Michael Hesse

Almost all systematic treatments agree that Calla is a puzzling case, being a highly autapomorphic taxon with obscure relationships. In molecular–based classifications the variable placements of Calla within Aroideae conflict strongly with those in morphologically and anatomically based systematic classifications, which treat the genus as a subfamily (Calloideae) of its own. We studied the pollen morphology and ultrastructure of Calla by light and electron microscopy, and mapped the relevant pollen characters as well as some flower characters to the proposed placements of Calla within the Araceae as indicated in the various molecular phylogenies. Calla pollen is extraordinary within the entire Araceae. Pollen grains are small, and basically disulcate or with a ring–like aperture. The ornamentation is psilate to perforate, and the pollen wall consists of a sporopolleninous tectate–columellate exine. These pollen characters are shared with those of several earlier–diverging aroid taxa, especially with those of subfamily Zamioculcadoideae, whereas pollen characters in members of subfamily Aroideae deviate significantly. These findings are in accordance with other floral characters. Therefore, we propose that Calla is best placed in a transition zone between either subfamily Zamioculcadoideae ( Stylochaeton clade) and subfamily Aroideae (Aroideae clade) or between subfamily Zamioculcadoideae ( Stylochaeton clade) and subfamily Lasioideae.

The heteropolar pollen grains of Dionaea and Drosera (Droseraceae) are united into permanent tetrads. Dionaea and Drosera tetrads have unique, complex internal set-ups. In both genera, apertures (pores) are situated equatorially (stephanoporate); neither Dionaea nor Drosera can be designated as examples for proximal polar situated apertures. In turgescent state, the tetrads show protrusions along the borders of adjoining pollen grains, which are enclosed by a thick intine and emerge from the apertures. One of these protrusions finally forms the pollen tube during germination. Both genera produce Ubisch bodies. In Drosera, the distal pollen wall above the apertures of each monad invaginates towards the centre of the pollen grain, forming an incomplete septum with a central opening. This septum divides the pollen grain into a larger distal and a smaller proximal chamber containing cytoplasm. In the literature, the septal opening was erroneously addressed as aperture (‘pore’). The proximal chambers collapse in the dry state (or after acetolysis) due to the harmomegathic effect. The septum becomes folded and partially in contact with the proximal wall, thus forming some sort of channels. The thick equatorial pollen walls of Dionaea muscipula show a row of pores, i.e. the apertures, from where the protrusions extend. In contrast to Drosera, there is no inner septum. Each protrusion in Dionaea comprises a massive operculum, which acts as a plug in dry pollen grains.

Pollen characters in Araceae accord well with recent DNA–based phylogenies, and here we provide a new example of “compass needle” quality in Araceae on the basis of two closely related genera, Schismatoglottis and Apoballis. All investigated Schismatoglottis pollen is psilate (smooth pollen surface) with calcium crystals covering the pollen surface. By contrast, pollen of species transferred to recently resurrected Apoballis (Apoballis acuminatissima and A. mutata) is distinctively echinate (spiny). A unique layer covers the endexine of Schismatoglottis, and the whole pollen surface of Apoballis. Our findings strongly suggest that ““Schismatoglottis” species with echinate pollen fall into the genus Apoballis. Moreover, all schismatoglottid taxa perform spathe movements during anthesis to control the movement of pollinators. The spathe movements of Apoballis acuminatissima clearly differ from those known in Schismatoglottis species, and indeed are so far unique for the entire family. This, together with differences in floral odour is strongly suggestive of differences in pollination ecology between the genera Schismatoglottis and Apoballis.

The mature pollen wall of gymnosperms and angiosperms consists in principle of two fundamentally different layers, the complex, thick sporopolleninous exine and the homogeneous, thin, single-layered pectocellulosic intine. In angiosperms, the typical exine is usually formed by a tectum, columellae, a foot layer, and an endexine. An exine reduction (minimally up to the complete absence) occurs in many unrelated seed plants, without consequences for pollen viability. The intine sometimes also deviates from its common form, being either extremely thick or appearing two- or even three-layered. Environmental factors or developmental constraints are highlighted as being responsible for the various deviating exine and intine forms. Die fertige Pollenwand der Gymnospermen und der Angiospermen besteht im Prinzip aus zwei fundamental verschiedenen Lagen, aus der komplexen, dicken und sporopolleninhältigen Exine, und der homogenen, dünnen, einschichtigen und überwiegend zellulosehältigen Intine. Bei Angiospermen ist die typische Exine aus einem Tectum, aus Columellae, aus einem Foot Layer und zumeist noch aus einer Endexine geformt. In vielen, nicht miteinander verwandten Angiospermen (seltener bei Gymnospermen) is die Exine mehr oder weniger stark reduziert, was allerdings keinen Einfluß auf die Keimungsfähigkeit des Pollens hat. Auch die Intine weicht manchmal von ihrer üblichen Ausbildung ab, ist entweder auffallend dick oder zwei bis dreischichtig. Sowohl Umweltfaktoren als auch embryologisch und entwicklungsgeschichtlich bedingte Hemmungen sind für die abweichenden Exine- und Intineformen verantwortlich.

