It was R. J. CAMERARIUS, professor of medicine at Tübingen and director of the botanical garden who discovered the sexuality of plants. He recognized that it was of enormous importance that the pollen reaches the pistil, though he could not explain, which processes it elicits. He wrote:
".....in order to settle this difficult question is it to be hoped that we learn from those equipped with optical instruments better than lynx eyes of what the grains of the anthers contain and how far they penetrate the female apparatus."
The question arose again at the end of the 18th century. Baron W. F. von GLEICHEN studied the pistils of a number of species with self-made magnifying glasses and microscopes. Due to its size did he concentrate on the tulip. His works, published in 1790 contain only little durable information. The observations of J. HEDWIG (1793) and later that of D. AMICI are much more precise. From them do we know that the pollen develops a tube that penetrates the style's transfusion tissue. Fertilization itself was analyzed by W. HOFMEISTER. E. STRASBURGER observed the disintegration of the pollen tube's tip. |
 |
It is easier to understand the interactions of pollen and stigma surface, if we learn previously a little about the surface properties of both structures.
The structures of the pollen grains of different plant species vary mostly in the nature of their walls called sporoderm. Structural details can only be made out in the microscope or the electron microscope. The surface can best be studied with a scanning electron microscope. The interest in the pollen structure has several causes:
 |
Surface properties of the pollen grain decide whether the 'right' (= species-specific) pollen germinates on a 'right' stigma. |
 |
Pollen that is distributed by the wind has to travel large distances. Pollen grains are rather small and their surface is smooth. Only rarely as in the case of Pinusand Picea, for example are they equipped with lateral airsacs. |
 |
Pollen that is distributed by insects (or other pollinators) has to be suitable for transport. The pollen grains have to stick to each other and to the bodies of the insects. |
 |
The outer layers called exines consist of a robust material (sporopollenin), so that fossil pollen is better preserved than other plant parts. Typical angiosperm pollen is the only proof that this plant group has already existed during the lower Cretaceous and perhaps even in the Jurassic period. |
Pollen analyses are also well-suited for the elucidation of the late past of floral history. Successions of plants representative of bog formation, for example, can be captured that way. The shape of angiosperm pollen is more variable than that of gymnosperm pollen. The sporoderm (the wall of the pollen grain) consists usually of two layers: the less robust intine forms the wall's inner part while its outer sporopollenin-containing layer is called exine.
The exine is subdivided into nexine and sexine. The sexine is composed of rod-, club-, cone-, wart- or net-shaped structures called columellae or bacculae. Their tip regions may be partially or completely connected so that they may form a tectum. The space between the columellae is often filled with oily or protein-rich pollen cement. Pollen with tectum is called tectate , while that without tectum is intectate. The columellae spring from the topmost layer of the nexine that is called foot-layer.
Usually is the exine perforated by apertures. These are the sites chosen by the growing pollen tube. The position and number of apertures are main classification features of pollen. Pollen with just one aperture is called uniaperturate, that with two apertures is named diaperturate, that with three apertures triaperturate, etc. Pollen grain with degenerate apertures is called atreme pollen. Pollen grains without apertures or with just indicated germination sites (leptomata) are inaperturate.
The pollen surface consists often of very elaborate, three-dimensional patterns. These patterns can be used as classification features. Pollen is produced in the anthers.
After the pollen mother cells went through meiosis can each of the newly formed haploid cells (gones) develop into a pollen grain. It is homologous to the gametophytes of algae and pteridophytes. Pollen grains have two to three nuclei, in rare cases even more, i.e. during pollen maturation goes the nucleus of the gones through mitosis. The daughter nuclei differentiate into a generative and a vegetative nucleus. Pollen with three nuclei develop by a further division of the generative nucleus.
During the maturation do the pollen grains separate from each other and the surrounding (diploid) tissue of the anthers. First of all do they remain in a container whose walls are lined by a layer of highly specialized cells: the tapetal layer. In many cases does it consist of secretory cells that disintegrate successively during pollen maturation. The substances thus set free (carotenoids, lipids, lipoproteins, etc.) are stored in the exine's caverns and on the exine's surface. In 1930 introduced F. KNOLL the term pollen cement (Pollenkitt) for this sticky material. Just like the viscin threads of some pollen (M. HESSE, 1980) helps the pollen cement to stick the pollen to the insect's body and to each other during transport by insects. In addition are at least some of its components involved in the interaction of pollen and stigma surface.
In addition to the pollen cement is the pollen surface studded with molecules (proteins and others) that are produced by the pollen itself. The pollen genome is haploid, that of the tapetal cells is diploid. The surface pattern of the pollen is therefore composed both of the products of the haploid gametophyte and those of the diploid sporophyte (anther = microsporophyll).
The stigma surface contains usually numerous papillae. It is always diploid and is often covered by more or less distinct layers of mucus. The latter stigmata are also called wet stigmata. In contrast are the cells of dry stigmata surrounded by a continuous cutin layer. Wet stigmata contain, too, a cutin layer, but it is often perforated or partially degraded. In both pollen and stigma surface were a number of enzymes detected (MASCARENHAS, 1989).
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de