. 1964 (18), pp. 111-18.

This series, with three species, is represented in cultivation mainly by R. edgeworthii (bullatum), which is easily recognised among lepidote rhododendrons by its rugose leaves densely tomentose beneath and its fragrant, white or rose-tinted, funnel-shaped flowers. Although many lepidote rhododendrons are bristly or downy, it is very uncommon for them to have such a dense indumentum. Indeed only two other species show this character, and both are on that account grouped with R. edgeworthii, namely, R. seinghkuense and R. pendulum (qq.v.), neither of which bears much resemblance to it in floral characters. The series is confined to the eastern Himalaya, upper Burma, and N.W. Yunnan.

s. Ferrugineum

Dwarf evergreen shrubs, Leaves up to 112 in. long, densely scaly beneath, crenulate in two species; scales entire. Inflorescence shortly racemose or umbel­late. Corolla tubular with a spreading limb, pink. Style short, included. This series, with three species, is the only one endemic to Europe (Pyrenees to Bal­kans) and not closely related to any other series.

R. ferrugineum is now taken as the type-species of the genus Rhododendron.

s. Glaucophyllum

Revision: R.Y.B. 1948 (3), pp. 79-93 (as Glaucum series).

This group is mainly represented in cultivation by the typical subseries, of which the leading characters are as follows: Small evergreen shrubs. Leaves up to 334 in. long, their undersides usually glaucous, with scales of two kinds, the smaller scales pale yellow, variously spaced, the larger dark brown, scattered. Inflorescence terminal with up to ten flowers. Calyx well developed, up to almost 12 in. long. Corolla campanulate, up to 1 in. long, pink, rose, slaty purple, or yellow. Style usually short, stout and bent (but often straight in R. glaucophyllum). Five species in the rainier part of Sino-Himalaya, from E. Nepal to N.W. Yunnan.

The other subseries of Glaucophyllum comprises R. genestierianum (q.v.) and R. micromeres (not treated).

s. Heliolepis

Closely allied to the Triflorum series and, according to Hutchinson, scarcely separable from it by any definite characters. At present nine species are recognised, some precariously based and likely to be sunk when the series comes to be revised. The series ranges from upper Burma through W. China as far north as Kansu. All the cultivated plants examined have proved to be polyploid.

s. Lapponicum

Revision: Melva N. and W. R. Philipson, ‘A Revision of Rhododendron section Lapponicum’, Notes Roy. Bot. Gard. Edin., Vol. 34 (1975), pp. 1-71.

Dwarf evergreen shrubs. Leaves usually less than 1 in. long, aromatic in most species, scaly on both sides, the scales of the underside of varying density and either uniform in colouring or bicolorous. Inflorescence terminal, few-flowered. Corolla usually less than 1 in. wide, funnel-shaped with a short tube, in various shades of lavender, pink, purple, or violet, rarely yellow, white only in albino forms. Stamens five to ten, usually hairy at the base, exserted (rarely included in the tube). Style variable in length, often even in the same species.

The main concentration of species is in the drier and bleaker parts of the Sino-Himalayan region, where they dominate over vast tracts in some areas, especially on the borders between Yunnan and Szechwan, in the mountains around the two great bends of the Yangtse river. In the rainier parts of Sino-Himalaya they are less common. Outside Sino-Himalaya there are two closely related species – R. lapponicum and R. parvifolium – which between them girdle the globe in high latitudes.

The Lapponicum series is taxonomically a difficult one, in which polyploidy is common and natural hybrids occur. The revision referred to above, in which the series is given the rank of a section of the subgenus Rhododendron, appeared in August 1975, when most of the present volume was in proof and shortly before it finally went to press. However, an effort has been made to incorporate or mention the authors’ main conclusions. They have stressed, and would certainly like the warning to be repeated, that the botanical key provided is based on herbarium specimens, in which the colouring of the scales – a character so useful for identification – is darker than on living plants. Leaves of the previous season’s growth should always be used when a living specimen is taken through the key.

s. Lepidotum

Revision: R.Y.B. 1948 (3), pp. 93-101.

A horticulturally unimportant group of three species, recognisable by the relatively long pedicels, the small, rotate corolla and sharply bent style. R. lepidotum is one of the most widely distributed of Sino-Himalayan species. The other two are confined to the Himalaya.

s. Maddenii

There has been no revision of this group since Dr J. Hutchinson constituted and discussed it in ‘The Maddenii Series of Rhododendron’, Notes Roy. Bot. Gard. Edin., Vol. 12 (1919), pp. 1-84. This is the basis of his treatment of the series in Species of Rhododendron (1930).

The species in this group are on the average much larger than other lepidotes, both in the size of the plant and in the dimensions of leaf and flower. The leaves are persistent, mostly over 3 in. long and up to 8 in. long in some species. Inflorescence terminal with up to seven flowers. Corolla funnel-shaped, tubular-funnel-shaped, or funnel-campanulate, rarely less than 2 in. long, white, pink, or yellow, often fragrant. Style nearly always scaly, at least in its lower part.

There are three subseries, all three reaching as far west as Sikkim or E. Nepal and as far east as Yunnan. The Ciliicalyx subseries, which is the most widely distributed of the three, is also represented in S.E. China, Thailand (where the ss. Maddenii also occurs), and in the former Indochina. Mostly the species occur at low altitudes, as epiphytes or on rock-ledges or by torrents. Very few of these beautiful shrubs can be grown outdoors near London, except most of the Maddenii subseries and a few members of the Ciliicalyx subseries such as R. fletcherianum, R. valentinianum, R. ciliatum, and R. johnstoneanum.

ss. Maddenii. – Medium-sized or large shrubs (R. manipurense sometimes a tree). Calyx variable. Corolla narrowly funnel-shaped, white or light pink. Stamens fifteen to twenty-five. Ovary-cells ten to twelve. Eight species are at present recognised but this is likely to be reduced when the group comes to be revised. The characters used to separate the species are poorly correlated, and too much weight has been given to size of calyx and the precise number of stamens. The hairs on the stamen-filaments, the presence or absence of which is made a leading character in Species of Rhododendron, are sparsely and irregularly developed, and sometimes reduced to a few watery excrescences. Polyploidy is rife in this group. Some plants have even been found to be dodecaploid, i.e., with six times the normal number of chromosomes. On the average the species in this subseries are hardier than in the other two, and all those in cultivation are late-flowering.

ss. Ciliicalyx. – Calyx mostly small, frequently ciliate. Most species have ten stamens and a six-celled ovary. Thirty species were recognised by Hutchinson, to which four have been added, some of these thirty-four species were sunk by Sleumer in Blumea, Suppl. IV (1958), pp. 40-7. The number is certainly excessively large, considering the limited range of variation in this group. Some species are founded on minor characters that probably fluctuate even in a local population.

ss. Megacalyx. – Calyx well developed, often large and leafy, not ciliate, except in R. lindleyi. Stamens normally ten. Ovary-cells five. Another character given by Hutchinson is that the midrib of the leaf is prominent above and the upper side of the petiole convex (slightly grooved in R. megacalyx). In the other two subseries the midrib is sunken and the petiole concave. The group is a small one, with about nine species.

s. Micranthum

This series consists of a single species (q.v.), not closely related to any other rhododendron. The flowers resemble those of Ledum, except that the segments are united at the base, and are borne in racemes. The seeds have a short appendage at each end. N. and N.W. China.

s. Moupinense

Of the three closely related species in this series, only R. moupinense (q.v.) is in cultivation. All three are frequently epiphytic and are confined to a small area of S.W. Szechwan.

s. Saluenense

Revision: R.C.Y.B. 1954, pp. 84-98.

