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; some details about this subject are explained by Stauffer et al. (2018). 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 (Fig. 1). The pollen of the genus has been described by Dransfield et al. (2008) as ellipsoidal (in the case of H. coriacea looking spherical), bi-symmetric, with a distal sulcus and an ectexine tectate, finely perforate-rugulate, with supratectal gemmae (Figs. 2-5). The female flower is 3-carpellate and a conspicuous septal nectary develops between flanking carpels (Figs. 6-10).
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. Male flower of Hyphaene guineensis at late bud stage. Note the well-developed endothecium and the abundant pollen within the pollen sacs.
Fig. 4. Deatl of anther of anther in Hyphaene guineensis.
Fig. 5. Spherical pollen grains observed on an anther of Hyphaene coriacea (photo L. Michon).
Fig. 6. 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).
Fig. 7. Detail of a young ovule in Hyphaene thebaica.
Fig. 8. Septal nectary in Hyphaene thebaica. This nectary producer tissue is good evidence of insect pollination in the genus Hyphaene, although wind may also play a role in open environments.
Fig. 9. Detail of epithelium in the septal nectary of Hyphaene thebaica. The epidermal cells look papillate and are quite elongate.
Fig. 10. Detail of a petal in a female flower of Hyphaene thebaica. Note the well developed raphide idioblasts acting as protective devices against herbivory.