QBARS - v35n4 Groping for Grouping
GROPING FOR GROUPING
Rhododendron Taxonomy
by Herbert A. Spady
The similarity of 'Yaku Fairy' and R. arboreum may be obvious to the modern rhododendron enthusiast. Such affinity would not have been obvious to any ancient Greek or Roman or even to a European scholar up to the early 17th century. The development of modern concepts of taxonomy was a very laborious process.
The learning inherited by the herbalists (Brunfels, Bock, and Fuchs) was nothing more than medieval superstitions. Their interest was to recognize medicinal plants named by ancient Greeks and Romans. Although their works were feeble by later standards, a new epoch of scientific botany began with these men. This was the first descriptive botany. Their efforts were naive and un-artistic, but they were made directly from plants as examined by the authors. The compilers of the herbals formed no general conclusions from their observations. The works were organized as herbs, shrubs and trees or compiled alphabetically.
The number of described plants rapidly increased from five hundred in 1542 to six thousand in 1632. Along with this increase in number came a vague sense of natural relationships within the plant kingdom. Although vague and unexpressed formally, this was a concept never glimpsed in classic antiquity or the middle ages. If not formally expressed, how was it expressed? Groupings that have natural affinities occur in the works of l'Obel in 1576 and more clearly in the works of Kasper Bauhin in 1632. These groupings are spotty and broken and still occur in the framework of trees, shrubs, under shrubs and herbs. If these men had worked with descriptions of rhododendrons, they no doubt would have had spotty groupings of similar rhododendrons under trees, shrubs and under shrubs. Their groupings gave heavyweight to plant habit and little thought to the sexual parts of plants.
The similarities between plants were unconsciously accepted through an association of ideas. This involuntary mental act created the feeling of a necessity for a clearer idea of the phenomenon of natural groupings. Many early botanists attempted to impose artificial criteria of classification that had no basis in scientific observation. Their criteria were based on a priori grounds. Using a few easily recognized features, each botanist tried to improve on the last botanist's system, but missed the mark. Even Linnaeus" sexual system was artificial. He recognized that fact. He was the first to state that there was a natural system of plants, which could not be easily established by the use of predetermined criteria and that the rules for framing that system were still undiscovered. Linnaeus was far from the first to use the binomial system, or to attach importance to the sexual parts of plants. He does not represent the beginning of a new era in botany, but rather a culmination of the old era.
The real significance of the affinities of plants could not be explained or understood in the face of the doctrine of the constancy of the species. It was not until Charles Darwin made it evident that the relationship of species is linear or branching that the muddy waters began to clear. We have not yet cast off the hang-ups of the pre-Darwinian botany. Our minds are muddled by plants not conforming to the type of the species or genus. We conveniently put them into some category like "natural hybrid." In populations of plants, as in populations of people, there are no types. That is, there is no average individual. Each living creature is individually different from every other. We seldom have any trouble distinguishing the people that we know well, but contrast that with ones that we know poorly. If the details about the individuals in a collection of plants of one species were well known, we would have little trouble separating them.
Let us begin with the end of the old botany and the system of Linnaeus. It was a system that he recognized as artificial. Based on the criterion of the number of stamens (the characteristics of the flower, i.e., the sexual system) he divided the nine rhododendrons known to him in two groups. Those with ten stamens, R. ferrugineum , R. hirsutum , R. dauricum and R. maximum were placed in genus Rhododendron . He placed those with five stamens, i.e., A. indica ( R. indicum ), A. pontica ( R. luteum ), A. viscosa ( R. viscosum ), A. lutea ( R. calendulaceum) , and A. lapponica ( R. lapponicum ) in genus Azalea.
Over the next two hundred years there was a tenfold increase in the number of species that would be included in the genus Rhododendron . The first few years saw the recognition and analysis of species mainly from the geographic and phylogenetic periphery of the genus. The general reaction of the botanists was to put each new species that did not fit the type of the genus into a new and separate genus. Linnaeus created genus Rhodora ( R. canadense ) in 1763. Blume named genus Vireya in 1826 and genus Hymenanthes ( R. metternichii ) in 1826.
George Don in his General History of Dichlamydious Plants (1834) established the first framework for a hierarchy of rhododendrons. He formerly united Azalea and Rhododendron as had been previously suggested by other botanists (Salisbury 1805-08 and Tate 1831). He subdivided the genus into sections. The names of some of these still apply: Tsutsusi , Pentanthera , and Pogonanthum . His section Lepipherum and the two sections Ponticum and Booram foreshadowed, but still somewhat missed the mark of the lepidote-elepidote division of the genus.
Don was still handicapped by the limited number of species and the lack of definition of those characteristics that displayed affinities and discontinuities. The next important step in the recognition of these features was by Maximovicz (1834-1870). He stressed the significance of the position of the flower buds and their relationship to leaf buds in defining the major subdivisions of the genus. His work, along with that of Planchon 1854, began to clarify that within "azalea" there were to be found several atypical groups. Although Clarke published a key in 1882 that depended on the presence or absence of scales as its major cleavage feature, no clear cut division into lepidote and elepidote divisions occurred until Koehne proposed this division in 1893. Vireya by Clarke and Rhodorastrum by Franchet were more accurately subdivided.
