. Un manuel de botanique pour les collèges et universités ... La botanique. REPRODUCTION ET DISPERSION 829 jamais, les fleurs possèdent des avantages, et elles seront désormais prises en compte. Le r61e de fleurs et les organes essentiels en cause. - La pollinisation. - La pollinisation, c'est, le transfert des grains de pollen à la stigmatisation (ou à l'ovule dans les gymnospermes) est le chef de l'activité associé avec des fleurs. Lorsque le pollen est transféré à partir d'une fleur d'une plante à une fleur d'une autre, le phénomène est appelé cross polli- nation ou xenogamy, et lorsque le pollen est transféré des anthères de la stigmatisation de la t

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. A textbook of botany for colleges and universities ... Botany. REPRODUCTION AND DISPERSAL 829 ever, flowers possess conspicuous advantages, and these will now be considered. The r61e of flowers and the essential organs involved. — Pollination. — Pollination, that is, the transfer of pollen grains to the stigma (or to the ovule in gymnosperms) is the chief activity associated with flowers. When pollen is trans- ferred from a flower of one plant to a flower of another, the phenomenon is termed cross polli- nation or xenogamy, and when pollen is trans- ferred from the anthers to the stigma of the same flower, it is termed close pollination or autogamy. Geitonogamy, in which pollen is transferred from one flower to another on the same plant, is intermediate between xenogamy and autogamy, and often is classed with the former, but in reality it is much closer to the latter. In many species autogamy is the only kind of pollination possible, and in other species (probably a greater number) only xenogamy is possible, but it is probable that in the great majority of plants both autogamy (or geitonogamy) and xenogamy are possible, though usually it is believed that the latter is the more advantageous. In all cases xenogamy is possible only through the action of external agents, of which wind and insects are the most important. In geitonogamy and autogamy (especially the latter) pollination may occur through the direct contact of anther and stigma, but gravity, wind, and insects often effect autogamy and geitonog- amy as well as xenogamy; in some cases in- sects are as necessary for autogamy as for xenogamy (as in Yucca).' The dehiscence of the anthers. — When the pollen grains are mature, the anther dehisces, usually by longitudinal slits (fig. 1142), but sometimes by transverse slits, by valves (fig. 1176), or by terminal pores (as in Solanum and. Fig. 1141. — A com- pound raceme of Coleus, the individual ilowers being arranged in paired cymes; c, calyx; tf, the sym

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• bookcentury1900
• bookdecade1910
• booksubjectbotany
• bookyear1910