Small, mainly dwarf shrubs, sometimes more or less prostrate. Leaves up to 112 in. long, densely coated beneath with crenulate scales. Inflorescence few-flowered, terminal. Calyx well developed. Corolla funnel-shaped or rotate, downy outside, purple or crimson. R. fragariiflorum, provisionally placed in this series, is anomalous, and near to the Lapponicum series. The other seven species, all very closely allied, differ from the Lapponicum series in the larger calyx, usually enclosing the capsule, and the crenulate scales. The series is distributed in the E. Himalaya, upper Burma, N.W. Yunnan and bordering parts of Tibet, in the alpine and subalpine zones. In this region the Lapponicum series is poorly represented.

s. Scabrifolium

Revision: R.C.Y.B. 1964 (18), pp. 118-33.

Small or medium-sized shrubs; branchlets downy or bristly (except in R. racemosum). Leaves persistent, up to 4 in. long, mostly downy or bristly above, green and downy beneath (but the undersides glaucous and glabrous in R. racemosum, glaucous and slightly downy in R. hemitrichotum). Inflorescences axillary on the upper part of the previous season’s growths, each cluster with one to four flowers but sometimes terminal only in R. spinuliferum. Corolla funnel-shaped to tubular (saccate in R. spinuliferum), white or pink, up to 1 in. long. Six species in S.W. China, mostly in rather dry habitats and apparently absent from the rainier parts of Sino-Himalaya.

In Sleumer’s classification, this series, with s. Virgatum, constitutes the subgenus Pseudorhodorastrum, characterised by: inflorescences axillary; new growths from pseudoterminal buds or from buds below the flower-bearing part of the shoot.

s. Trichocladum

The eleven species at present recognised in this group are for the greater part distinct from most other lepidotes in their deciduous leaves and precocious flowers, and have on that account been given by Sleumer the rank of a subgenus – Pseudazalea. But some species are evergreen, e.g., R. viridescens (which is also late-flowering), and a completely deciduous habit is found in some other lepidote species. Furthermore the number of deciduous species in this series is greatly inflated and could be reduced to but one, R. trichocladum itself (q.v.). A more distinctive feature of the group is the rimless, bun-shaped scales on the undersides of the leaves.

s. Triflorum

Revision: R.C.Y.B. 1963 (17), pp. 156-222.

Lightly branched shrubs mostly of moderate size, rarely small trees. Leaves persistent, rarely semi-deciduous or deciduous, up to 6 in. or so long, usually of rather thin texture, scales on undersurface of variable density. Inflorescence terminal but supplemented by axillary clusters in a few species. Calyx small. Corolla (except in the Hanceanum subseries and the anomalous R. concinnoides) funnel-shaped, zygomorphic (‘butterfly-shaped’), usually speckled in the upper part, white, yellow, greenish yellow, pink, lavender, purple, or deep magenta. Style long, slender (except in R. afghanicum). The largest subseries is the Yunnanense, with seventeen species; the Augustinii subseries (four species) is similar, but differs in having the leaf-midrib hairy beneath. The Triflorum subseries (eight species) differs in its constantly yellow or greenish-yellow flowers. R. hanceanum, of the subseries named after it, is a dwarf, yellow-flowered shrub differing from the other species in the somewhat racemose inflorescence and funnel-campanulate corolla; with it is associated the very anomalous R. afghanicum.

The typical, yellow-flowered subseries is widely distributed, from E. Nepal to W. Szechwan, and has one species in Japan. The Augustinii and Yunnanense subseries are essentially Chinese in distribution, being absent from the Himalaya and rare west of the Salween; mainly they are thicket-forming shrubs of open places or light woodland at moderate elevations.

s. Uniflorum

Revision: R.Y.B. 1948 (3), pp. 51-60, 101-9.

Dwarf shrubs. Leaves not much more than 1 in. long, their undersides glaucous, with small uniform scales. Inflorescence terminal, with one or two flowers. Calyx very small, except in R. ludlowii. Corolla campanulate or funnel-shaped, purple, pink, or rosy pink (yellow in R. ludlowii), hairy outside. Style slender and straight. This description excludes the anomalous R. monanthum, a taller shrub with leaves up to 2 in. long, their scales not uniform, and with the corolla not hairy outside. Apart from this there are six species in the eastern Himalaya and upper Burma.

s. Vaccinioides

This series, as defined in Species of Rhododendron, comprises the few northern outliers of a large group of lepidote species native mainly to the Malayan peninsula, Java, Borneo, Sumatra, the Celebes, the Philippines, and New Guinea (one species, R. lochae, in Australia). Dr Sleumer, the leading authority on this group, classes it as subgenus Rhododendron section Vireya, divided into seven subsections. The species treated in Species of Rhododendron, and listed under s. Vaccinioides in the present Rhododendron Handbook (1967), belong, with one exception, to the subsection Pseudovireya (C. B. CI.) Sleumer, of which R. vaccinioides is the type. This is in cultivation, and so too is the newly discovered R. santapaui.

The bulk of the section Vireya, which contains almost 300 species, occurs within the region covered by the great Flora Malesiana, and it has become usual to refer to them as the ‘Malesian rhododendrons’ (the English word ‘Malaysian’ has a narrower circumscription, and is not appropriate). The Malesian species are not hardy in Britain and not treated in this work. A detailed and well-illustrated account of the group by Dr H. Sleumer has been published in Flora Malesiana and is available separately (Series I, Vol. 6, part 4 (1966)). The Malesian Vireyas have been represented in cultivation since the 1840s. In that and the next decade several species from the western part of the region were introduced to the nurseries of Messrs Veitch by Thomas Lobb, including R. javanicum (Bot. Mag., t. 4336); others were sent in the late 1870s by the Veitchian collector Charles Curtis. Between 1874 and the early years of this century Messrs Veitch raised and put into commerce about 200 hybrids from these introductions, some of great complexity, which were familiar glasshouse plants before the first world war. In recent years there has been a revival of interest in the Malesian Vireyas, thanks to the introduction of species that tolerate cooler conditions than the old Veitchian species and hybrids, which came mostly from low altitudes. Many of the recent introductions are from New Guinea, which is the richest of all the islands in rhododendrons, having some 155 species, forty of which were introduced to Europe and America in the 1950s through the Rijksherbarium, Leyden, Holland. For further information see the section on Malesian rhododendrons in The Rhododendron Handbook and the articles by Michael Black in the Rhododendron and Camellia Year Book for 1966, 1967, and 1970. Also, for figures of recently introduced species, see Botanical Magazine, new series, tt. 552, 575, 600, 610, 653.

s. Virgatum

Revision: R.C.Y.B. 1964 (18), pp. 106-7, 130-3.

