An Error in the Glossary of the New Rhododendron Register
Donald H. Voss
The newly published second edition of The International Rhododendron Register and Checklist includes a glossary. I looked first for two terms ("apiculate" and "mucronate") that have for long confused those using descriptions of leaf apices. I was shocked to see that the glossary defined an apiculate point as being formed only from the midrib. This contradicts definitions found in well-known botanical reference works (see References at end). A central thread of the definitions in these botanical glossaries is that an apiculus is flexible, not stiff, and (in a leaf ) comprises leaf-blade tissue. In contrast, the mucro is a hard point comprising vascular tissue at the terminus of the midrib.
Definitions printed in the Register (Leslie, 2004a) and in Stearn (1992) are:
• Register: terminating in a small, abrupt point (the point formed only from the vein or midrib)
• Stearn (terminating abruptly in a little point: differing from mucronate in the point being part of the limb, and not arising wholly from a costa [costa = midvein, limb here refers to the leaf-blade tissue DHV]
• Register: terminating abruptly in a short, hard point (the point formed from the limb and vein/midrib)
• Stearn: abruptly terminated by a short, hard point
Believing that the Register definitions are incorrect as printed, I contacted Dr. Alan C. Leslie, International Rhododendron Registrar since 1983 and editor of the new Register. He informed me that an error in placement of parenthetical explanations for the two definitions was missed in checking the Glossary prior to publication (Leslie, 2004b). The definitions for "apiculate" and "mucronate" were formulated to follow the interpretation of Stearn (1992) but the parenthetical explanations were inadvertently reversed. The intended definitions are:
• Apiculate: terminating in a small, abrupt point (the point formed from the limb and vein/midrib)
• Mucronate: terminating abruptly in a short, hard point (the point formed only from the vein or midrib).
In my observation of rhododendrons and azaleas in gardens (as well as several hundred herbarium specimens at the U.S. National Arboretum), I have not encountered leaves that terminate in a flexible point of leaf-blade tissue. The common feature of leaf apices in the genus is a hard point or terminus formed by a thickening of the end of the midrib. In working with herbarium specimens and checking them against descriptions published in Cullen (1980) and Chamberlain (1982), it became evident that even among botanists who specialize in the study of Rhododendron usage of the subject terms varies widely.
Dr. Leslie (2004b) stated that there is inconsistency in the way that botanists (not to mention registrants of Rhododendron names!) have used these terms. He also mentioned that - in some cases where the definition of an individual word has varied or its application has varied between different authors - using several words to describe a structure more adequately might be better than using one.
The root of confused usage of mucronate and apiculate in Rhododendron literature may lie in the attempt to distinguish two character states for each of two leaf tip characters (the geometry of the point and its physical composition) with only two descriptive terms. By including a reference to the midrib (costa), Stearn’s definition of apiculate ("the point being part of the limb, and not arising wholly from a costa") points to structural detail that I believe to be the source of differing usage.
The terminus of the midrib at the apex of the leaf blade in Rhododendron is often flanked laterally by leaf-blade tissue forming a point that tapers (part or all of the way) toward the distal limit of the midrib. Rather than attempt to distinguish terminologically the degree to which leaf-blade tissue flanks the protruding end of the midrib, I believe it would be better to accept that in Rhododendron the presence of a hard, inflexible termination of the midrib protruding beyond leaf-blade tissue characterizes a mucronate (or mucronulate) tip. For cases in which a point of leaf-blade tissue includes termination of the midrib (Stearn and Register definitions of apiculate), an explicit description of the size, shape, and composition of the point would be more useful than a single term.
Chamberlain, D. F. 1982. A Revision of Rhododendron: II. Subgenus Hymenanthes. Notes from the Royal Botanic Garden Edinburgh 39(2):209-486.
Cullen, J. 1980. A Revision of Rhododendron: I. Subgenus Rhododendron sections Rhododendron and Pogonanthum. Notes from the Royal Botanic Garden Edinburgh 39(1):1-207.
Jackson, Benjamin Daydon. 1928. A Glossary of Botanic Terms with Their Derivation and Accent (4th ed.). London: Gerald Duckworth.
Kiger, Robert W. & Porter, Duncan M. 2001. Categorical Glossary for the Flora of North America Project. Pittsburgh: Hunt Institute for Botanical Documentation, Carnegie Mellon University.
Lawrence, G. H. M. 1951. Taxonomy of Vascular Plants. New York: Macmillan.
Leslie, Alan C. 2004a. The International Rhododendron Register and Checklist (2nd ed.). London: Royal Horticultural Society.
Leslie, Alan C. 2004b. Personal communication.
Stearn, W. T. 1992. Botanical Latin (4th ed.). Devon, UK: David and Charles.
Donald Voss is a member of the Potomac Valley Chapter and District 9 Director.