Pollen Grain


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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

References in periodicals archive ?
Pollen sterility/ fertility was determined by staining pollen grains with 1% I-KI solution (1% iodine in 2% potassium iodide) followed by examination under a microscope (COIC-BA2303, Chongqing Optical Electrical Instrument Co.
Description: Pollen grain, polyplicate, overall amb elongate oval, extremities sharply to broadly rounded, exine polyplicate with 3-5 or more massive folds on the proximal face running parallel to the long axis of grain and associated with fold is a narrow linear longitudinal cleft in the exoexine detectable under oil immersion.
After entrainment, the movement of pollen within large airflow patterns is generally considered in terms of the terminal settling velocity of pollen grains (which depends on the density and the size of pollen grains) and on ambient meteorological conditions.
2009), the viability of the pollen grain may vary significantly throughout its development; as time goes on, the viability of the pollen grain diminishes, reducing its efficiency in fertilization.
The present study was aimed to describe the most unique pollen found in the honey collected in protected landscapes of Lithuania and to show digital images of pollen grains found in honey.
Accessory pollen types (AP, comprising 15 to 45% of the nectariferous pollen grains counted) included pollen types of Arecaceae, Eucalyptus, Myrcia and Schinus in samples of Tetragonisca (Amparo-T, Lins-T), characterizing bifloral honeys when two of these taxa occurred simultaneously.
Sometimes, pollen grains that contact the pistil are triggered to grow rapidly, not into the style but on the medium (Figure 3D, E, black arrows).
But she's a good sport about discussing whether her research suggests that tree pollen grains have their own versions of nose-punching brawls over female favor.
Based on information related to the number, position, and type of aperture in pollen grains, described in the literature and from personal observations, Shridhar & Singh (1990) classified Cucurbitaceae pollen grains in nine main groups or morphotypes, where grains of the species in the subtribe Sicyoeae are multicolpate and circular, and in the subtribe Cyclantherinae are multicolporate.
We are working with Aberystwyth University on a project that can track pollinator movements, using DNA barcoding to identify the pollen grains carried on pollinators' bodies," she said.
One technique that is occasionally used to track movement in a landscape is the identification of pollen grains collected from flower-visiting insects (Silberbauer et al.
To identify the pollen grains adhering to the bodies of the visitors, adhered pollen grains after removing from the body of the visitors with the help of a fine brush, moistened with sterile distilled water, were mounted in 50 % glycerine and observed under a bright-field microscope.