What is a stigma in a plant
The sexuality of plants was discovered in 1694 by the Tübingen professor of medicine and director of the botanical garden, R. J. CAMERARIUS. He recognized the importance of the pollen on the pestle scar, but could not clarify which processes were triggered by it. He writes:
"... in order to solve this difficult question it would be very much to be hoped that we should learn from those who, through their optical instruments, have more than lynx eyes, what the granules of the anthers contain and how far they penetrate the female apparatus."
The question was taken up again at the end of the 18th century. Freiherr W. F. von GLEICHEN, called Rußworm, examined the pestle of a number of species using self-made magnifying glasses and microscopes. Because of its size, he focused on the tulip blossom. But apart from many flowery words, there is little that can be found in his work, published in 1790. The observations of J. HEDWIG (1793), and later also those of G. D. AMICI, are much more precise. Through them we know that the pollen forms a tube that grows through the stylus tissue (transfusion tissue). The fertilization process itself was analyzed by W. HOFMEISTER, the dissolution of the pollen tube tip was observed by E. STRASBURGER.
In order to understand the interactions between pollen and the surface of the scar, we first have to deal with the surface properties of both structures.
The pollen grains of the various plant species differ structurally primarily through the nature of their walls (sporoderm). Details of the structure can only be seen under the microscope or the electron microscope, images of the surface can be obtained with the aid of a scanning electron microscope. There are several reasons why one should study the pollen structure in more detail:
Firstly, surface properties determine whether the "correct" (= native) pollen germinates on a "correct" stigma.
Second, the pollen that is spread by the wind must be such that it can cover great distances. The pollen grains are relatively small, their surface is smooth. Rarely, e.g. at Pinus, Picea Among other things, they are equipped with laterally arranged air bags.
thirdly, pollen that is spread by insects (or other pollinators) must be transportable. In other words: the pollen grains have to adhere to each other as well as to the body of the insect.
fourthly, it has been shown that the outer layers (exine) consist of resistant material (sporopollenin) and that pollen can therefore be preserved in fossil form better than any other plant part. Typical angiosperm pollen is the only indication that this group of plants must have already existed in the Lower Cretaceous (possibly in the Jura?).
Pollen analyzes are also suitable for elucidating the history of flora in the recent past. In this way, for example, successions (successions) of certain (typical) representatives in the course of a bog formation can be recorded and dated. The shape of angiosperm pollen is more variable than that of gymnosperm pollen. The sporoderm (the wall of the pollen grain) usually consists of two superimposed layer complexes: the less resistant inner intine and the sporopollenin-containing outer exine.
These in turn are divided into Nexine and Sexine. The sexine is subdivided in many ways, it is composed of rod, club, cone, wart or net-shaped structures (columellae, baccula), which can be wholly or partly connected to one another in their tip region and thus form a tectum. Oily or protein-rich pollen cement is often stored between the columellae. The pollen with a tectum is called tectat, those that lack it are called intectat. The Columellae arise from the top layer of the nexine, which is called the foot-layer.
As a rule, the exine is broken at certain points by openings or apertures through which the pollen tube that is produced during pollen germination grows. The position and number of these apertures are essential determinants of the pollen. With only one aperture the pollen is mono- or uni-aperture, with two apertures one speaks of diaperture pollen, with three openings of triaperture, etc. Pollen grains with receding apertures are called atreme pollen. Pollen grains without apertures or only indicated germination sites (leptomata) are inaperturated.
As proven by numerous studies, the pollen surface is abundantly sculpted. The pattern can be used as a determining feature. The anthers are the origin of the pollen grains.
The pollen mother cells go through meiosis, and a pollen grain can develop from each of the haploid cells (gons) that are created in the process. It is homologous to the gametophyte of algae and pteridophytes. Pollen grains are two- to three-nucleus, more rarely multinucleated, i.e. the nucleus of the gons goes through mitosis during pollen maturation. The resulting daughter kernels differentiate themselves into the generative and vegetative kernels. Trinuclear pollen is created by further dividing the generative nucleus.
During maturation, the pollen grains separate from each other and from the surrounding (diploid) tissue of the anther. At first, however, they remain in a container, the wall of which is lined with a layer of highly specialized cells: the tapetum. In many cases it consists of secretory cells that gradually dissolve as the pollen ripens. The substances released through separation and dissolution (carotenoids, lipids, lipoproteins, etc.) are stored in the caverns of the exine and deposited on the surface. In 1930 F. KNOLL called this sticky material pollen putty. It serves to hold the pollen grains together during transport by insects and to attach them to the insect body; the viscine threads found in some types of pollen also serve this purpose (M. HESSE, 1980). In addition, at least parts of the pollen cement contribute to the interaction between pollen and the surface of the scar.
In addition to the pollen cement, molecules (proteins, etc.) that are produced by the pollen itself are exposed on the surface. The pollen genome is haploid, the genome of the tapetum cells is diploid. The surface pattern of the pollen is therefore composed on the one hand of products of the haploid gametophyte and on the other hand of those of the diploid sporophyte (anther = microsporophyll).
The surface of the scar is usually covered with numerous papillae. The tissue is always diploid, and in many cases it is covered by more or less pronounced layers of mucus. One then speaks of wet scars. This contrasts with the dry scars, in which the cells are surrounded by a cohesive layer of cutin. A cutin layer is also present in the moist scars, but there it is often broken through or partially dissolved. A number of enzymes have been localized on both the pollen and the scar surface (MASCARENHAS, 1989).
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