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The family ERICACEAE presently consists of 8 subfamilies, 126 genera and about 3,995

species that are distributed worldwide from temperate and subarctic regions to high elevations of tropical regions. Rhododendron L. belongs to the subfamily ERICOIDEAE

7 _{Also known as Carolus Clusius, a Flemish doctor and pioneering botanist. He is renowned for his} extensive study of the Spanish flora

8 _{A Scotsman from Aberdeen, who worked for the East India Company, and was renowned as a botanist} and the author of the publication Malayan Miscellanies.

Endl. (Gillespie & Kron 2010; Kron et al. 2002) and was previously placed under the subfamily RHODODENDROIDEAE (Copeland 1943; Cox 1948; Stevens 1971) and tribe RHODOREAE G Don. RHODODENDROIDEAE was divided by de Candolle, which he recognized as a tribe, into two groups depending on whether or not the corolla lobes were free (de Candolle & de Candolle 1838). Klotzsch (1851) did not use this character, and recognized MENZIESIACEAE (with squamose buds and anthers dehiscing by more or less

elongated slits) and RHODORACEAE (with strobiliform leaf buds and anthers dehiscing by pores, the pollen being mixed with viscin threads).

Copeland (1943) recognized 20 genera and four tribes within the subfamily RHODODENDROIDEAE (Kron et al. 2002). These 20 genera are currently recognized in the

subfamily ERICOIDEAE (Kron 1997). Copeland (1943) based his circumscription of the

subfamily on anatomical and embryological characters. He defined the ERICOIDEAE by

the presence of deciduous corollas, anthers without awns, and septicidally-dehiscent capsular fruits. The subfamily ERICOIDEAE presently includes 19 genera in five

recognized tribes, with the tribes separated by the pattern of anther dehiscence. Cox (1948) studied the wood anatomy of RHODODENDROIDEAE and described five tribes, four

of which including RHODOREAE were classified based on anatomical characters similar

to those of Copeland’s study.

The formal classification of the rhododendrons began when Linnaeus first instituted the genus Rhododendron9 in 1753 and he also created a separate genus Azalea which contained 6 species. The division of these species into Rhododendron and Azalea was based on the number of stamens, 10 and 5 respectively. Salisbury (1796) remonstrated that Azalea and Rhododendron could not be maintained as distinct genera. In early 19th century, George Don (1834) recognized 57 species of Rhododendron and divided the genus into 8 sections based on floral and foliar characteristics, as shown below:

9 _{The name ‘Rhododendron’ referred to the rhododendrons however appeared in literature as early as the} year 1535.

Order10_{ERICACEAE G. Don}

Subfamily RHODOREAE G. Don Genus Rhododendron L. Section Ponticum G. Don Section Boòram G. Don Section Pogonanthum G. Don Section Lepípherum G. Don Section Chamaecístus G. Don Section Tsutsutsi G. Don Section Pentanthera G. Don Section Rhodora G. Don

Genus Vireya Blume

Don differentiated the genus Vireya from the genus Rhododendron with the following characters: (i) having flowers with small calyces, (ii) the stamens not being attached to the corolla in any way, (iii) the majority of the species are epiphytic shrubs, (iv) leaves scattered and verticillate, (v) leaf lamina with margins quite entire, coriaceous, and covered with scales, and (vi) flowers borne in terminal fascicles.

Indiscriminate naming of rhododendrons continued, with authors placing species in both Rhododendron and Azalea. Planchon (1854) reviewed and placed all the 25 then known taxa from the Far East in Rhododendron, increasing the number of species considerably. Until 1860 all the known species of azaleas introduced from the Far East were from gardens, the majority from China. In 1870, botanist and plant collector Carl Johann Maximowicz11 (1870) used living collections and herbarium material to develop a series of new diagnostic characters (firmness of the testa and the persistence or otherwise of the bracteoles), which enhanced Don’s work. Their combined natural system of classification much improved and revised the delimitation of the genus. Maximowicz recognized two subtribes, EURHODODENDREAE corresponding to the RHODORACEAE of Klotzsch, and

PHYLLODOCEAE corresponding to the MENZIESIACEAE.

In 1882 Charles Baron Clarke published a treatment (with keys) of Indian Rhododendron in J D Hooker’s Flora of British India (Clarke 1882), in which the genus was divided into the four subgenera, as shown below:

10 _{Classified as an ‘Order’ as opposed to a ‘Family’, containing 5 tribes.}

11 _{Curator/Chief Botanist of St Petersburg Botanic Gardens; Keeper of the Herbarium at the Imperial} Botanic Garden, St Petersburg. He is also recognized as the first person to enumerate the flora of Japan.

Genus Rhododendron L.

