Some Thoughts On Rhododendron Species
David W. Goheen, Ph.D.
Species - a word that is both singular and plural, used to describe rhododendrons collected in the wild. In reference to one particular type of rhododendron, one often hears the incorrect use of a singular form of the word, "specie". The word "specie" generally refers to money and the phrase from Latin and French, "in specie" is money in coin for payment. However, the word "specie" really means with the same form or kind and, as such, can refer to many things and even abstract ideas. An example might be "his work is, in specie, similar to mine."
Species is almost an obvious concept to us but it is hard to come up with a concise and rigorous definition that all can agree with. In botany, one can say that a species is a group containing all of the individuals of a particular kind of plant that exist now or that existed in the past no matter where they were or where they may now be found. One famous French botanist, de Jussieu, long ago defined a species as the perennial succession of similar individuals perpetuated by generation. The rub here is the phrase "similar individuals. "This is difficult to quantify and is quite subjective but is, of course, based on the study of the morphology and significant and common characteristics shown by large numbers of individuals.
We still are having some trouble with the definition so it might appear that recourse to Webster's dictionary would help. On reference to such a resource*, the following was found under species: "A group of intimately related and physically similar organisms that actually or potentially interbreed and are less commonly capable of fertile interbreeding with members of other groups that ordinarily comprise differentiated populations limited geographically (as subspecies) or ecologically (as eco-types) which tend to inter-grade at points of contact and that, as a group, represent the stage of evolution at which variations become fixed through loss of ability to exchange genes with members of other groups although formerly conceived to be the total progeny of a single distinctive, specially created, pair." This extraordinarily complex sentence surely must set some sort of record for its fog index! It does emphasize that, although we have little difficulty in thinking about what a species is, we have a great deal of difficulty in precisely defining one.
In our particular case, rhododendrons, we are dealing with a singular, almost unique assemblage of individuals. As noted above, the ability or inability to interbreed with other individuals to produce fertile offspring is an often used trait to help separate one species from another. By careful study, specialists have named hundreds of species. (The number in the genus Rhododendron can be as high as 1200 or as low as, perhaps, 600 depending on whether one is a "splitter" or a "lumper.") The genus has also been grouped into several generally similar morphological groups, for example lepidotes (scaly leaved), elepidotes (non-scaly leaved), azaleas (I can't tell you how but you know an azalea when you see one) and vireyas (tropical and subtropical rhododendrons). The remarkable thing is that, within these groups, individual species freely cross and interbreed to produce hybrid individuals that, of all things in contrast to the Webster's definition, are fertile and can freely interbreed! Thus, the possible numbers of fertile combinations become almost astronomical in size. I have no idea why rhododendrons are such unchaste, wanton creatures, but the fact that they are so is a delight to fanciers and hybridists and offers nearly unlimited possibilities for formation of new and unusual varieties.
Now to the subject of native rhododendron species. The early and, I must say, great and brave collectors such as Kingdon Ward, Rock, Forrest and others, must have read Webster. They were reluctant to believe that rhododendrons were unchaste creatures. They professed to believe that hybridization in the wild was either nonexistent or very rare. They naturally collected the so-called best or superior forms of the varieties they encountered and these were then selected and named as the types for various species. In general, this turned out to be all right but occasionally has resulted in much confusion.
It is my belief that, in the wild, natural hybridization actively occurs, that is speciation is the norm rather than the exception. Thus, the so-called superior forms of species may very well have hybrid characteristics. A good example of this was found during our 1986 expedition to Tibet. Near Doshong La, we encountered large populations of R. uvariifolium and R. wardii in close proximity to each other in bloom and with active insect populations visiting the flowers.
|R. wardii, Doshong La
Photo by David Goheen
|R. uvariifolium, Doshong La
Photo by David Goheen
Now we know uvariifolium as a large plant related to R. fulvum with flowers with a prominent blotch and wardii as a plant with rounded leaves related to species in the Thomsonia group with clear yellow flowers - no sign of blotching. Well, what to make of the wardii collected and introduced by Ludlow and Sherriff as L. and S. wardii? Herr D. Hobbie considered this as one of the finest species in his collection. My belief is that this is a natural hybrid between uvariifolium and wardii with the blotch coming from uvariifolium. A plant was found near Doshong La that is very much like the L. and S. wardii. I consider the plant in Tibet to be a hybrid.
| A natural hybrid between R. uvariifolium
and R. wardii, Doshong La
Photo by David Goheen
| Hobbie's Ludlow & Sherriff R. wardii
Photo by David Goheen
Likewise, we found other specimens with obvious hybrid nature. One of these was a R. campylocarpum with a faint red blotch. Another was R. hookeri with suspiciously variant flowers.