This series contains only R. virgatum (oleifolium), characterised by the one-flowered, axillary inflorescences, and seeds with a fairly well-developed appendage at each end. It is part of Sleumer’s subgenus Pseudorhodorastrum (see s. Scabrifolium).

Elepidote (Hymenanthes) Rhododendrons

The species grouped here are what most gardeners would think of as ‘typical’ or ‘true’ rhododendrons, for they far outnumber the lepidote rhododendrons in cultivation and, because of their greater bulk, they are more conspicuous. But, as already pointed out, it is the lepidote species R. ferrugineum, the alpine rose, that is the type of the genus Rhododendron. The elepidote rhododendrons are very uniform in their essential botanical characters and in the estimation of most botanists constitute a single subgenus, for which the correct name is subgen. Hymenanthes (Bl.) K. Koch. The principal synonyms are: Hymenanthes Bl. (the type of which, and hence of the subgenus, is R. degronianum f. heptamerum); Rhod. sect. Eurhododendron Maxim., in part; Rhod. subgen. Eurhododendron Series a C. B. Clarke (excl. R. edgeworthii and R. pendulum); Rhod. sect. Leiorhodium Rehd.; Rhod. subgen. Leiorhodium (Rehd.) Pojarkova.

The term ‘elepidote’ applied to these rhododendrons refers, of course, to the fact that the modified glands, or scales, characteristic of the subgenus Rhododendron, do not occur in this group; glands, when present (as they often are on one part or another of the plant), are of simple structure. A further distinction, which marks the group off from all other subdivisions of the genus, is that the non-glandular hairs on the leaves and stems are, with few exceptions, branched and often of elaborate structure. In many species these hairs form a dense coating on the underside of the leaf, the nature of which depends on the structure of the hairs and their density. The Hymenanthes rhododendrons are mostly medium-sized to large shrubs or trees, very rarely dwarf. The leaves are always persistent, and usually last for two years. The inflorescence is terminal (though in some young and vigorous garden hybrids the uppermost axillary buds may also produce flowers). The corollas usually have the normal five lobes, but seven-lobed corollas are common in s. Fortunei, mostly eight-lobed in s. Falconeri and s. Grande, and there are exceptions in other series also. The number of stamens is usually double that of the corolla-lobes, though often only approximately so when the latter are in excess of five. The number of chambers in the ovary is commonly more than five in this group, though rarely more than twice the number of corolla lobes. The seeds are very uniform, all being of what Kingdon Ward termed the ‘forest type’, i.e., more or less winged and with a frilly protrusion at one end.

Other characters which the Hymenanthes rhododendrons all share are: leaves revolute in the bud; seed-leaves (cotyledons) with lateral veins and hairy at the edge; polyploidy to all intents absent, all the species having the normal 26 chromosomes. The Philipsons have shown that the Hymenanthes rhododendrons are unique in this genus in their nodal anatomy.

The Hymenanthes rhododendrons have more or less the same distribution as the genus as a whole, except that they are scarcely represented in the Malesian region and do not extend so far north as the lepidotes. The group as a whole has a narrower ecological range than the lepidotes. No species has developed an epiphytic habit and the group is consequently rare in the warm-temperate rain-forests of Sino-Himalaya. Also they have failed to evolve more than a very few dwarf species, whence their rarity in the alpine zones and moorland regions of Sino-Himalaya, which the lepidotes, with less baggage to carry, have colonised with such remarkable success. On the other hand, where conditions are suitable for their growth, they may form dwarf forests or thickets of vast extent.

In Sleumer’s classification, the subgenus Hymenanthes contains but a single section, divided into numerous subsections. For the most part these subsections correspond to the series of the Edinburgh classification; thus the Ponticum series becomes subsect. Pontica. But in some instances he has, usually with good reason, partly or wholly broken up those series that are divided into subseries, giving to the latter the rank of subsection. His more important conclusions are mentioned in the appropriate place.

s. Arboreum

The species of the typical subseries, of which about eight are at present recognised, are mostly trees with a distinct bole, or large shrubs. Leaves up to 10 in. long, leathery; indumentum of the undersurface bistrate, with an upper layer of dendroid hairs and a lower layer of rosulate hairs (but one or the other sometimes lacking in R. arboreum). Inflorescence a dense, many-flowered truss. Calyx small. Corolla campanulate or tubular-campanulate, with nectar-pouches at the base, usually deep red but sometimes rose or white (forms of R. arboreum), or in shades of purplish blue to purple-lilac (R. niveum). Apart from R. niveum and R. lanigerum (qq.v.) the subseries is made up of R. arboreum and its allies, a taxonomically difficult complex with a wide range from the W. Himalaya to Yunnan, south to S.W. Peninsular India, Ceylon, middle Burma, Thailand, and the former Indochina.

ss. Argyrophyllum. – This subseries was originally set up as an independent series, and its reinstatement was proposed by Cowan (Rhodo. Leaf, p. 78). The inflorescence is laxer than in ss. Arboreum and usually fewer-flowered; the corolla is narrowed at the base and nectar-pouches are absent. In most of the species the leaves are covered beneath with a thin but continuous covering of rosulate hairs (the so-called ‘plastered’ indumentum), but in R. insigne the indumentum is silvery and burnished. In a few species, however, notably R. floribundum and R. hunnewellianum, the covering is woolly; these (group aa. in the key in Species of Rhododendron) are separated by Sleumer as subsect. Floribunda. The subseries Argyrophyllum (including the R. floribundum group) is Chinese in distribution, from Kansu to Yunnan, east and south to Hupeh, Chekiang, Kwangtung, and Fukien (but a rhododendron akin to R. coryanum occurs at the eastern end of the Himalaya). The cultivated species are from Szechwan and Hupeh.

s. Auriculatum

This series now contains one species – R. auriculatum (q.v.). At one time R. griersonianum was incongruously associated with it, but is now placed in a separate series, also monotypic.