Subgenus Vireya Blume

Subgenus Pseudovireya G. Don

Subgenus Eurhododendron (DC.) Maxim. Series A (non-lepidote species)

Series B (lepidote species) Subgenus Rhodorastum Maxim.

The subgenera were separated according to the characters: capsule and seed morphology, capsule dehiscence, phyllotaxy, and inflorescence arrangement. The species of Subgenus Eurhododendron were separated into the two series (A and B) mainly according to foliar characters (Photo 3):

Series A

● Elepidote (non-lepidote): leaves with abaxial surfaces glabrous, or tomentose without glandular scales.

● Plants are usually large shrubs or trees. Series B

● Lepidote: leaves with abaxial surfaces covered with sessile, round, glandular scales (which in R. anthopogon coalesce, forming a tomentum).

● Large or small shrubs.

Photo 3 Examples of elepidote and lepidote rhododendrons. (a) Elepidote (Series A) species:

Rhododendron arboreum ssp. zeylanicum, growing wild on the Horton Plains (Sri Lanka), at ~2,000 m altitude. (b) Lepidote species (Series B): R. lindleyi, cultivated at the Pukeiti Rhododendron Garden (New Zealand).

In 1889, Leopold Dippel12 published an account of trees and shrubs cultivated in Germany, which included several species of Rhododendron and its allies (Dippel 1889). The classification he followed was mainly that of Maximowicz, but also utilized additional morphological characters such as inflorescence, floral and foliar to demarcate the various subgroups, and the classification used is as shown below:

Suborder RHODODENDRINEAE J. Presl. Family ERICACEAE G. Don

Tribe RHODODENDREAE (Juss.) Colla Subtribe EURHODODENDREAE Maxim. Genus Rhododendron L.

Subgenus Osmothamnus Maxim. Subgenus Eurhododendron Maxim. Section Candalbra Dippel

Section Chrysantha Dippel Section Lepidota Maxim. Subgenus Therorhodion Maxim. Subgenus Azalea L.

Section EuazaleaMaxim. Section Rhodora G. Don

Subgenus Tsusia (Planch.) Maxim. Subgenus Rhodorastrum Maxim. Subgenus Azaleastrum Planch.

Dippel described 39 species of Rhododendron using the above classification, and his work became an excellent account of the majority of species of Rhododendron in cultivation at that time.

The treatment of ERICACEAE by Drude (Drude 1897) in the monumental work on plant taxonomy and phytogeography, Die Natürlichen Pflanzenfamilien (‘The Natural Plant Families’ by Adolf Engler and Karl von Prantl) is notable. This treatment, even though minor compared to some of the previous works such as those by Maximowicz, forms an integral part of the proposed plant classification system (the ‘Engler System’), which is still in use by many herbaria and authors.

Smaller number of species were included in various subsequent publications such as the works by Koehne (1893), Zabel (1902) and Schneider (1906) all adopting the

classification by Maximowicz. The classification system of Dalla-Torre and Harms (von Dalla-Torre & Harms 1903) provided a comprehensive synonymy of Rhododendron L. and its subgenera. The hierarchy is arranged as below:

Family ERICACEAE DC.

Subfamily RHODODENDROIDEAE Drude Tribe RHODODENDREAE Spreng.

Genus Rhododendron L.

Subgenus Eurhododendron Drude Section Eurhododendron Drude Section Vireya Hook. f.

Section Osmothamnus Maxim. Subgenus Azalea Planch.

Section Rhodora G. Don Section Azaleae Drude Section Tsusia Planch.

Subgenus Rhodorastrum Maxim. Subgenus Azaleastrum Planch. Subgenus Keysia Drude

Subgenus Choniastrum Franch. Subgenus Therorhodion Maxim.

In the above classification Vireya was re-introduced as a section, and Pseudovireya is reduced to a synonym of Vireya. There are a few notable hierarchical changes compared to Maximowicz’s and Dippels’s classification, such as Osmothamnus and Tsusia.

The years that followed saw the discovery of numerous new species of Rhododendron, many of them described by Ernest Henry Wilson. In 1913, E H Wilson collaborating with Alfred Rehder published an account of the genus Rhododendron (Wilson 1913). The classification he followed was principally that of Maximowicz, but differed widely in the conception and limits of certain sections, mainly the arrangement of the species with revised nomenclature. The main characters used to classify the subgenera were: (i) scaliness and hairiness of the leaves, (ii) leaves persistent or deciduous, (iii) ovary surface (iv) ovary locules 5 or more than 5, (v) number of corolla lobes, and (vi) number of stamens either 5–10 or 10–20.

Family ERICACEAE G. Don

Genus Rhododendron L.

Subgenus Lepidorhodium Koehne. Section Pogonanthum G. Don.

