Glossary: Dimorphic and Deciduous
Donald H. Voss
Vienna, Virginia

        The terms discussed describe characteristics encountered in a wide range of plants. Application of these terms in the genus Rhododendron occurs primarily in the subgenera Tsutsusi and Pentanthera.
        Dimorphic - Literally, dimorphic means occurring in two forms. In botany the term describes plant organs that appear in two distinct forms or shapes on the same plant or in closely related plants, such as those in a species. The Oxford English Dictionary (1933) attributes to Darwin early English use of the term. It appeared in two chapters on "Heterostyled Dimorphic Plants" (Darwin 1862). Much of Darwin's discussion centered on Primula: some plants in a species had long pistils and short stamens, while others had short pistils and long stamens. Darwin (1878) also included "dimorphic" in the glossary appended to the sixth edition of The Origin of Species.
        When used in descriptions of evergreen azaleas (Rhododendron subgenus Tsutsusi), the term dimorphic refers to the two forms of leaves found on a given plant, not to the sequence of their emergence and senescence. Wilson (1921) described two leaf forms in evergreen azaleas and highlighted their significance in his discussion of R. obtusum (not a "good" species; now recognized as a mélange, primarily of R. kiusianum, R. kaempferi, and their hybrids): ... Too little attention has been paid to the fact that the red-flowered species of this section of Rhododendron all have dimorphic leaves. Those which form immediately after the flowers open are from lanceolate to ovate or elliptic in shape, acute, light green in color and membranous in texture. The "spring leaves" are followed by "summer leaves," which are smaller, more or less obovate in shape, obtuse or rounded at the apex, dark green and fairly coriaceous. These spring leaves are normally deciduous and the summer leaves persistent, but climate exercises a strong influence, the colder it is the more deciduous the leaves, and vice versa. In the early summer when the larger spring leaves predominate, the plant presents a very different appearance to what it does in late autumn or early spring when the small obovate summer leaves only are present. The overlooking or lack of appreciation of this simple fact has resulted in much confusion in the classification of the species and forms.

        Deciduous - Wilson's (1921) statement that the spring leaves of the above evergreen azaleas are deciduous introduces the second term to be discussed. In the context of trees and shrubs, deciduous usually refers to the falling of leaves at the end of the growing season; it can also apply to the falling of other organs such as flowers.
        Many temperate-zone plants, including azaleas in subgenus Pentanthera (the so-called deciduous azaleas), adapt to the changes in insolation (day-length and intensity of sunlight) and temperature that occur as winter nears by dropping all of their leaves. By eliminating a source of moisture loss through transpiration, leaf drop contributes to the plant's preparation for survival under freezing conditions that could damage plant tissues and reduce or prevent the absorption of water by the roots. The physiology of the deciduous condition was treated by Palmer (2008).
        The leaf-drop cycle in the plants of subgenus Tsutsusi is more complicated. Their spring leaves normally drop in winter, while their summer leaves persist through the winter and drop during the following spring or early summer. While Wilson stated that the spring leaves are normally deciduous and the summer leaves persistent, this distinction is based on the arbitrary convention that the life-cycle period is coterminous with the calendar year. Although summer leaves persist through the winter, they drop during the following calendar year and thus are, at some point in the plant's growth cycle, deciduous. The timing and extent of the leaf drop may vary among species and is, of course, strongly influenced by environmental conditions such as temperature and drought.
        It is inappropriate to associate the meaning of the term deciduous with "bimorphic." If bimorphic were a word accepted for use in botany, it would refer to the form or shape of plant organs and be a superfluous synonym of dimorphic. The prefix bi- is Latin meaning "twice, having two": di- is Greek for "twofold, twice"; the stem morphe is Greek meaning shape or form. In contrast, deciduous refers to the falling off of organs (such as leaves or flowers) at some point in the growth cycle. Whether the leaves on plants traditionally classified as deciduous are uniform or differ in form, when winter comes they all fall off! "Bimorphic" does not appear in major English dictionaries or in several botanical dictionaries and is not recognized by four professional botanists consulted by the author of this note.

References
Darwin, C. R. 1862. On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. J. Linnean Soc., Bot. 6:77-96.
Darwin, Charles. 1878. Glossary in the "The Origin of Species", 6th ed.. Reprinted 1979, along with the original (1859) edition of "The Origin of Species". New York: Avenel Books: 460+16 pp.
Palmer, Bruce. 2008. The Word: Deciduous. J. Amer. Rhododendron Soc. 62: 46-47.
The Oxford English dictionary. 1933 (reprinted 1978). Oxford: Clarendon Press, Vol. 3: p. 372
Wilson, E.H. 1921. The Azaleas of the Old World. In E.H. Wilson and A. Rehder. A Monograph of Azaleas. Cambridge, MA: Harvard University Press: 1-105.