Palynological characters and their phylogenetic signal in Rubiaceae

Botanical Review, The, July-Sept, 2005 by Steven Dessein, Helga Ochoterena, Petra De Block, Frederic Lens, Elmar Robbrecht, Peter Schols, Erik Smets, Stefan Vinckier, Suzy Huysmans

d. Protruding Onci and Pollen Buds

In many angiosperm species, a lens-shaped structure that is not resistant to acetolysis and occurs beneath the aperture is formed. In Rubiaceae, these onci sometimes protrude through the apertures, forming papillae (Fig. 1). This feature has been discussed for Oldenlandia nudicaulis Roth (Farooq & Inamuddin, 1969), Stephegyne parviflora Korth. (? = Mitragyna sp.) (Ramam, 1954), and Mitriostigma axillare Hochst. (Hansson & El-Ghazaly, 2000). In many other publications, similar structures are seen in illustrations of unacetolyzed pollen grains of other taxa but are not discussed in detail; as, for example, in Atractocarpus (Puttock, 1992), Gardenia Ellis (Puttock, 1992), Scyphiphora C. F. Gaertn. (Puff & Rohrhofer, 1993), and Strumpfia Jacq. (Igersheim, 1993a).

In SEM, these protruding onci look very similar to pollen buds, which were reported in Mussaenda L., Ophiorrhiza L., Pseudomussaenda Wernham, and Schizomussaenda Li (Chennaveeraiah & Shivakumar, 1983; Igersheim & Weber, 1993; Mathew & Philip, 1987; Philip & Mathew, 1975; Puff et al., 1993b). The protoplasmatic vesicles seen in Myrmecodia (Robbrecht, 1988a) are probably also pollen buds. Pollen buds are larger than the protruding onci and differ in the fact that they possess a vacuole. According to Igersheim and Weber (1993) and Weber and Igersheim (1994), they also differ in separating from the pollen grains before shedding. Tilney and Van Wyk (1997) report protrusions of intine, often in addition to protoplasm in Canthium Lam., Keetia, and Psydrax. Although their work includes TEM observations, they do not distinguish protruding onci from pollen buds.

5. Sexine

Rubiaceae show a wide array of morphological variation in sexine patterns, including psilate (Fig. 29), perforate (Fig. 30), foveolate, (micro)reticulate (Figs. 31-34), rugulate, rugulate-clavate, articulate, hamulate, striate, and double reticulate (Fig. 32). The majority of the species have a tectum perforatum or are (micro)reticulate. In Mycetia javanica (Blume) Reinw. ex Korth., the thick tectum is very irregular, with elongated or rounded processes beset with unequally spaced microspines (Fig. 35). Another strange sexine is observed in Gouldia terminalis (Hook. & Am.) Hillebr., in which the bacula are interconnected and beset with microspines (Fig. 36).

[FIGURES 29-36 OMITTED]

A double or complex reticulum characterizes the tribe Coccocypseleae (Piesschaert et al., 2000b). It also occurs in Pavetteae (De Block & Robbrecht, 1998), Spermacoceae s.1. (Dessein, 2003; Groeninckx, 2005; Pire, 1997a; Pire & Cabral, 1992), and the genus Metabolos (Puff & Igersheim, 1994a). A double reticulum consists of a smooth "suprareticulum" and a spinulate "infrareticulum" situated at a level slightly below the suprareticulum (Fig. 32). At present, it is unclear whether all structures named "double reticulum" in Rubiaceae are structurally identical. An in-depth TEM study is needed to verify this hypothesis.

Atectate grains are rare but do occasionally occur in Psychotrieae and in Versteegia Valeton (Pavetteae; De Block & Robbrecht, 1998). Versteegia cauliflora (K. Schum. & Lauterb.) Valeton has free-standing bacula on the foot layer.