Mature pollen usually has a conspicuously thick intine in the aperture regions, whereas in the interapertural area(s) the intine is thin. In Carex (and in other Cyperaceae with pseudomonads), usually five to six circular or elongated apertures (poroids) are present, one situated distally and the others equatorially. In the species studied here the situation is opposite: the apertures (poroids) are characterised by a very thin intine, whereas in the interapertural area(s) the intine is conspicuously thickened. A similar condition is only known for Vinca (Apocynaceae). A second unusual sporoderm feature in Carex relates to the foot layer. In the interapertural area(s) the foot layer is thin and discontinuous. The equatorial poroids exhibit a compact, thick and homogeneous foot layer, whereas the distal poroid is distinguished by an elaborated foot layer, structurally similar to a transfer-cell wall labyrinth. This unique elaborated foot layer in the distal poroid probably functions as a nutrimental passage between the tapetum and the maturing pollen. The pseudomonads are usually pear-shaped, sometimes spherical. Ornamenting elements are microechini, granules, and perforations, all sometimes placed on verrucae.

Beschorneria yuccoides (Agavaceae) microspores are arranged mostly in planar tetrads. Later on, the pollen grains of the tetrad usually fall apart, but sometimes remain loosely connected by ektexine elements. The ektexine consists of a tectum, of short columellae, and of a thin, discontinuous foot layer. An endexine is absent. The bilayered intine is without any additional thickening that would usually indicate an aperture region. From this point of view the pollen grain might be considered as omniaperturate. The pollen ornamentation is reticulate with wide lumina and robust, smooth muri. The pollen grains show an indistinct sulcus characterised by a loose reticulate ornamentation. The sulcus is not exactly at the distal pole, but shifted towards the equator. No pollen tubes are formed regularly at the sulcus. Instead, pollen tubes are normally formed at the proximal pollen face. The proximal area, indicating a large germination field, is morphologically and functionally clearly an aperture (a germination zone); however, it does not represent a sulcus. The proximal face of all pollen grains appears as ornamented, with some exine lumps. Asimina triloba (Annonaceae) pollen is shed in permanent planar or decussate tetrads. The distal sides are microreticulate to foveolate, and do not show an aperture; the psilate proximal sides are the germination areas of A. triloba. The presence of apertures placed at the proximal pole was reported for distinct taxa of several angiosperm families. For Drosera, Dionaea (Droseraceae) and most probably for the diaperturate Cuphea species (Lythraceae) the existence of polar germination areas can be excluded. However, in some Annonaceae taxa with permanent tetrads (Annona cherimola, Asimina triloba) a situation similar to Beschorneria might be present, and indeed a proximal polar pollen tube is formed. Beschorneria yuccoides, Annona cherimola and Asimina triloba are unequivocal examples of angiosperm pollen with an exactly proximal aperture (germination area).

In the family Araceae, the members of all subfamilies except Aroideae follow the conventional mode of exine formation pattern, which conforms with the textbook view of sporoderm stratification and chemistry (sporopollenin ektexine formed before the endexine). Only members of the subfamily Aroideae show a quite uncommon mode of exine formation pattern, with an endexine formed prior to the nonsporopollenin, polysaccharidic outer exine layer. The intine is formed simultaneously with this non-sporopollenin layer. From the differing timetable and especially from the different origin it is concluded that this outer exine layer is not homologous to the angiosperm ektexine. The fundamental question, why members of the Aroideae lack an elaborated sporopollenin ektexine, is discussed in terms of functionality of the nonsporopollenin outer exine layer. It seems that a major change in aroid evolution took place at the point when the family phylogenetically and ecologically shifted from bisexual (most subfamilies) to unisexual flowers (Aroideae only). The hypothesis is that ephemeral spathes and the absence of sporopollenin are the consequence of an adaptive syndrome for a short pollination time window in many members of the Aroideae, with short-lived pollen, an energetically not costly pollen wall, rapid germination of pollen tube, and brief receptivity of stigma.