Those Exciting Rhododendron Epidermal Appendages
Part I: Subgenus Hymenanthes
Arthur P. Dome
Seattle, Washington

The Subgenus Hymenanthes contains plants which have no scales, and are referred to as being "elepidote." However, many of them have interesting indumentum or hairs on certain parts of their leaves depending upon the species. There is just one Section ( Ponticum ) in this Subgenus which has 24 Subsections.

I'm sure you were really intrigued the first time you looked at certain rhododendron leaves and then turned them over and saw something new and unexpected. What happened to the smooth green surface that was customary? With a second look it appeared that this unusual substance or growth was actually a part of the leaf. Depending upon the leaf it could have been scales, bristles, hairs of various types, or a felty-type of growth of varying degrees called indumentum. In the case of the Subgenus Hymenanthes they would always be hairs and/or a felty type of growth of some type of indumentum.

John Macqueen Cowan states, "These structures are minute organs, each of which is capable of performing definite functions and their real significance is in relation to the well being of the plant. Together, in a large part, they regulate the water supply and thus enable the rhododendrons to withstand the extremes of climate they are called upon to endure. There can be no doubt that, without them, some rhododendrons would have failed in the habitats where they prevail."

These epidermal appendages on the leaves of rhododendrons have always been one of the determining factors in the classification and naming of these plants. Now, newer developments in DNA technology and the use of chemical analysis comparisons in classifying and reclassifying other genera and those within the family Ericaceae are very important. Those of you that recently attended The Fifth International Rhododendron Species Symposium, in Bellevue, Washington, had the opportunity to listen to the world's leading authorities discuss the newer DNA technology and other comparisons. However, it was determined that these epidermal appendages will still play an important role in classifying and identifying plants. These eight photographs, and the two on the cover of this issue, illustrate some of the different hairs and indumentum types that one might find on the leaves of plants in Subgenus Hymenanthes . Age and environment can affect the sizes and colors of these appendages. In some cases there were discrepancies between references in identifying a specific hair or indumentum type. In this case the reference with the latest publication date was favored.

R. auriculatum , Section Ponticum , Subsection Auriculata . Young leaf underside pale green covered with a tomentum of isolated flexuous hairs some of which may be gland tipped. Photo by Arthur P. Dome		R. beanianum , Section Ponticum , Subsection Neriiflora . Mature new leaf underside covered with thick woolly indumentum in the form of dendroid hairs. Photo by Arthur P. Dome

R. bureavii , Section Ponticum , Subsection Taliensia . Mature new leaf underside covered with a thick woolly indumentum of ramiform hairs. Photo by Arthur P. Dome		R. campylocarpum ssp. caloxanthum , Section Ponticum , Subsection Campylocarpa . Young leaf upper side - glabrous except for short glandular hairs that disappear when older. Photo by Arthur P. Dome

R. campanulatum ssp. campanulatum , Section Ponticum , Subsection Campanulata . Mature new leaf underside covered with a felted indumentum of capitellate hairs. Photo by Arthur P. Dome		R. roxieanum var. roxieanum , Section Ponticum , Subsection Taliensia . Mature new leaf underside with woolly bistrate indumentum of ramiform and rosulate hairs. Photo by Arthur P. Dome

R. lanatum , Section Ponticum , Subsection Lanata . Young leaf upper side is glabrous except when covered with a floccose indumentum which usually disappears when mature. Photo by Arthur P. Dome		R. lanatum , Section Ponticum , Subsection Lanata . Young leaf underside covered with a dense tomentum of dendroid hairs that turn deep brown when mature. Photo by Arthur P. Dome

Acknowledgments

The author is indebted to Steve Hootman, Co-Director-Curator, Rhododendron Species Foundation, Federal Way, Washington, for proofreading the text of this article.

References

Chamberlain, David, and Roger Hyam, George Argent, Gillian Fairweather, Kerry S. Walter. 1996. The Genus Rhododendron, Its classification & synonymy . Edinburgh: Royal Botanic Garden Edinburgh.
Cowan, John Macqueen. 1950. The Rhododendron Leaf . Edinburgh: Oliver and Boyd.
Davidian, H. H. 1989. The Rhododendron Species, Vol. II, Elepidotes . Portland, OR: Timber Press.
Davidian, H. H. 1992. The Rhododendron Species, Vol. III, Elepidotes . Portland, OR: Timber Press.
Chamberlain, D. F. 1982. Notes from the RBGE, Vol. 39 . Edinburgh: Royal Botanic Garden Edinburgh.

Arthur Dome, a member of the Seattle Chapter, has written articles on alpine rhododendrons and ericaceous companions for the Journal. He will continue this series on epidermal appendages in the next issue.
In photographing the leaves in this series, Dome has used plants growing at the Rhododendron Species Botanical Garden. Magnification varies from x10 to x50.