s. Barbatum

Medium-sized shrubs or small trees, mostly characterised by the presence of bristly glandular or eglandular hairs on the petiole, extending in some species to the midrib and even to the underside of the blade; other types of hair occur in this series, but only in a few species do the leaves have a close, continuous indumentum. The leaves are 3 to 8 in. long. In all the subseries except ss. Maculiferum the calyx is well developed and, in some species, coloured. The corolla is five-lobed, tubular to more or less campanulate, never truly funnel-shaped.

ss. Barbatum. – Young shoots and petioles coarsely bristly (except in R. imberbe). Blade glabrous to loosely woolly beneath. Truss dense. Corolla tubular-campanulate or campanulate, red, with conspicuous nectar-pouches. Four species in the Himalaya.

ss. Crinigerum. – The two closely related species placed here are related to the Glischrum subseries, but the leaves have a continuous brown felt-like indumentum beneath.

ss. Glischrum. – Young stems and petioles clad with bristles which in some species extend to the midrib beneath, in others cover the whole undersurface of the blade. Compound hairs may also be present, and two species, R. hirtipes and R. vesiculiferum, have peculiar bladder-like hairs on the veins. Corolla white, rose, more rarely crimson. Ten species in the Assam Himalaya, upper Burma, N.W.Yunnan and bordering parts of Tibet.

ss. Maculiferum. – Small or medium-sized shrubs for the most part. Leaves rarely more than 6 in. long, glabrous beneath except for hairs or glands on the midrib. Inflorescence lax, with up to fifteen flowers. Calyx small. Corolla white or rose (except in the red-flowered R. strigillosum, which, according to Cowan, is out of place here and should be transferred to the Glischrum subseries). The distribution is similar to that of s. Arboreum ss. Argyrophyllum – Szechwan, Hupeh, Anwhei, and Formosa. In Sleumer’s classification this group is detached from the other subseries (subsect. Barbata) as subsection Maculifera.

s. Campanulatum

Revision: R.Y.B. 1949 (4), pp. 159-82.

As Cowan and Davidian point out in their revision, this group is a heterogeneous assemblage of Himalayan species, having little of significance in common and ‘no more closely allied to each other than are some of them to certain other species now in other series’. However, they retained it in its original form until all the groups with which its members have some kinship have been revised. Apart from the Himalayan habitat, the species have in common a more or less elliptic leaf, obtuse at both ends (though R. wallichii frequently has oblanceolate leaves, tapered at the base). In all the species the leaf is more or less coated with hairs beneath, though glabrous in some forms of indumented species, and also in R. succothii. The hairs are of diverse structure, four different types being represented in the series. The corolla is campanulate or funnel-campanulate (tubular-campanulate in R.fulgens), yellow and spotted in R. lanatum, lilac, rosy purple, pink or white in R. campanulatum, R. wallichii and R. tsariense, deep red in R. fulgens, R. sberriffii, and R. succothii. Except in R. lanatum and R. tsariense the ovary is glabrous or almost so. R. tsariense is dwarf in some forms and R. fulgens is usually a fairly small shrub; the others are of larger size. R. campanulatum ranges farther to the north-west than any other elepidote species; the others are confined to the region east of E. Nepal and most do not extend west of Bhutan.

s. Falconeri and s. Grande

These two series, although well distinguished by a character mentioned below, have so much in common that it is preferable to treat them together.

Large shrubs or trees, with stout, tomentose young stems. Leaves leathery, large to very large, indumented beneath. In both series there are species in which the petiole is very short and is winged on the upper side owing to the decurrence of the leaf-blade (R. basilicum in s. Falconeri and R. praestans, R. coryphaeum, and R. semnoides in s. Grande). The two series differ in the nature of the hairs that compose their indumentum. In s. Falconeri the upper stratum is made up of peculiar funnel- or cup-shaped hairs, which produce a thick indumentum; in a few species these hairs are sparse and there is a thinner indumentum composed mainly of the lower stratum of rosulate hairs. In the Grande series a thin, skin-like or plastered unistrate indumentum is the rule rather than the exception, and where an upper layer is present, giving a woollier indumentum, the hairs are of a more conventional kind. In both series the inflorescence is a many-flowered terminal truss with a well-developed rachis sometimes over 2 in. long. The calyx is always very small. The corolla is predominantly campanulate with a broad, rounded base, and is often oblique. Its colour is white (though rarely), creamy white, yellow, or in various impure shades between red and purple. The lobes are commonly eight (rarely the standard five) and are unusually short for the genus. The number of stamens is around double that of the lobes. The ovary is nearly always tomentose, with between eight and sixteen chambers (occasionally up to twenty). It is interesting that in both series there are anomalous species in W. Szechwan with glabrous ovaries (see R. galactinum and R. watsonii).

The two series have much the same distribution. Mostly they are confined to the rainiest parts of Sino-Himalaya, from N.W. Yunnan through upper Burma to N.E. India, where they extend through the Himalaya to E. Nepal and are represented in the Naga Hills by R. macabeanum in s. Grande. But R. fictolacteum ranges as far as S.W. Szechwan (where its relative R. rex is also found); and there are two (perhaps three) species in W. Szechwan (see above). The maximum development of the two series in the monsoonal region is between 8,000 and 10,000 ft, where they often form forests of their own on steep slopes where the soil is too thin to support forest trees; R. arizelum extends into the subalpine zone.

s. Fortunei

Large shrubs or trees. Leaves 4 to 10 in. long (except in ss. Orbiculare and ss. Oreodoxa), glabrous when mature, usually rounded or obtuse at the apex. Inflorescence terminal, often with a well-developed rachis. Corolla funnel-shaped to campanulate, 2 in. or more wide, white, pink, or rose, commonly six- or seven-lobed. Stamens twelve to twenty-five, rarely ten. Style slender (except in ss. Calophytum). The series has its main distribution in China and is rare west of the Mekong (but see ss. Griffithianum).

ss. Fortunei. – Calyx small, usually edged with glands. Style glandular to the tip. Eight species in China, two extending into bordering parts of Burma and one into Siam.

ss. Griffithianum. – R. griffithianum (q.v.) is the sole member of this subseries. It is exceptional in its mainly Himalayan distribution and in its large calyx.

ss. Calophytum. – Of the two closely allied species in this subseries only R. calophytum is cultivated. It is anomalous in having a stout style with a large discoid stigma.

ss. Davidii. – The main species in this subseries is R. sutchuenense (q.v.) which differs from ss. Fortunei in the glabrous style and differently shaped leaves and from ss. Calophytum in the funnel-campanulate corolla and capitate stigma. The type-species is probably not in cultivation and is little known botanically. R. planetum, the only other species, is not known in the wild. All the species are Chinese (Hupeh and Szechwan).

ss. Orbiculare. – The two species in this subseries are characterised by broad-elliptic to roundish leaves, cordate or auriculate at the base, a shape not met with in the other subseries. Both are Chinese.

ss. Oreodoxa. – The two main species in this subseries, R. oreodoxa and R. fargesii, are unique in the series by reason of their rather small elliptic leaves and perhaps represent two races of a single species, ranging from Hupeh to Szechwan and Kansu. The other two species were described from cultivated plants.

s. Fulvum

Revision: R.Y.B. 1949 (4): pp. 159-64, 176-82.

