Architecture, Anatomy, and Functional Morphology of the Pseudopetiolate Species of Calymperes and Syrrhopodon (Musci, Calymperaceae)

Abstract

World-wide, three species of Calymperes and two of Syrrhopodon (Musci, Calymperaceae) share the foliar feature of a pseudopetiole. This structure, which is unique among mosses, is an interval of naked costa connecting the summit of the leaf base (with its waterretaining cancellinae) to the proximal end of the expanded photosynthetic upper lamina. The primary function of the pseudopetiole appears to be that it elevates and exposes the photosynthetic portion of the leaf above the moss turf In two of these mosses the pseudopetioles form a cagelike system between the leaf bases and the expanded portions of the laminae which functions to retain water for brief periods. The basic functional morphology of the highly derived plants of the moss family Calymperaceae is focused on capture and retention of water. The evolution of the pseudopetiole-a petiole-like interval of naked costa between the leaf base and the upper lamina (Fig. 1)-in five species of these mosses is of considerable interest as an example of extreme specialization in this largely tropical and largely corticolous family. Reese and Tan (1983) presented an overview of what they referred to as the petiolate Calymperaceae. They described and illustrated the condition as they observed it in four species of Calymperaceae, but did not offer suggestions as to the presumed evolutionary origins or possible functions of the pseudopetioles. One purpose of the present article is to add a fifth species-Syrrhopodon aristifolius-to the roster of pseudopetiolate Calymperaceae and to describe features of the condition in greater detail than presented by Reese and Tan. In addition, suggestions are offered for the functional roles of the pseudopetioles. Some considerations possibly bearing on the evolutionary origin of the pseudopetiolate condition are also noted. The mosses discussed in this article occur in two of the three genera of the Calymperaceae. They comprise the paleotropical species Calymperes aeruginosum Sande Lacoste, C. robinsonii Tan & Reese, Syrrhopodon aristifolius Mitten, and S. loreus (Sande Lacoste) Reese, and the neotropical C. venezuelanum (Mitten) Pittier. See Figure 1, and Eddy (1990), Reese and Tan (1983), and Reese et al. (1986) for illustrations of these mosses. It is doubtless noteworthy that pseudopetioles are not known in Mitthyridium, the most highly derived taxon of the Calymperaceae, which is confined to the paleotropics and closely related to Syrrhopodon. There are degrees of the pseudopetiolate condition. For example, not all specimens of C. aeruginosum and S. aristifolius are obviously pseudopetiolate, but some populations of these species are remarkable for strong development of the pseudopetioles. In some collections of both species the plants may have the leaves mostly lacking pseudopetioles, but in other collections the reverse may be true. On the other hand, populations of species such as C. robinsonii, C. venezuelanum (Fig. 1), and S. loreus seem to be rather uniformly pseudopetiolate. The pseudopetioles vary in length among and within the species: C. aeruginosum, 0.0-1.2 mm; C. robinsonii, 1-3 mm; C. venezuelanum, 2-3 mm; S. aristifolius, 0.0-5 mm; S. loreus, 2-5 mm. Specimens of morphologically similar species of Calymperaceae e.g., C. lonchophyllum Schwaegrichen, C. serratum A. Braun, and S. croceus Mitten, often show an approach to the pseudopetiolate condition. However, I have not seen the completely elaminate pseudopetiolate condition in these moss-