From the time of Maximovicz to 1930 came a flood of rhododendron species from southwestern China. This ultimately helped to establish a more defined hierarchy, but at the same time contributed to its obscuration and frustration. This was largely due to the way that these species were handled. This cumulative 175 years of work to establish a hierarchy of the genus was largely ignored by Professor Isaac Bayley Balfour and associates when they attempted to break down the huge flood of species into more manageable units. They clustered the species into Series. These Series were listed alphabetically in The Species of Rhododendron without in any way grouping them into larger units. This publication had a profound influence in educating and providing a means of communication for gardeners. Balfour and his associates acknowledged that this expedient was a stop-gap measure. Revisions were contemplated, but never undertaken by English speaking botanists.
The lack of a systematic treatment of the genus Rhododendron led Hermann Sleumer to concern himself with the problem. Beginning in 1934, he attempted to fuse the Series as published and revised by Balfour and others into the hitherto accepted subgenera and sections. His work was ultimately published in German as A System of the Genus Rhododendron , L. 1949. It was not published in English until 1980! "The system of Sleumer, as modified by Cullen and Chamberlain and the Philipsons" was adopted at the International Rhododendron Conference in 1978 as "a useful framework for taxonomic organization of Rhododendron at the level of section and subgenera."
That brings us to the most modern revision of the genus Rhododendron by Cullen, Chamberlain, and others. This revision essentially dismembers the Series concept. Because the Series concept has long been accepted among hobbyists and gardeners, the revision has met with a considerable degree of resistance and unpopularity. Who wants to change their way of thinking or lose their favorite species as a synonym? As pointed out by Cullen in Contributions Toward a Classification of Rhododendron , The usefulness of the Series concept had been stretched beyond reason by new scientific knowledge and methods. The publication includes modern applications of population studies to the definition of species. Microscopic and molecular characteristics never before considered are used as taxonomic marks. It points the direction of future work and technologies. We cannot know the future, but one can visualize the day when the genetic material (DNA) of a species may be the fingerprint of that species.
Although there is considerable diversity in the genus Rhododendron , there is also considerable homogeneity. As a group they come very close to the description of the average dicotyledon as described by Ronald Good in Features of Evolution in Flowering Plants :
These are "plants which, though on the one hand having a fully woody habit, do not, on the other, possess a main axis or trunk. In short, the kind of plant which is called a shrub, and whose stature within that definition, may be thought of as somewhere between five and fifteen feet." The foliage is "represented by medium sized, more or less lanceolate, space separated blades..."' The segmentation of the foliage is "neither hairy nor completely glabrous, but exhibits some kind of pubescence, at least in their younger parts." "The flowers are of small to medium size, grouped in a semi-compacted inflorescence: regular: hermaphroditic: diclamydeous: pentamerous: and either mono- or diplo-stemonous."
The genus does stretch some of the points of the description.
Within subgenera, there is considerable homogeneity. One of the most important illustrations of this similarity is that the species, within limitations, will hybridize and form fertile progeny. This points out the dilemma facing the systematic botanist in delineating the species in many plant groups. If we were discussing animals, we would consider these mutually fertile species to be one species with subspecies and races. In some genera, species are quite distinct in appearance and transitional forms between species are absent; but in rhododendrons, species are outwardly very similar or identical. Modern concepts of population genetics were not well formulated at the time of the plant explorations of Forrest, Ward, Wilson, etc. In fact, they set out with the very purpose in mind to find new species. We are familiar with the confusing mass of material which they sent back, and the constant reduction in the number of species that has gone on since the original descriptions. Some of the species described seem hardly more distinct than races or subspecies of a single species. Races and subspecies may be incipient species. This does not mean that every race will some day turn into a separate species. It means only that some of them may do so if the genetic divergence proceeds far enough.
That divergence must proceed to the point that the species become genetically reproductively isolated. These isolating mechanisms may take two forms, one before formation of the seed (prezygotic) and one after the formation of seed (post-zygotic). Prezygotic mechanisms are:
1. Ecological or habitat isolation - Different habitats in the same territory (sympatric).
2. Seasonal or temporal isolation - Sexual maturity at different times.
3. Ethological or sexual isolation - In plants, represented by pollinating mechanisms dependent on specific insects.
4. Mechanical isolation - Differing flowering parts preventing pollen from reaching the ovary.
5. Gametic isolation - Non-mechanical incompatibility of pollen.
Post-zygotic isolating mechanisms are:
1. Hybrid inviability - Progeny die before they can reproduce.
2. Hybrid sterility - Progeny do not produce viable seed.
3. Hybrid breakdown - Viability decreases or sterility increases in subsequent generations of progeny.
Geographic isolation is deliberately excluded from prezygotic isolating mechanisms. There is no doubt that races or subspecies of sexually reproducing forms can be kept apart by the simple fact of being geographically isolated. An example of this is R. cinnabarinum and R. xanthocodon . The same is true of allopatric species, some of which easily hybridize when brought together in an artificial environment. However, geographic isolation is not a reproductive isolating mechanism stemming from the genetic constitution of the population. Geographically isolated populations may be genetically identical, or if they are different, the differences may be a consequence rather than a cause of the isolation. A rigorous test of efficient reproductive isolation is the ability of the populations to coexist sympatrically with little or no gene flow between them. Species, not races of sexually reproducing and out breeding forms, can coexist sympatrically without gene exchange and eventual fusion. Many plant species can diverge greatly without becoming separated by permanent barriers of reproductive isolation. They also remain morphologically different even when sympatric over large areas, but in localized regions, often in the presence of habitat disturbance, they form hybrid swarms. This is well illustrated by the native "azaleas" of the southeastern U.S.
The problem of clearly defining a species is related to the very manner in which speciation occurs. The origin of a new species requires a succession of events that are usually distributed over a considerable period of time, and almost impossible to comprehend in terms of a human life span. The classical theory of speciation fits well in to the geography of southwest Asia. This theory holds that speciation requires: first, spatial isolation of a subpopulation from the remainder of the ancestral species and second, divergent selection pressures that establish new barriers of reproductive isolation based upon a large number of genetic differences. The principal arguments in favor of this theory are as follows:
1. Numerous conditions intermediate between subspecies and full species are known among plants.
2. Nearly all full-fledged species, even closely related ones, are separated from each other by more than one kind of reproductive isolating mechanism, each of which is determined by differences at a large number of gene loci. Hence, the divergence that produced the differentiation of these species must have involved successive steps. This gradual differentiation process could not continue in the presence of gene flow from the parental population.
3. Most closely related species differ from each other by morphological and physiological characteristics that are only quantitatively, not qualitatively different from the characteristics that separate fully interfertile subspecies and partially interfertile semi-species. No evidence exists for the postulate that special genetic differences must arise before the species can evolve, except for those that contribute to reproductive isolating mechanisms.
Because of the slowness with which barriers of reproductive isolation accumulate in many plant groups, semi-species are particularly common in higher plants. They consist of completely allopatric populations separated on different continents which are distinguished from each other by numerous, easily recognized and constant morphological differences, but which form fully fertile hybrids when brought together and crossed artificially. How better to describe rhododendrons?
Is it any wonder that many botanists are skeptical of the value of the biological species concept in plants?
The long held concept that the area centering in Yunnan is the center for dispersal for the genus Rhododendron is not necessarily correct. Fossil records show that rhododendrons existed in essentially their present form on the American and European land masses 50 million years ago. The collision between India and southwestern Asia that produced the Himalayan mountains supposedly did not occur until 40 million years ago. Hence, rhododendrons of some type had a worldwide distribution before the many unique altered environments of that area developed. It would be assumed that the existing populations of rhododendrons in southwestern Asia were offered a challenge of diverse and demanding environments that has led to an ongoing process creating new species, semi-species and races.
Out of this mass of confusing rhododendron material, the botanists have painstakingly and slowly synthesized a hierarchical system for the genus Rhododendron. Although not always perceived or stated, this hierarchy is based on the recognition of discontinuities between the various populations of plants, whether these are represented by species, subsections, sections or subgenera. An excellent discussion of the use of correlated discontinuities may be found in Cullen's article, "Rhododendron Taxonomy and Nomenclature," in Contributions Toward a Classification of Rhododendron, 1980. The hierarchical system utilizing discontinuities is natural, so far as the discontinuities are observable. Such a hierarchical system is the basis of phylogenetic systematics. Discontinuity and hierarchical ordering are universal in the living world.
The subgenera of the genus Rhododendron represent:
1. Very homogenous groups, the species among which considerable hybridization is possible.
2. Subgenera in which many species as previously described represent biologically and genetically subspecies, semi-species or races.
3. Subgenera in which species concepts as presently studied and presented represent very useful methods of identifying characteristics of relatively morphologically isolated groups of plants.
4. Subgenera in which species identities are requiring and will require revision as more plant materials and populations are available for study.
There does not appear to be much ground for hostility to taxonomic revisions based on sound scientific principles. The species concept is a great abstract leap from the real world of plant populations. This does not detract from its usefulness to the scientist and amateur. Regardless of its usefulness, as it applies to a favorite species or classification, it is not cast in stainless steel, but rather like a flowing river must constantly change shape in an effort to conform to the realities of its surroundings. So the species of Rhododendron must as nearly as possible be described and classified in terms of the natural populations and the advancing scientific facts about their differences.
Footnote: This paper was prepared for a course of study on species rhododendrons. The course has been made available by mail by the Education Committee of the Rhododendron Species Foundation. Direct inquiries to the Rhododendron Species Foundation, P.O. Box 3798, Federal Way, WA.