A small group, in which five species were recognised in Species of Rhododendron, reduced to two by Cowan and Davidian in their revision. It is closely allied to s. Campanulatum, differing in the oblanceolate to obovate leaves and the narrow, cylindrical seed-capsule. In both species the ovary is glabrous. It has a wide range, from S.W. Szechwan to the eastern end of the Himalaya.

s. Griersonianum

R. griersonianum (q.v.), the only member of this series, was grouped with R. auriculatum until removed to a series of its own by H. H. Davidian in 1963 (R.C.Y.B. 1964, pp. 109-11).

s. Irroratum

The typical subseries consists for the most part of species with the following characters: Medium-sized shrubs or small trees. Leaves of hard texture, glabrous at maturity, with a cartilaginous edge, usually acute at the apex, mostly 3 to 5 in. long. Inflorescence terminal with seven to fifteen flowers. Corolla tubular-campanulate and up to 2 in. long, more rarely saucer-shaped, white, pink, magenta, or red, often spotted or with basal markings. Ovary glandular, or tomentose or both glandular and tomentose, or glabrous. Style glandular to the tip in some species, in others glabrous, in one hairy. R. agastum and two allied species are out of place in this series, having leaves with a persistent indumentum beneath, and in Cowan’s view are more akin to s. Arboreum ss. Argyrophyllum. At present about twenty-five species are recognised, ranging from Bhutan to Yunnan and Kweichow, south to Siam and the former Indochina. Remarkably there is also a species in the Malayan peninsula (but belonging to the R. agastum group, see above) and two in N. Sumatra.

The Parishii subseries, which should probably be given independent status, comprises a few species in which (excluding R. venator) the young stems and leaf-undersides are clad with a floccose tomentum of stellate hairs which gradually wears away. The leaves are obtuse or rounded at the apex, of softer texture than in ss. Irroratum. All have deep red, tubular-campanulate, fleshy corollas and are confined to Burma and bordering parts of Yunnan and N.E. India.

s. Lacteum

Revision: R.C.Y.B. 1956 (10), pp. 122-55.

Large or medium-sized shrubs (rarely dwarf). Leaves 3 to 7 in. long (longer in R. beesianum, shorter in the dwarfer species), indumentum of the undersides commonly suede-like or felted or powdery, sometimes reduced to a thin veil of hairs or almost wanting. Inflorescence rather dense, with rarely fewer than eight flowers on pedicels mostly less than 1 in. long (but laxer in the anomalous R. wightii). Calyx very small (except in one species not in cultivation). Corolla campanulate to funnel-shaped, 1 to 2 in. long usually blotched or spotted, white, pink or yellow. Style usually glabrous. A series of some fourteen species, none common in cultivation, ranging from S. Szechwan through N.W. Yunnan and S.E. Tibet to the Tibetan side of the E. Himalaya and Bhutan; R. beesianum extends into northernmost Burma. R. wightii, provisionally included in this series, is Himalayan.

s. Neriiflorum

Of this series H. F. Tagg wrote: ‘At first sight this assemblage under one series has a heterogeneous appearance, but analysis shows that it represents an unmistakable phyletic grouping. Three characters are of special importance: the inflorescence is never congested; the corolla is almost invariably unspotted; the ovary is usually densely hairy or hairy and glandular or very rarely entirely glandular’ (Species of Rhododendron, p. 507).

ss. Neriiflorum. – Small or medium-sized shrubs. Leaves rarely more than 4 in. long, papillose beneath in most species and either glabrous or with a more or less detersile indumentum. Corolla tubular-campanulate, 114 to 134 in. long, fleshy, scarlet or crimson, with darker nectar-pouches. Ovary narrow, tapered into the style. About seven species from N.W. Yunnan and bordering part of Tibet through upper Burma to the E. Himalaya as far west as Bhutan.

ss. Forrestii, – Dwarf or prostrate shrubs. Leaves up to 2 in. long, those of the cultivated species glabrous, broad-elliptic to obovate, rounded at the apex (but narrower and thinly hairy beneath in others). Corolla red or pink, fleshy, tubular. Ovary tomentose or glandular or both. The distribution is similar to that of the typical subseries except that it does not extend so far into the Himalaya.

ss. Haematodes. – Small or medium-sized shrubs to about 10 ft (R. mallotum sometimes a small tree). Leaves on the average larger than in the other subseries, up to 6 in. long, obtuse to rounded at the apex, clad beneath with a brown to fawn woolly indumentum (but becoming rather thin in some forms of R. chaetomallum). Corolla tubular-campanulate, 114 to 2 in. long, fleshy, nearly always red or scarlet (white in the anomalous R. chionanthum), with conspicuous nectar-pouches. Calyx cup-shaped, coloured and fleshy in some species. Ovary relatively broad, hairy or glandular. Nine species (at present) in the rainiest part of Sino-Himalaya, from N.W. Yunnan (mostly west of the Mekong) through upper Burma to the Mishmi Hills of Assam. A taxonomically difficult subseries, much in need of revision. Cultivated plants are sometimes difficult to place even though they are known to have been raised from wild seed.

ss. Sanguineum. – ‘This subseries is one of the most distinctive of the genus, but, at the same time, one of the most complex; the species intergrade and overlap in bewildering confusion; among elepidote Rhododendrons no other assemblage of plants is so difficult to arrange in phyletic sequence.’ (J. M. Cowan, ‘Rhododendrons of the Rh. sanguineum Alliance’, Notes Roy. Bot. Gard. Edin., Vol. 20 (1940), pp. 55-91.) The species grouped in this subseries are closely allied, but the plants show different combinations of characters usually considered to be of taxonomic importance, e.g., to take some of Dr Cowan’s examples: flower-colour; corolla thin or fleshy; ovary glandular or eglandular; indumentum thin, woolly or none. Altogether, there are some 200 possible character-combinations, of which 120 are known from herbarium specimens to actually exist. Some thirty of these combinations had been given specific status before a halt was called; the rest, which logically should have been given the same status, were placed under this or that “species” according to which of its characters was thought to be of most importance (Cowan, op. cit., pp. 56-7, 60). In his revision Cowan recognised eight species based mainly on leaf-shape and leaf-indumentum, relegating the other described species to the rank of subspecies and giving the same rank to some other combinations not previously named. The treatment was rather elaborate and not wholly consistent, since the numerous combinations existing in R. aperantum were left unnamed.

