phytotaxa - Magnolia Press

the morphology of several attributes of Desmoncus not used in delimiting species is discussed. Sources of
morphological variation are also discussed.
Stems are mostly clustered. Only one species, Desmoncus giganteus has solitary stems, although this is
recorded from only two specimens. Stems in all but one species, D. stans, are not free-standing, and climb by
means of acanthophylls. However, one subspecies of D. mitis, subsp. ecirratus is recorded on several
specimen labels as being non-climbing. Despite the climbing habit of most species, stems of Desmoncus
seldom reach great heights. The mean plant height of all climbers is only 6.2 m. Isnard et al. (2005) have
discussed the biomechanics and development of the climbing habit of two species of Desmoncus, D. horridus
subsp. horridus (as D. orthacanthos) and D. polyacanthos.
Leaves of Desmoncus are spiny. These spines occur on sheaths, petioles, rachises, cirri, and pinnae and
provide several useful taxonomic characters. However, there is considerable variation in spininess of leaf
parts both amongst and within species. Spininess also appears to vary according to age of the stem and habitat
(Wessels Boer 1965).
Leaves are arranged all along the stems, and are recorded on labels as being distichously arranged. The
leaf consists of sheath, petiole, rachis, pinnae, and cirrus. The elongate, basal sheaths tightly enclose the stem
and persist even after the leaf has died. Sheaths have well-developed ocreas, extending for several centimeters
above the insertion of the petiole. Petioles are usually rather short, and only in D. cirrhifer (7.0–16.5 cm long),
D. costaricensis (8–15 cm long), and D. giganteus (15–30 cm long) are they well-developed. Rachises are
well-developed in all species, and range from 8–190 cm long. Rachis spines are useful taxonomically and
species can be divided into two groups based on these spines. In one group, rachis spines are straight with
briefly swollen bases, usually more than 1 cm long, and mostly adaxially or laterally placed on the rachis (Fig.
1A). In the second group, rachis spines are recurved with markedly swollen bases, usually less than 1 cm long,
and mostly abaxially placed on the rachis (Fig. 1B).
The rachis is extended into a cirrus. This climbing organ can be defined as a distal section of the rachis
which does not bear pinnae but rather acanthophylls. These are pinnae which are modified into reflexed
hooks. Cirri can take various forms. They may be well-developed with acanthophylls (Fig. 1C); poorlydeveloped
with the rachis terminating in a short cirrus with or without acanthophylls; virtually absent with the
rachis terminating beyond the distal most pair of pinnae in a short ‘stub’ (Fig. 1D); or poorly-developed with
the rachis terminating in a short cirrus with some small, acanthophyll-like pinnae present (Fig. 1E).
Arrangement of spines on the cirri differs somewhat, and is useful in identification. In one group of species,
the cirri have few spines abaxially, mostly on proximal part of the cirri only (Fig. 1C); in a second group the
cirri have spines abaxially throughout (Fig. 1F); in a third group cirri have many, usually paired spines (Fig.
1E); and in a fourth group the cirri are without spines abaxially (Fig. 1G).
FIGURE 1. A. Rachis spines usually >1 cm long, mostly adaxial or lateral, straight with briefly swollen bases; pinnae
with an adaxial beard of spines at the bases (D. chinantlensis; Steyermark 44526). B. Rachis spines