JARS v62n3 - The Word: Lepidote

The Word: Lepidote
Bruce Palmer
Cutten, California

Most of the flowers on our rhododendrons are gone until next spring. Now it's time for enjoyment of the other features of the genus that cause our addictions. The leaves are quite diverse and many of us collect new plants as much for their foliage as for their flowers. Among the more interesting leaf structures in the genus Rhododendron are those that give the surfaces of the leaves distinctive appearances. Let's concentrate on one type of structure this time: scales. The word, then, is lepidote . It's from the Greek lepidos , scale. Originally the term applied to the scales of snakes and fish. The prefix is used in a number of places in biology. Lepidoptera (scale wing), for example, is the order of insects to which butterflies and moths with their scaly wings belong. Lepidodendron (scale tree) is used by paleontologists to describe a long extinct group of large primitive trees whose poorly developed conifer-like leaves left scars on the fossil surfaces that resemble snakeskin. The fossil scale marks were so distinctive that early collectors mistook Lepidodendron fossils for fossilized snakes. Lepidodendrons contributed heavily to the formation of the coal deposits in Pennsylvania and elsewhere.

Scales are one of many types of structures on the surfaces of leaves. To the naked eye they generally appear as dots on the leaf underside. Under a microscope they appear as minute cups with flat covers that project beyond the edges forming wide rims (Leach). They fall in a category collectively called trichomes (Greek, trichos , a hair). The term trichome is probably preferable to the word hair in plants because plant hairs do not arise from multi-celled subsurface follicles as they do in mammals but are modified surface cells. Scales don't resemble other plant hairs, but that is what they are. In some plants other than rhododendrons, olives for example, they are usually called peltate (Latin, peltatus , a shield) hairs. Trichomes come in various shapes and have a number of distinct functions. Many aromatic leaves such as rosemary, sage and geranium carry their scent inside swollen hairs that burst open to release the smell. Nettles have hairs containing an irritant. Some succulents such as ice plant have expandable surface vesicles (Latin, vesicula , a small bladder) for storage of water in dry conditions. The tiniest structures that absorb water and nutrients for higher plants, the root hairs, are trichomes. The trichomes that we rhododendron enthusiasts are most familiar with are those we term indumentum (Latin: hair covering). Plants living at high altitude and in deserts often have silvery hairs on their surfaces for sun protection and water retention. Indumentum is a general botanical term for hairs on leaves and stems, along with numerous other terms (a page and a half in Stern's Botanical Latin ). Applied to the genus Rhododendron many of us tend use the term restrictively to mean coverings of soft hairs of various kinds on the undersides of leaves. They might repel insects also. Whether indumentum on rhododendron leaves does any of these things is an interesting question.

Scales on lepidote rhododendrons are less conspicuous than indumentum and don't contribute a great deal to the appearance of the leaves. Their function is problematic. Leach in Rhododendrons of the World indicates that they are for water retention. Others believe that they repel insects. A quick, very unscientific survey of our yard shows that of about 120 rhododendrons growing near redwoods and Sitka spruce, those without scales, with or without indumentum, are twice as likely to have leaf damage from root weevils and cutworms. The sample is not large enough or sufficiently diverse to be a statistically significant finding, but I thought it interesting.

For our purposes, the importance of scales has to do with classification. Classification of the genus is the next best thing to a nightmare for taxonomists, but the presence of scales or lack of them stands out distinctly from others. H. H. Davidian in all four of his volumes of The Rhododendron Species devotes the first full page of text to classification, with nearly all of his emphasis on the differences between lepidote (scaly) and elepidote (non-scaly) rhododendrons. Lepidote and elepidote rhododendrons are dramatically different. Their genes are sufficiently different that it is extremely difficult to hybridize between the groups. The seeds are different. The flowers open differently. There may be more of a tendency toward polyploidy in lepidotes, though that idea is still being tested as shown in the recent detailed studies by Jeff Jones et al. published in the fall 2007 issue of the Journal American Rhododendron Society . A large majority of all of the beautifully fragrant rhododendrons we grow in the benign coastal climate of northwestern California and southwestern Oregon are lepidotes in the subgenus Rhododendron .

In summary, lepidote rhododendrons, distinguished by scales, are a distinct grouping from elepidotes and our gardens would be seriously impoverished without them. Let's appreciate them this summer while we have no blossoms on most of our rhododendrons and add scales to the list of interesting features of members of the genus Rhododendron in our gardens.

References

Leach, David, 1961. Rhododendrons of the World . New York: Charles Scribner's Son.

Davidian, HH. 1982. The Rhododendron Species . Portland, Oregon, Timber Press.

* Name is not registered.

Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.