The species are mostly small shrubs. Leaves mostly between obovate and oblong, up to about 4 in. long, with a covering beneath of rosulate hairs giving a plastered, mealy or cobwebby indumentum according to their density; but in R. citriniflorum the indumentum is woolly and bistrate, and in some species the undersides are glabrous and usually papillose. Calyx variable, from small to large, fleshy in R. dichroanthum and R. parmulatum. Corolla variable in colour even in the same species, and sometimes darker red than in any other elepidote rhododendron; except in R. parmulatum it is unspotted.

s. Ponticum

Medium-sized to large shrubs, rarely dwarf. Leaves up to 10 in. long and rarely as short as 3 in., mostly with an indumentum beneath, but glabrous at maturity in a few species. Inflorescence terminal with rarely fewer than eight flowers; rachis usually well developed; pedicels rarely less than 1 in. long, elongating after flowering, the uppermost more or less erect or sometimes all the pedicels erect and fastigiate; the posture of the pedicels, helped by the longish rachis, gives to the inflorescence a shape usually termed ‘candelabroid’. Corolla funnel-shaped to campanulate, rather more deeply divided than in most Hymenanthes rhododendrons, though this character tends to be obscured in some species by the overlapping of the lobes at the base; except sometimes in the Japanese species the corolla is five-lobed and somewhat zygomorphic. Stamens commonly ten and ovary chambers five (except in flowers with extra lobes).

The geographical distribution of the series is unique among the Hymenanthes rhododendrons, but is parallelled by that of the Luteum azaleas. There are no species in the Sino-Himalayan region proper, the only Chinese species – R. adenopodum – being a native of Hupeh and bordering parts of E. Szechwan. The other species range from Japan (and Formosa if R. hyperythrum is included) through N. Asia to western Eurasia and N. America. Two subseries are recog­nised. In the typical subseries (R. ponticum and the three American species) the leaves are stated to be glabrous, though in some forms of R. maximum they have a persistent thin indumentum beneath. In ss. Caucasicum, with about seven species, the leaves are indumented beneath except in R. aureum. It seems to be generally agreed that R. brachycarpum should be excluded from this series, and the Formosan R. hyperythrum is only provisionally included in it. The nearest ally of ss. Caucasicum would seem to be s. Arboreum ss. Argyrophyllum.

s. Taliense

Although poorly represented in cultivation, the species of the Taliense group are an important constituent of the rhodoreta of Sino-Himalaya, where they have their maximum development in the bleaker parts and are virtually absent from areas that receive the full weight of monsoonal rain. The leaves are mostly rather thick, clad beneath with a thick, woolly or felted indumentum which is usually brown or reddish (or, if paler, then spongy), and in most species is composed of ramiform hairs with an undercoating of rosulate hairs (but see ss. Wasonii). Inflorescence terminal with a very short rachis. Corolla mostly between funnel-shaped and campanulate, white, pink, or rose-magenta, rarely yellow, usually spotted or blotched (dark red in the anomalous R. gymnocarpum).

ss. Taliense. – The ten species in this subseries have flowers with a very small calyx, and the ovary is glabrous. Except in the little-known R. purdomii the leaf-indumentum is woolly or felted, and the hairs are often bound together by a sticky secretion. This subseries extends farther west than the others, occurring in the eastern Himalaya on both sides of the range; if R. purdomii (a native of Shensi) is excluded, the north­eastern limit is in S.W. Szechwan.

ss. Adenogynum. – Calyx well developed, mostly 14 to 38 in. long, deeply lobed, glandular (but very small in R. alutaceum, included because in other respects it resembles the type). Ovary always glandular, sometimes tomentose also. R. prattii is anomalous, for although the calyx is large, the leaves are uncharacteristic in their greater than average width and thin indumentum. Eighteen species are at present recognised, ranging from N.W. Yunnan (mostly east of the Mekong) through Szechwan to Kansu.

ss. Roxieanum. – The species grouped in this subseries are for the most part of dwarf stature and confined to high elevations in N.W. Yunnan and bordering Szechwan. But R. roxieanum itself sometimes attains 10 ft in the wild, and there is one species (R. recurvoides) in Burma. Apart from R. forrestii, R.pronum (not treated here) is the dwarfest of the Hymenanthes rhododendrons and attains perhaps the highest elevation (15,000 ft). The majority are slow-growing shrubs with narrow, often strongly recurved, heavily indumented leaves and small, dense inflorescences. The calyx is small. For two anomalous species at present placed in this series, see R. gymnocarpum.

ss. Wasonii. – The seven species in this subseries, all natives of Szechwan or Kansu, have flowers with a small, eglandular calyx (against usually large and glandular in ss. Adenogynum) and a tomentose ovary (glabrous or almost so in ss. Taliense). The leaves are always flat and comparatively broad, so confusion with ss. Roxieanum is unlikely. Although the indumentum is usually woolly, there is great diversity in hair-structure in this series (Cowan, op. cit., pp. 66, 82).

s. Thomsonii

Revision: R.Y.B. 1951-2 (6), pp. 116-83.

The Thomsonii series is the most difficult of all to comprehend, for the species grouped in it seem to have so little in common that it is impossible to define the group except in the vaguest terms. However, there is no need to belabour this difficulty, which is fully discussed by Cowan and Davidian in their valuable revision of the series. The group is more easily understood if it is realised that the type, R. thomsonii, is not the central species but rather an extreme. The middle position is occupied by the Campylocarpum subseries, which has obvious affinity with ss. Thomsonii in one set of characters (leaf- and corolla-shape) and with ss. Selense in another (notably the partly glandular style and the slender, often curved seed-capsule). In Cowan and Davidian’s words: ‘The Thomsonii series represents an alliance which cannot be strictly defined, a loosely linked yet recognisable natural association.’

Nearly all the species are medium-sized shrubs. Leaves thinly leathery, obtuse to rounded at the apex, obtuse or cordate at the base, undersides glabrous or with an inconspicuous veil of hairs (see also R. hookeri). Inflorescence terminal, loose, with rarely more than twelve flowers. Corolla-shape variable (see subseries). Ovary commonly glandular (though not in some species of ss. Thomsonii). Style glandular at least at the base (except in ss. Thomsonii). Capsule variable (see subseries).

ss. Thomsonii. – Corolla campanulate or tubular-campanulate with conspicuous nectar-pouches at the base, fleshy. Calyx variable, often very large. Style without glands. Capsule short and stout, or oblong. Ten species from N.W. Yunnan to E. Nepal.

ss. Campylocarpum. – Leaves 1 to 4 in. long, mostly relatively broad. Calyx small. Corolla campanulate, yellow, pink, or white. Ovary glandular; style glandular in the lower part. Capsule slender, cylindric or oblong, sometimes curved. Six species, perhaps reducible to four, from N.W. Yunnan to E. Nepal.

ss. Cerasinum. – R. cerasinum (q.v.) resembles R. thomsonii in shape of corolla but differs markedly in foliage and in having the style glandular to the tip. The only other species in the subseries is known only from a single collection and is not cultivated; it is geographically far removed from R. cerasinum (W. Szechwan).

