

CHAPTER VI 



POLLINATION 



Pollination is the transfer of pollen grains to the stigma, or, 

 in the case of gymnosperms, to the ovule. In the very great majority 

 of cases it is brought about by some external agency. The principal 

 agents of pollination are animals, wind and water. The cases in 

 which pollination is brought about through the agency of animals 

 represent our commonest examples of reciprocal nutritive dis- 

 junctive symbiosis and these will be discussed in Chapter VII. The 

 present chapter treats of pollination through non-living agencies 

 and, therefore, does not deal with symbiosis. It is interpolated here 

 between two chapters on symbiosis for the sake of convenience in 

 discussing all types of pollination in sequence. 



41. Cross and Close Pollination.— When a flower is pollinated 

 with pollen derived from a flower of a separate plant the phenomenon 

 is called cross pollination or xenogamy, while, if the flower is pol- 

 linated with its own pollen, it is called close pollination or autogamy. 

 A condition that is intermediate between autogamy and xenogamy is 

 that in which the pollen is derived from another flower on the same 

 plant. This is called geitonogamy. 



In some plants autogamy is the only kind of pollination that is 

 possible. This is obviously true of flowers that never open, such as 

 the cleistogamous flowers produced by several species of violets and 

 by a number of other plants. In other plants, and probably a much 

 greater number, only xenogamy is possible. In the great majority 

 of plants, however, it is probable that both autogamy, or geitonog- 

 amy, and xenogamy are possible. Xenogamy is never possible 

 except through the aid of some external agent and with relatively 

 few exceptions this is true also of geitonogamy. Even in autogamy 

 gravity, wind, water, or animals often bring about pollination and 

 in many cases one of these agencies is absolutely essential. In some 

 cases of autogamy, however, pollination is brought about by actual 

 contact between the stamens and stigmas. Strictly speaking these 

 are the only cases of self-pollination. In practice, however, all cases 

 of autogamy and geitonogamy are ordinarily called self-pollination 

 as opposed to cross pollination which includes all cases of xenogamy, 

 that is, all cases in which the pollen is derived from a separate plant. 

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