Pollen Grain

Pollen Grain 

the male gametophyte of a seed plant. The pollen grain develops from a microspore in the microsporangium and completes its development after pollination, that is, its transfer into the pollen chamber of the ovule (in gymnosperms) or to the stigma of the pistil (in angiosperms). It has two membranes: the external one, or the exine, is tough and sturdy, and the internal one, or the intine, consists mainly of cellulose and pectin substances. The exine usually has furrows or pores, through which, upon sprouting of the pollen grain, the protoplast, which is covered with the intine, protrudes. (This process is the formation of a pollen tube.)

In gymnosperms the pollen grains consist of several living cells and, sometimes, the remains of dead cells by the time of pollination. The cells include a vegetative cell (haustorial), and a generative cell (antheridial). The vegetative cell forms a pollen tube in the pollen sac; the tube embeds itself in the nucellus. The generative cell divides to form a spermatogenic, or spermiogenic, cell and a sister cell (the foot layer). The spermatogenic cell then forms gametes—multiflagellate spermatozoids (for example, in sago palms) or nonflagellated spermatozoids (for example, in conifers)—which travel to the archegonia of the female prothallium along the pollen tubes. By the time it falls on the stigma of the pistil, the pollen grain of gymnosperms consists either of a syphonogenic cell with a spermatogenic cell inside it (bicellular or binuclear pollen) or, if the spermatogenic cell has already divided, of two spermatozoids inside the siphonogenic cell (tricellular or trinuclear pollen). When a tricellular pollen grain sprouts, the nucleus of the siphonogenic cell and both spermatozoids enter the pollen tube. When a bicellular pollen grain sprouts, the nucleus of the siphonogenic cell and the spermatogenic cell enter the pollen tube; the spermatogenic cell divides into two spermatozoids in the tube. Development of the pollen grain is completed when the pollen tube reaches the embryo sac, into which both spermatozoids participating in double fertilization enter.

In most seed plants the pollen grains are single (monads). In some angiosperms the microspores and the pollen grains that develop from the microspores are united in twos (dyads; for example, in Scheuchzeria) or fours (tetrads; for example, in many Ericaceae and some Orchidaceae) or in eights, 12’s, 16’s, or 32’s (polyads; for example, in Mimosaceae). In Asclepiadaceae and some Orchidaceae all the pollen grains in one or two chambers of the anther remain in a coherent mass (pollinium).

There are variations in the shape, size, and structure, and ornamentation of pollen grains; in the structure of the exines; and in the structure and positioning of the apertures. However, these features are constant in plants of the same species. In representatives of different taxa, the closer the taxonomic relationship, the greater the similarity of features. Hence, the study of pollen grains is important in plant taxonomy (seePALYNOLOGY). Since the exine is often preserved in sedimentary rock, the study of pollen grains is one of the basic methods of paleobotanical research.

A. N. SLADKOV