Section Lepipherum G. Don (Syn: Osmothamnus Maxim.) Section Rhodorastrum Maxim.

Section Lepidota Maxim.

Subgenus Eurhododendron Maxim. Subgenus Azalea Planch.

Section Chionastrum Franch. Section Azaleastrum Planch.

Section Tsutsutsi G. Don (Syn: Tsusia Planch.) Section Pentanthera G. Don (Syn: Euzalea Maxim.)

Following on from their previous work, Wilson and Rehder (1921) published a monograph on azaleas for the Subgenus Anthodendron, which was divided into two major parts, one for azaleas of the Old World (the Asiatic sections) and the other for the azaleas of North America. In this monograph in addition to the description of novel species and a new Section Sciadorhodion, they rectified previous errors and misconceptions:

Azaleas of the Old World Genus Rhododendron L.

Subgenus Anthodendron Endlicher Section Tsutsutsi G. Don

Section SciadorhodionRehder & Wilson Section Rhodora G. Don

Section Pentanthera G. Don

Azaleas of North America Genus Rhododendron L.

Subgenus Anthodendron Endlicher Section Rhodora G. Don Section Pentanthera G. Don

The work carried out by Schlechter (1919) for the Rhododendron species of New Guinea and Copeland (1929) for the Rhododendron ofPhilippines, showed similar trends in their classifications. Notably, these two studies also suggested subdivisions for the vireyas (which are discussed further in Section 2.2.1).

In the early part of the 20th century a large number of plants were acquired by the Royal Botanic Garden Edinburgh (UK) from several collectors including Sir Isaac Bayley Balfour13, Peter Hadland Davis14, Reginald John Farrer15, George Forrest16, Henry Halcro Johnston17, Frank Kingdon-Ward18, and Frank Ludlow19, leading to another dramatic rise in the number of Rhododendron species known. The genus grew immensely due to the influx of the new species, and some sections such as Hymenanthes grew very large. To break down these very large sections, Sir Isaac Bayley Balfour began to group many of the species into series (Balfour 1916, 1917, 1919, 1920).

Cataloguing the new living material of the many taxa that came to the garden led to the development of an artificial classification system, culminating in the publication of The Species of Rhododendron by the Rhododendron Society and edited by J B Stevenson (1930) under the auspices of the Regius Keeper of the garden, Sir Isaac Bayley Balfour. The morphological characters used for this classification system included: leaf-shape and size, leaf punctuation, presence or absence of hairs on the pedicels, ovary shape and surface features, etc. The new classification system gave rise to a very significant revision of the genus, in which all the temperate and subtropical species of Rhododendron known at that time were split into 39 series and several subseries (Hutchinson 1919, 1930; Rehder 1930; Stevenson 1930, 1947; Tagg 1930). This classification has been adopted by consequent botanists and researchers until recent developments in molecular systematics (Goetsch et al. 2005; Goetsch & Hall 2002; Kron 1997; Kron & Judd 1990), which have now begun to assist in the unravelling the many systematics issues associated with the

13 _{Sir Isaac Bayley Balfour (1853–1922) collected in Rodriguez and Socotra Islands, describing over 300} new species of plants in his Botany of Socotra.

14 _{Peter Hadland Davis (1918–1992) collected plants from almost all of the countries of the Mediterranean,} and many other countries of the world. He was renowned as a major contributor to the Flora of Turkey and the East Aegean Islands.

15 _{Reginald John Farrer (1880–1920) explored the Himalayas and the greater China, Burma and Ceylon.} His collections together with illustrations, field notes, botanical specimens and seeds, provided valuable information to the Royal Botanic Garden Edinburgh, where the Regius Keeper Sir Isaac Bayley Balfour, took a special interest in Sino-Himalayan plants.

16 _{George Forrest (1873–1932) ranks amongst the greatest of all collectors of rhododendrons, introducing} hundreds of species from China and Tibet to the Royal Botanic Garden Edinburgh, notable species including R. giganteum and R. sinogrande.

17 _{Henry Halcro Johnston (1856–1939) made significant contributions to botany and horticulture through} his detailed collection of plant species.

18 _{Frank (Francis) Kingdon Ward (1885–1958) went on around 25 expeditions over a period of nearly fifty} years, exploring the Himalayas, Tibet, North Western China, Burma (Myanmar) and Assam (North Eastern India).

19 _{Frank Ludlow (1895–1972) is well known for his discovery of rhododendrons on expeditions to Tibet,} and also found better forms of many previously discovered rhododendron species.

genus Rhododendron. The work on the revision of the classification resumed with the revision of several series by J M Cowan and H H Davidian (Cowan & Davidian 1947, 1948, 1949, 1951; Davidian 1954, 1963, 1964).