The Structure of Pollen

Abstract

In plants, the life cycle is an alternation between a diploid, spore-producing multicellular generation (the sporophyte) and a haploid, also multicellular generation (the gametophyte), producing sperm and/or egg cells [1, 2]. Pollen is a biological structure functioning as a container, in which is housed male gametophyte generation of the angiosperms and gymnosperms. Such a container is an evolutionary adaptation for life out of water because it protects male gametes from adverse atmospheric influence while transferring from anthers to pistils. Each pollen grain contains vegetative (nonreproductive) cells and a generative (reproductive) cell. In flowering plants, the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm cells. A pollen grain contains the male gametophyte of the seed plant, i.e. the sperm-producing generation (gymnosperms and angiosperms). In seedless plants, gametophytes are generally independent from the sporophyte and consist of hundreds of cells; evolution of the seed-bearing habit involved dependence of the gametophytes upon sporophytes and reduction of their size. Thus, in gymnosperms, the male gametophyte has up to ten cells, whereas in angiosperms, there is only one vegetative cell apart from the two sperm cells. Each grain has an internal limiting cellulose membrane, the intine, and a two-layered external covering, the exine, composed of a durable substance called sporopollenin which is primarily a high molecular weight polymer of fatty acids. The male gametophyte originates from a spore, generally called a microspore, and is still surrounded by this spore’s sporopollenin outer wall, the exine, at maturity. The gametophyte’s own wall is called the intine.