JARS v50n4 - Commentary: What's the Point?

Commentary: What's the Point?
Donald H. Voss
Vienna, Virginia

In the Rhododendron literature, "mucronate" and "apiculate" both appear in descriptions of the leaf-tip structure. This structure typically includes the thickened terminus of the midvein, usually extending somewhat beyond the leaf margin. Those turning to published descriptions in connection with plant identification would certainly benefit from clarification of definitions and from consistency in usage.

In view of definitions cited below, the author believes that "mucronate" (or "mucronulate") should be the term of choice. A joint effort - by the Registration Authority, the various Rhododendron societies, and the botanists who prepared the Edinburgh revisions of the genus - to clarify definitions and contribute to consistent usage in the future would be salutary.

The terms used to describe leaf apices in genus Rhododendron have long confused the author and some of his acquaintances in the "rhododendron world." On reflection, the source of difficulty is two-fold:

• The terms "mucronate" and "apiculate" are not clearly distinguished in many descriptive works; both terms appear to be applied to the structure that includes the thickened terminus of the midvein at the very tip of the leaf.

• "Mucronate" and "apiculate" sometimes appear as alternatives to generalized descriptors of leaf apex shape such as "obtuse," "acute," "acuminate," etc.

If the terms are alternatively applicable to leaves in Rhododendron , the definitions of the precise structures denoted need clarification. Application of the term (or terms) describing detail of the leaf-tip structure should, moreover, be used separately from terms used to describe the general outline of the apex, i.e., as an additional character.

Jackson specifies that "apicula" denotes "a sharp and short, but not stiff point " [emphasis added]; while "mucro" is "a sharp terminal point" (4). Porter describes "apiculate" as "a small point which is laminar in nature" and "mucronate" as "a short, hard point which is a continuation of the primary vein" (6). Stearn states that "apiculatus" means "terminating abruptly in a little point, differing from mucronate in the point being part of the limb [lamina, leaf-blade tissue), and not arising wholly from a costa [midvein]"; and "mucronatus," as "abruptly terminated by a hard short point" (8). More precise delineation of these terms is generally lacking, and illustrations purporting to assist visualization of the structures are notoriously confusing.

A common thread is that the structure defined by "apiculate" is laminar in origin and not stiff. "Mucronate" describes a point that is short, hard, and originates from the midvein. Rickett commented that the notion of an apiculate structure "being part of the limb, and not arising wholly from a costa" dates back to John Lindley (7). (What, though, is a structure that is stiff and arises from the thickened terminus of a midvein but is flanked by small wedges of laminar tissue?) Rickett concluded that "precision of description would best be served by getting rid of 'apiculate' with 'cuspidate' and using 'mucronate' with suitable qualifying words, for any abrupt point short of 'aristate' or 'caudate'." We are confronted with the glass-half-full, glass-half-empty conundrum: apiculate laminar, not stiff; mucronate arising wholly from the midvein - but Rhododendron leaves usually neither if one is a strict constructionist!

If one accepts the views of Jackson, Porter, Stearn, and Rickett cited above (whether interpreted narrowly or broadly), some descriptions in the Edinburgh revisions of Rhododendron would require elucidation. For example, Cullen refers to leaf apex shapes, generally using "rounded," "obtuse," "acute," "acuminate," and, in R. lutescens , "drip-tip" (3). "Mucronate" and "mucronate-callose" appear in the keys. But "acute or mucronate" appears in at least one description. Ironically, R. mucronulatum (for which one expects to see some evidence of "mucronateness") is described only as "acute to acuminate at the apex."

In Chamberlain's section key for Subgenus Hymenanthes, leaf-apex shapes are cited as "acute to apiculate," "acute to cuspidate," "rounded, apiculate to shortly acuminate," "rounded and apiculate," and "rounded and apiculate to acuminate" (1). The leaf apex is described as "mucronate" in R. fortunei inter alia. For a long list of other species, however, the term "apiculate" is used. The author examined specimens of a number of these in the U.S. National Arboretum herbarium. In all of those examined, a hard, swollen structure terminates the midvein, in some cases flanked by small wedges of laminar tissue. A flexible point, laminar in nature, was not seen. (Admittedly when working with exciccata, an inference of "flexibility" is at least partly a matter of faith.) The species examined were: auriculatum, sinogrande, macabeanum, souliei, campylocarpum, campanulatum, caucasicum, maculiferum, brachycarpum and degronianum .

What appear to the author to be further examples of mixing descriptors of generalized apex shape with those of the apex-tip structure include: R. oldhamii , "acuminate to mucronate"; R. nakaharae , "acute or mucronulate" - but (gratefully) R. mucronatum is "mucronate." In the treatment of Section Pentanthera, Kron appears to distinguish apex shape from tip structure: "apex may be acute to obtuse; often it is also [emphasis added] mucronate" (5). To the author, this is a useful distinction.

References
1.  Chamberlain, D.F. A revision of Rhododendron: II. Subgenus Hymenanthes. Notes RBG Edinb. 39(2):209-486; 1982.
2.  Chamberlain, D.F.; Rae, S.J. A revision of Rhododendron: IV. Subgenus Tsutsusi. Edinb. J. Bot. 47(2):89-200; 1990.
3.  Cullen, J. A revision of Rhododendron: 1. Subgenus Rhododendron. Notes RBC Edinb. 39(1):1-207; 1980.
4.  Jackson, B.D. A glossary of botanic terms. Fourth ed. London: Gerald Duckworth & Co. Ltd.; 1928.
5.  Kron, K.A. A revision of Rhododendron section Pentanthera. Edinb. J. Bot. 50(3):249-364; 1993.
6.  Porter, D.M., et al. A guide for contributors to Flora North America, Part II. Glossary of terms. Provisional edition. Washington, DC: Smithsonian Institution; 1973.
7.  Rickett, H.W. Materials for a dictionary of botanical terms—IV. Terms to describe apices. In Bull. Torrey Bot. Club 83(5):348-349; 1956.
8.  Stearn, W.T. Botanical Latin. Fourth ed. Devon, UK: David & Charles; 1992.