Pollen or pollen is the flour-like mass formed in the anthers of seed plants. At the time of release, the pollen grains had already developed into the male gametophytes. Pollen grains serve to bring the male germ cells protected to the female receiving organs and thus to ensure pollination and, subsequently, fertilization. Microspores are the pollen grains, as male and female spores are of different sizes in seed plants.
Pollen grains are very diverse in size, shape and surface structure and can often be assigned to the respective species or at least genus on the basis of these characteristics.
Intine and Exine
Pollen grains have a resistant wall, which is called Sporoderm. The intestine completely surrounds the cell, but is usually delicate and not particularly resistant. It often consists of two to three layers, with the outermost layer containing a high proportion of pectin, which makes it easy to detach from the exine. The inner layers consist mainly of cellulose fibrils.
When the pollen grain germinates, the inline grows out into a pollen tube. In gymnosperms, the endexins have a lamellar structure. The ectxins in turn consist of an inner foot layer and an outer compact layer covering a granular or alveolar middle layer. In angiosperms, endexins have a granular structure.
The endexins and the dense foot layer of the ektexins are combined to form nexins. The rest of the ectxins form the sexins, which are usually very strongly structured. In tectate pollen grains, the columns on the outside are connected to form a layer, the tectum.
Various substances are stored or deposited in the cavities of Tectum
Pollen putty is an oily substance consisting of lipids and carotenoids and causes the pollen grains to adhere to the pollinators.
The pollen grains have one or usually several germination openings. A distinction is made between mono-, tri-, stephano- and panto-aperturate pollen according to the number of germination openings. Structures located in the equatorial plane are designated with the syllable zono-. Mono-aperturate pollen predominates in monocotyledons and basal dicotyls.
The aperture may be covered by an operculum, a structure completely separate from the rest of the sexins.