![]() ![]() ![]() The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. The vegetative and generative cells are surrounded by a thin delicate wall of unaltered cellulose called the endospore or intine, and a tough resistant outer cuticularized wall composed largely of sporopollenin called the exospore or exine. ![]() Structure Tulip anther with many grains of pollen Closeup image of a cactus flower and its stamensĮxcept in the case of some submerged aquatic plants, the mature pollen grain has a double wall. In some flowering plants, germination of the pollen grain may begin even before it leaves the microsporangium, with the generative cell forming the two sperm cells. ĭuring microgametogenesis, the unicellular microspores undergo mitosis and develop into mature microgametophytes containing the gametes. The predominant form amongst the monocots is successive, but there are important exceptions. While there may be a continuum with intermediate forms, the type of microsporogenesis has systematic significance. In simultaneous microsporogenesis meiotic steps I and II are completed before cytokinesis, whereas in successive microsporogenesis cytokinesis follows. Two basic types of microsporogenesis are recognised, simultaneous and successive. The exine is what is preserved in the fossil record. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. In a process called microsporogenesis, four haploid microspores are produced from each diploid sporogenous cell (microsporocyte, pollen mother cell or meiocyte), after meiotic division. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. The fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. As the flower develops, four groups of sporogenous cells form within the anther. In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. Pollen microspores of Lycopersicon esculentum at coenocytic tetrad stage of development observed through oil immersion microscope the chromosomes of what will become four pollen grains can be seen. Some pollen grains are based on geodesic polyhedra like a soccer ball. In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm nuclei. Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell. It is a gametophyte, something that could be considered an entire organism, which then produces the male gamete. Because of agricultural practices, it is often contaminated by agricultural pesticides. Pollen is infrequently used as food and food supplement. In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower. Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollination. The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archaeology, and forensics. Individual pollen grains are small enough to require magnification to see detail. If pollen lands on a compatible pistil or female cone, it germinates, producing a pollen tube that transfers the sperm to the ovule containing the female gametophyte. Pollen grains have a hard coat made of sporopollenin that protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants, or from the male cone to the female cone of gymnosperms. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen is a powdery substance produced by flowers of seed plants. Colorized scanning electron microscope image of pollen grains from a variety of common plants: sunflower ( Helianthus annuus), morning glory ( Ipomoea purpurea), prairie hollyhock ( Sidalcea malviflora), oriental lily ( Lilium auratum), evening primrose ( Oenothera fruticosa), and castor bean ( Ricinus communis).
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