ss. Selense. – A polymorphic group, uncommon in cultivation. Leaves mostly oblong or elliptic. Branchlets slender, bristly-glandular or sometimes glabrous. Corolla usually near to funnel-shaped (but campanulate in R. eurysiphon and in some forms of R. esetulosum), pink, rose, or white, often spotted or blotched. Ovary and lower part of style glandular. Capsule slender, usually curved. Eleven species in N.W. Yunnan, bordering Tibet and upper Burma.

ss. Souliei. – Very near to ss. Campylocarpum, in which it was included by Balfour, but with a cup-shaped corolla and the style glandular throughout. Four species from W. Szechwan to the eastern end of the Himalaya.

ss. Williamsianum. – The one species in this subseries (q.v.) resembles R. souliei and its allies in having the style glandular to the tip, and R. callimorphum (in ss. Campylocarpum) in its pink, campanulate corolla and short broad leaves. It differs from nearly all species in the Thomsonii series in its dwarf habit and few-flowered inflorescence.

Azaleas

In Species Plantarum (1753) Linnaeus distributed the species of Rhododendron known to him between two genera, according to the number of stamens (a character of great importance in his system of classification). In his genus Rhododendron he placed the species with ten stamens (R. ferrugineum, R. hirsutum, R. dauricum, and R. maximum). His genus Azalea comprised species with five stamens, of which four are members of the Azalea series of Rhododendron-. R. indicum (A. indica L.), R. luteum (A. pontica L.), R. viscosum (A. viscosa L.), and R. periclymenoides, better known as R. nudiflorum (A. lutea L., altered to A. nudiflora in the second edition of the Species Plantarum). The two remaining founder-members of Linnaeus’ Azalea were R. lapponicum, a lepidote rhododendron that happens to have five stamens instead of the normal ten (A. lapponica L.); and a distant relative of Rhododendron, named by Linnaeus A. procumhens, now known as Loiseleuria procumbens (L.) Desvaux.

In splitting Rhododendron (as now understood), Linnaeus showed less taxonomic insight than his predecessor the French botanist Tournefort, whose genus Chamaerhododendros contained originally both the azalea R. indicum and the two lepidote rhododendrons R. ferrugineum and R. hirsutum (Institutiones Rei Herbariae (1700), p. 604), to which he added, after seeing them in the wild in Turkey, R. ponticum and the common yellow azalea R. luteum (Corollarium (1703), p. 42). As soon came to be realised, Tournefort and not Linnaeus had the rights of it. The number of stamens in the genus Rhododendron is not generally a character of much significance, and may fluctuate even among closely allied species. It happens that the azaleas of the Luteum subseries, of which Linnaeus knew three, have consistently five stamens; but R. indicum, also with five stamens, belongs to a group – the Obtusum subseries – in which the number is mostly between six and ten. In the century or so after the publication of Species Plantarum botanists either gave the genus Azalea half-hearted acceptance or transferred the known azaleas to Rhododendron. The coup de grâce to Azalea as a separate genus was given by Maximowicz in his authoritative work Rhododendreae Asiae Orientalis (1870), which laid the foundation for all later classifications of the genus. In gardens the genus Azalea was kept distinct from Rhododendron long after it had become botanically disreputable to do so. At the present time the word ‘azalea’ is still used in an informal sense for the species and hybrids of the Azalea series, and quite properly so, and still has standing as a subgeneric name (see below).

A leading character of the azaleas is that not only are scales absent from all parts of the plant (the species are ‘elepidote’) but, as Almuth Seithe has pointed out, their hairs are always unbranched. Also, the majority of the species are deciduous or (as in many species of the Obtusum subseries) drop the leaves formed in spring while retaining through the winter a cluster of thicker and smaller leaves at the apex of the shoot (the so-called summer-leaves). A deciduous habit is elsewhere found in Rhododendron only among the lepidote species – commonly in s. Dauricum and s. Trichocladum, rarely in s. Triflorum and s. Lepidotum. The azaleas are slenderly branched shrubs. The inflorescence is always terminal and umbellate (even when it is many-flowered the rachis is very short). The corolla is usually funnel-shaped or rotate, with a fairly deeply divided, more or less zygomorphic limb; the markings, when present, are on the upper-central or three upper lobes, as is usually the case with zygomorphic corollas. The number of stamens is often below the standard ten, and very rarely more than that. The ovary is consistently five-celled. The seeds are winged or unwinged, never tailed.

In Species of Rhododendron the Azalea series is divided into six subseries, which in a strictly botanical classification should probably be regarded as representing two subgenera (only the principal synonyms are given):

subgen. Azalea (L. emend. Desv.) Planch. Azalea L., in part; Rhodora L.; Anthodendron Reichb.; Rhod. sect. Pentanthera G. Don; Rhod. a. Anthodendron Reichb. ex Endl.; Rhod. sect. Azalea (L.) Maxim., in part; Azalea subgen. Pentanthera (G. Don) Koch; Rhod. subgen. Anthodendron Endl. ex Rehd. & Wils., in part; Rhod. subgen. Pseudanthodendron Sleumer; Rhod. subgen. Pentanthera (G. Don) Pojarkova – The subseries or sections of this subgenus have terminal buds of the usual form characteristic of most members of the genus, i.e., they are composed of numerous perulae and are sharply demarcated from the surrounding leaves; the upper axillary buds are subtended by fully developed leaves, as again is normal in the genus (cf. subgen. Tsutsia).

ss. Luteum. – Erect deciduous shrubs. Inflorescences with up to about fifteen often fragrant flowers. Calyx mostly rather small. Corolla funnel-shaped, the tube as long as the limb or longer, the outside downy or glandular, rarely glabrous, white, pink, yellow, orange, or red, Stamens five, exserted. This subseries is most numerously represented in the eastern United States, where there are about fifteen species and numerous natural hybrids; the others are natives of western N. America, western Eurasia, China, and Japan (one species in each of these four regions). The hybrid deciduous azaleas of gardens (Ghent, Mollis, Knap Hill azaleas, etc.) all derive from crossings within this group.

