From anther and pollen ripening to pollen presentation
The events and processes occurring between pollen maturation, opening of the anther and presentation of pollen to dispersing agents are described. In the final phases of pollen development, starch is always stored; this occurs before the anther opens. Dep
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The reproductive structures of animals usually develop in elongated cavities open at one end; at maturity they emerge from the end into the outside world. In plants , the reproductive structures develop in closed cavities which open at maturity to enable dispersal. During development, spores and pollen are immersed in a liquid that conveys nutrients from the tapetum, that is, from the innermost part of the sporophyte (Pacini and Franchi 1992) to developing spores or pollen. When the pollen is almost ripe, the tapetum disappears, having fulfilled its function . The locular fluid remains for a short period of time (Pacini 1997) and then disappears enabling pollen dispersal at anther opening . The pollen itself normally loses water just before and /or just after the anther opens ; in this way it becomes dormant thus resisting the stresses of the external environment (Fig . 1, Table 1). There appears to be a progressive reduction in water content from the spores of early land plants to gymnosperm pollen . Similarly spores of early land plants have chloroplasts whereas angio sperm pollen has proplastids or amyloplasts . At dispersal , the spore s of Bryophytes often have chloroplasts, vacuoles and a high water content (Clarke 1979); Pteridophytes may have spores with : (a). proplastids or amyloplasts, a low water content (about 5%) and no vacuoles; (b). chloroplasts as the spores of Equisetum and Matteuccia have (Cran 1979,
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E. Pacin i: Anther opening and pollen presentation
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pollen development the time period of this phase is genetically programmed and only slightly influenced by the environment
pollen presentation and dispersal only the reduction in duration of pollen viability is programmed but this is greatly affected by the environment
Fig. 1. Graph showing changes in volume of pollen grains during development, presentation to dispersing agents, and dispersal. A decrease in volume occurs after release of the microspores; this is followed by a constant increase proportional to vacuole formation. Vacuole formation is followed by storage of starch by plastids; depending on the species, vacuoles may form once or twice. Just before the anther opens, all or part of the starch may be hydrolysed, depending on the water content at dispersal and whether water retention mechanisms exist. Depending on the capacity of pollen to retain water, the following categories can be identified: 0 0 0 0 0 0 0 pollen that is not dehydrated when the anth er opens and is dispersed in the sea; submarine pollination; pollen that has completed the process of dehydration prior to anther opening; anemophilous and entomophilous pollination; - - - - - pollen that has not completed the process of dehydration when the anther opens but completes it during exposure or dispersal; anemophilous and entomophilous pollination ; ••••••• pollen that is n
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