Morphology and anatomy
Hyphaene is pleonanthic, dioecious and includes species with small to massive and tall, solitary or basally clustered, erect or creeping stems (Figs. 1a-b); costapalmate leaves (Fig. 1d), often persistent when old; and conspicuously polymorphic, brown to orange fruits (Fig. 1f). The presence of once or twice, much rarely 3 times, dichotomously branched stems in some species of the genus, particularly in H. thebaica and less common in H. coriacea and H. compressa (Fig. 1a), is almost unique in the palm family. Branched stems in palms have been occasionally reported in species of Allagoptera, Borassus, Dypsis, Manicaria, Nannorrhops (Fisher & Maidman, 1999); whether the physiological and structural basis of this phenomenon is similar across this diverse palm groups remains unknown. The anatomical traits associated with leaf blades in the group are largely unstudied, other than a few efforts with H. dichotoma (Tomlinson et al., 2011). Such studies, however, will be critical to identify the main anatomical and histological adaptations of the leaves to the arid ecological conditions where most Hyphaene grow. Hyphaene is a dioecious genus and the unisexual flowers are inserted in separate inflorescences (Figs. 2-4). The pollen of the genus has been described by Dransfield et al. (2008) as ellipsoidal (in the case of H. coriacea looking spherical, Fig. 3), bi-symmetric, with a distal sulcus and an ectexine tectate, finely perforate-rugulate, with supratectal gemmae (Fig. 3).
Fig. 1. Growth habit and morphology in Hyphaene. A. Dichotomously branched stems of H. thebaica, Israel; B. Solitary, unbranched habit in H. petersiana, Namibia; C. Leaf sheath splitting in H. thebaica, Ivory Coast; D. Leaf blade of H. petersiana, Zimbabwe; E. Inflorescence of H. coriacea (Madagascar) bearing male flowers; F. Infructescence of H. coriacea showing ripe fruits, Madagascar.
Fig. 2. Male flower of Hyphaene coriacea at anthetic stage. Upper left: upper view of the male flower showing the 6 equally developed stamens (two anthers lacking); Upper right: side view of the male flower; Lower left: upper view of the poorly differentiated pistillode; Lower right: side view of the pistillode (photos L. Michon).
Fig. 3. Spherical pollen grains observed on an anther of Hyphaene coriacea (photo L. Michon).
Fig. 4. Female flower of Hyphaene coriacea at pre-anthetic stage. Upper left: upper view of the female flower showing the sepals and the petals; Upper right: side view of the female flower; Lower left: inner side of the petal showing one portion of the staminodial ring, note the presence of at least 3 staminodes; Lower right: detail of the uppermost part of the filament of the staminode, the anther looks clearly sterile (photos L. Michon).
We are currently exploring new characters to accurately distinguish Hyphaene in living and herbarium material. This is interesting as abundant material in international herbaria is only represented at vegetative stage and living juvenile individuals of some related taxa (i.e. Borassus) do strongly resemble. Additional features, such as presence and distribution of epidermis trichomes (Fig. 5) and morphology of the phytoliths (= silica bodies, Fig. 6) can be employed as alternative tools to identify Hyphaene samples. The latter study is product of a collaboration between the Universidad Nacional de Colombia (Gaspar Morcote and Lauren Raz) and Fred Stauffer (Conservatory and Botanical Garden of Geneva)
Fig. 5. Leaf surfaces showing trichomes (red arrows) in représentatives of Borasseae. Hyphaene compressa (left), specimen collected in Tanzania; Medemia argun (center), specimen cultivated at the Montgomery Botanical Center (Florida, USA); Borassus aethiopum (right), specimen collected in Ivory Coast.
Fig. 6. Phytoliths (known also as silica bodies) of Medemia argun (left), specimen cultivated at the Montgomery Botanical Center (Florida, USA), and Hyphaene compressa (right), specimen collected in Tanzania, displaying different morphologies (Photos Gaspar Morcote and Lauren Raz, Universidad Nacional de Colombia).