The botanical name for this group (as a section) is sect. Azalea (sect. Pentanthera G. Don).

ss. Canadense. – Deciduous erect shrubs. Inflorescence with two to eight flowers. Corolla widely campanulate, white, pink, purple, or purplish red, two-lipped in R. canadense and to a lesser degree in R. vaseyi. Stamens ten (fewer in R. vaseyi). The two species mentioned are natives of eastern N. America. The other two members of the subseries, R. albrechtii and R. pentaphyllum, are natives of Japan and differ in some respects from the American species. The botanical name for this subseries is sect. Rhodora (L.) G. Don, and the type is R. canadense, for which Linnaeus created the genus Rhodora in 1762.

ss. Nipponicum. – This subseries, which consists of a single species (q.v.), is of uncertain taxonomic status and perhaps wrongly placed in the subgenus Azalea. Its sectional name is sect. Viscidula Matsum. & Nakai.

subgen. Tsutsia Planch. Rhod. sect. Tsutsutsi G. Don, in part; Rhod. sect. Tsusia Planch, ex Maxim.; Rhod. subgen. Tsutsutsi (G. Don) Pojarkova; Rhod. subgen. Anthodendron Endl. ex Rehd. & Wils., in part – Here are placed two subseries, ss. Obtusum and ss. Schlippenbachii, in both of which the buds show unusual features, usually stated, for both groups, to be that the flowers and young shoots are produced from the same bud. So far as the Schlippenbachii subseries is concerned, judging from the living plants examined, this is a crude over-simplification. The flower-buds and growth-buds are distinct, as is usual in the genus, but the uppermost axillary buds are subtended by leaves which have become partly modified into bud-scales (perulae). The bases of these persist through the winter, forming an outer covering to the true bud, but fall off as the flowers expand in spring. The statement ‘flowers and young growths from the same bud’ is true only if this compound structure is regarded as a single bud.

The first to note that the buds of the Obtusum subseries are unusual was Maximowicz, who, in the work mentioned above, gives ‘Young growths from the same bud as the flowers’ as the diagnostic character of his section Tsusia [so spelt by him], which comprised the then known species of the Obtusum subseries. However, in the supplement to his monograph, published in 1871, he withdrew this diagnosis, confessing that it had been based on inadequate study. The peculiarity of this group, as he pointed out in the supplement, is that the flower-buds are surrounded by very few fully developed scales, and these are concealed by the bases of the uppermost leaves, which form a tuft at the apex of the shoot and persist through the winter. The innermost of these leaves are partly converted into bud-scales, as can be seen from their flattened, etiolated bases. According to Maximowicz, whose observations were based on plants grown under glass at St Petersburg, the young growths spring from these intermediate leaves, but on plants grown out-of-doors, judging from those examined, the growth-subtending leaves are more or less normal. Another character remarked on by Maximowicz in his supplementary note is that the axillary growth-buds are not formed, or at least do not become visible, until the flowers are over, so that the processes of bud-formation and shoot-expansion are more or less simultaneous; also, these buds consist of very few scales.

ss. Obtusum. – Shrubs mostly of small size, sometimes prostrate; young stems and (more or less densely) the leaves, clad with erect or appressed hairs. Leaves partly deciduous, but at least the uppermost persistent. For buds see above. Inflorescence few-flowered. Corolla funnel-shaped or funnel-campanulate, mostly 112 to 2 in. wide, white, in various shades of purple, pink, or bright red (never yellow). Stamens five to ten (very rarely more or fewer). Ovary hairy and sometimes glandular. Seeds plain. This group has its greatest development in the maritime parts of E. Asia; in the Sino-Himalayan region it occurs only as an intrusion at comparatively low altitudes. About thirty species are recognised, mostly little known. Only a few are cultivated, these mostly Japanese. The numerous garden hybrids are also mainly derived from Japanese species (and the mainland R. simsii).

The botanical name for the Obtusum subseries, regarded as a section, would be sect. Tsutsia.

ss. Schlippenbachii. – Erect shrubs or sometimes small trees. Leaves deciduous (except sometimes in R. farrerae), usually forming whorls at the ends of the branchlets (but scattered on long shoots). For buds, see introductory note to the subgenus. Inflorescence terminal, one- to six-flowered. Corolla campanulate to funnel-shaped with a spreading limb, white (rarely), pink, purple, or red, usually spotted. Stamens six to ten. Seeds plain. Seven or eight species in Japan and continental E. Asia, where the subseries ranges from S. China through Hupeh to Manchuria and Korea.

The botanical name for this subseries, as a section, is sect. Brachycalyx Sweet (sect. Sciadorhodion Rehd. & Wils.).

ss. Tashiroi. – This subseries consists of a single species, R. tashiroi, not treated here. It is discussed by Rehder in Species of Rhododendron, p. 121.

Azaleastrums

The azaleastrums, or ‘false azaleas’, are a rather heterogeneous assem­blage of species, grouped together in the subgenus Azaleastrum Planch, ex Koch (Rhod. sect. Azaleastrum Planch, ex Maxim.), the type of which is R. ovatum. The group is elepidote (without scales) and the hairs when present, are always unbranched – a combination they share with the azaleas. Apart from that, the leading character of the azaleastrums is that the inflorescences are always axillary and few-flowered (solitary in some species). The young growths spring from terminal or pseudo-terminal buds, or from buds below the inflorescences. There are four series, each of which ranks as a section in Sleumer’s classification.

s. Albiflorum. – The only member of this series is R. albiflorum (q.v.), a native of western N. America (sect. Candidastrum Sleumer).

s. Ovatum. – Evergreen small shrubs. Leaves glabrous, with a pronounced mucro. Flowers solitary from many-scaled axillary buds. Calyx-lobes large and broad. Corolla in most species deeply lobed, with a short tube and spreading limb (but tubular-funnel-shaped in R. vialii), white, pink, or crimson. Stamens five, exserted. Capsule very short. Five species, perhaps reducible to two, natives mainly of central and south China, rare in Sino-Himalaya. Since this group contains the type of the subgenus it takes the botanical name sect. Azaleastrum (syn. sect. Euazaleastrum Sleumer).

s. Semibarbatum. – The one species in this subseries, a native of Japan, differs from all other members of the genus in its dimorphic stamens (see description). It was given generic rank by a Japanese botanist as Mumeazalea semibarbatum whence the sectional name sect. Mumeazalea (Makino) Sleumer.

s. Stamineum. – This, the largest and finest group of azaleastrums, takes the botanical name sect. Choniastrum Franch., the type of which is R. stamineum, though the first species to become known to science was R. moulmainense Hook., not treated here. They are shrubs or small trees up to 40 ft high, with evergreen glabrous or more rarely hairy leaves, which often form ‘pseudo-whorls’ at the ends of the shoots. Inflorescences with up to about five flowers, which are fragrant in some species; bud-scales numerous, often persistent. Calyx mostly small or obsolete. Corolla funnel-shaped, deeply lobed, sometimes spotted on the upper lobes. Stamens ten, usually exserted. Capsule long and narrow, the valves sometimes adhering at the apex. About fifteen species in S.E. Asia, occurring in Sino-Himalaya only at low elevations; the southern limit is in the Malayan peninsula, the northern in Hupeh, and the western in the Assam Himalaya. Unfortunately the finest species are very tender.


Footnotes

The revised diagnosis appears in Bull. Acad. Imp. Sc. St Petersb., Vol. 16, pp. 412-13 (Mel. Biol., Vol. 8, pp. 166-7).

Species articles

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