Now, I don't want anyone to say that I claim all superior forms of species are hybrids, but I do believe that many traits that we find make certain individuals stand out from others in their species, may be traits that have been introduced some time in the past by hybridization. Thus, a red form of R. falconeri which I saw growing at Stonefield Castle in Scotland can be called a falconeri because of its many traits similar to others in the species falconeri, but its red color, alas, points to an infusion of foreign genes in the past and thus this superior plant certainly shows some hybridity.
Let us then enjoy our wanton, unchaste rhododendrons with now and then a recognition that species are not necessarily cast from a rigorous and totally similar mold. Superior species forms are different (vive la difference!) from run of the mill specimens. The differences are often delightful and their occurrences result in endless discussions among enthusiasts. Just as we can't totally define what exactly is a species, we can't really say for sure whether any given superior form is or is not a representative of a given species!
From all this, one might get the impression that hybridization is very prevalent in the wild. Such is not the case and I should be alarmed if anyone believes and tells others that Dave Goheen says most species are really hybrids! Even though I maintain that some superior forms are hybrids, it is clear that, in the main, species are largely prevented from crossing with others by a variety of isolating mechanisms. Among these are:
1. Most individuals in any locale are of the same or similar genetic make-up.
2. Since rhododendrons are cross-fertilized primarily by insects, it is highly probable that they will exchange pollen with near-by individuals. This is a spherical volume situation and you may remember that the volume of a sphere is proportional to the cube of the radius. With a larger and larger radius, the volume increases rapidly and it is unlikely that an insect will locate a different species to exchange pollen over a great distance.
B. Different ecological adaptations
1. Blooming times are often different.
2. Species may have different soil and chemical requirements.
C. Reproductive factors
1. Pistil and stamens may not mature at the same time.
2. Gametes from other species, even though they may be carried to the pistil, may not be chemically attracted and will not fuse.
3. Even if a cross does result, gene incompatibility may prevent the individual from growing to maturity.
4. Offspring of inter-specific crosses may be infertile and thus are end of the line situations.
The above isolating mechanisms do much to prevent hybridization, but even if it does occur, there is always the fact of hybrid sterility to consider. Hybrids are frequently larger or more vigorous than either of their parents. This enhanced vigor is known as heterosis and is well-known to most of us in the vigor that is displayed by a mule, the result of the crossing of a horse and a donkey. In the main, although vigorous, hybrids show a tendency toward sterility and inability to have offspring. Hybrid sterility is a relative, not an absolute matter. Some hybrids are fully fertile while others are semi-fertile and others completely sterile. Sterility is generally related to closeness of relationship of the parents. If the species are widely variant in genetic make-up, sterility is almost certain. Species may differ in the structural arrangement of their chromosomes with the result that in the hybrid offspring, the chromosomes are unable to pair normally during meiosis and formation of gametes is upset.
In some plants, hybrid swarms do occur and consist of species, hybrids, F2s and backcross progeny. Hawthorns, serviceberries, violets and columbines are examples of plants that may show hybrid swarms. The majority of variants in a hybrid swarm will prove ill adapted for any given habitat and will eventually be eliminated by natural selection. Some may inherit a favorable combination of characters. These will be preserved by natural selection and may become progenitors of new species.
Various factors operate to divert the course of reproduction in natural hybrids away from formation of a natural swarm. Only one or a few individuals that are hybrids will usually arise at any one time and these will be surrounded by many individuals belonging to the parental species. Most of the offspring of the hybrids will be the result of crossing between the hybrids and parental species. Thus, backcrosses will preponderate over F2s. Since hybrids are usually partly sterile whereas parental species usually are fully fertile, more progeny will consequently result from a cross between a species and a hybrid than from the cross of a hybrid with another hybrid. The third factor that operates to channel hybrid reproduction in the direction of back-crossing is the greater viability of the back-cross progeny. Each species has passed through many generations of natural selection and in the process, it has become adapted to its habitat and has achieved an internal balance and harmony in its genetic constitution. In the F2s, the favorable combinations are broken up into numerous untried mixtures which tend to lose their adaptation to the habitat.
One of the principal effects of natural hybridization in plants is the flow of genes from one species into another closely related one by means of repeated backcrossing. This pattern is important in the plant kingdom and has been given a special name, introgressive hybridization, or simply, introgression.
A number of superior forms of rhododendron species may not be first or second generation hybrids, but well may be the result of introgression. My guess is that the blotch in the L. and S. wardii collected from Doshong La is the result of introgression. Most of the genetic make-up is the same as the type wardii but there is a remembrance of the genes from uvariifolium from some long ago hybridization.
* Websters Third New International Dictionary of the English Language, C. and C. Merriam Co. Springfield, Mass., 1976.
Dave Goheen, Portland Chapter member, received the ARS Gold Medal award at Williamsburg, Virginia, May 1988. Dr. Goheen is also active in the Rhododendron Species Foundation as past president and board member.