Report on an Exhaustive Study of Hair Types in Rhododendrons
Dr. Alexander Martin, Los Gatos, California
The following is a rough and rather imperfect translation of the summary ("Zusammungfassung") of Almut Seithe's report on: "Hair Types in The Genus Rhododendron And Their Taxonomic Significance"; in the Botanische Jahrbuch, May 1963, pages 297-393, with 7 tables.
Microscopic examination of 265 kinds of Rhododendrons resulted in recognition of 43 hair types. As far as possible, the studies included various parts of the plants as well as different stages of hair development. This made it evident that some kinds of hairs are related to each other through intermediate stages. There are, however, two main kinds of hairs on every plant.
It was concluded that hairs that are similar in location and in stage of development have similar genes and belong to the same general classification -though there are exceptions.
It was found that the two primary forms of hairs in the genus Rhododendron are closely related, the one kind being dissected and the other largely not dissected. Both of these two primary types occur in two varieties: the dissected hairs which are known as scales or glands, and those which are largely not dissected and are called spout-hairs and branched or woolly hairs.
In each main subdivision of Sleumer's 1949 classification system, a combination of two hair types is recognized. On this basis, three major groupings of subgenera were established in the genus Rhododendron, and the subgenus concept was introduced. Four subgenera bear scales in combination with spout-like hairs; three divisions of the subgenus azalea (this name has been validated by recent international nomenclature) bear glands and spouts; and the subgenus Hymenanthes bears glands and branched hairs.
On seedlings, as far as observed, only four hair types occur, with two kinds on each plant. They are four types recognized as primitive.
In developmental stages of hair growth, the four main varieties are quite distinct. Following the somewhat similar two-celled stage in all hairs (two enlarged epidermal cells), the first partitions in glands and spouts are on a slant, while in scales and branched hairs they are horizontal. Subsequent partitions in glands and spouts are horizontal, then turn outwardly from the horizontal (whereby the hair body is divided into sections). In scales, however, the partitions are inclined inwardly and compacted. Branched hairs and spouts differentiate later during elongation of the upper cells. By contrast, stretching of the upper cells occurs in glands and scales, and in scales, it separates the outer, walls, causing the main axis to become more and more horizontal.
For several reasons, glands appear more primitive than scales, spouts more primitive than branched hairs. The forerunners of hair types, described as developmental hairs, have probably originated as gland-like and spout-like forms. There is much to indicate that the developmental types are from similar origins, the simplest evidence being in the doubling of genes. Studies on the genus Solanum support this conclusion. For this genus, the diploid kinds were chosen. The assumed antecedents of the two primitive hair types were sectioned. Glandular hairs rank first in pubescence on the Ericaceae, and in second place are spouts.
Fig. 16. Young growth on R. rex in
Cecil Smith's garden.
Cecil Smith Photo
Regarding the pubescence and geographic distribution of Rhododendron and its close relatives (Theororhodion, Ledum, Menziesia, and Tsusiophyllum) the province of origin is assumed to be eastern China and Japan. From this common origin, the shelf scales probably split off (in the subgenus Rhododendron) and spread to western China in the Himalayas, and further. Next followed the branched hairs of the Hymenanthes subgenus. Several indications point to the probability that the genus was originally deciduous. A similar picture of development of the genus can be constructed from other characteristics. This is supported by various authors, using different approaches, in the literature.
So far as possible, the preceding explanation of genetic and phylogenetic relationships of hairs provides a foundation for correct application of characteristics in the taxonomy of the genus. Form of hair is very important for suitable interpretation of Rhododendron taxonomy; its chief value being in the significance of hair-type combinations for the major subdivisions of the genus. At a lower taxonomic level are some differences in hairs which are difficult to distinguish as to whether they are simply similarities or closely related.
A summary of the genus according to the major subdivisions of Sleumer has been inserted. Information about the pubescence of 265 species investigated has been assembled. The text is supplemented by 194 drawings. The work of earlier authors, especially of Cowan, is discussed critically.