A foundation monograph of Ipomoea (Convolvulaceae) in the New World

Androecium

The stamens are of little taxonomic value. They are always five and may be included or exserted. If they are included they are unequal with two noticeably longer than the other three but, if exserted or near exserted, they are subequal in length. The filaments are slightly expanded near the base but are occasionally thickened and subtriangular as in Lepistemon and some forms of Ipomoea batatoides. The filaments are always glandular pilose at the base. In a few species hairs are reported to extend upwards along the filament and this has been used as a diagnostic character in the Batatas Clade. (Austin 1978b).

Pollen (Figures 9, 10)

The pollen of Ipomoea is always globose and pantoporate with large supratectal elements that form acute or blunt spines. The presence of these echinulate supratectal elements is the diagnostic synapomorphy for Ipomoea within Convolvulaceae. Within this general pollen-type subtle variations are visible in the size of the pollen grains, in the number and shape of pores, in the number and structure of the supratectal elements, in the structure of the area surrounding the pores, the presence or absence of ‘basal cushions’ sensu Wilkin (1993) at the base of the supratectal elements and the extent of columellae in different parts of the pollen (Sengupta 1972, Pedraza 1983, Wilkin, 1993). In addition, individual pollen grains may look very different depending on whether the opercula or aperture membranes remain intact (Figure 10A, C–H) or not (Figure 10B) after acetolysis.
Our own survey of Ipomoea pollen confirms these previous studies demonstrating continuous variation in pollen morphology with little, if any, discrete variation that correlates with phylogeny. The attempt by Wilkin (1993) to correlate results with a broader infrageneric classification of Ipomoea was made in the pre-molecular era and does not correspond closely with our molecular results. Nevertheless, although pollen in itself is of little phylogenetic or taxonomic value within Ipomoea, a few broad generalisations can be cautiously made. The pollen of species in Clade A (Figure 9, A–C) usually has fewer supratectal elements (spines) than the pollen of species in other clades. Pollen in Clades B and C (Figures 9D, E, 10C–H) often shows a regular pattern of 4–6 supratectal elements per pore as exemplified by Figure 10C and 10G but this is not always the case (Figure 10A, H). The pollen of I. alba (Figure 10D) and related species in the Calonyction Clade (species 271–274) have characteristic stout, rounded gemmiform spines rather than the usual acute spines, but similar blunt spines are also found in other more distantly related species such as I. dumosa (Figure 10E).
In summary, the pollen of Ipomoea is characterised by echinulate supratectal elements, showing a number of features that vary continuously and some specific morphologies that are homoplastic.