124 PLANT PHYSIOLOGY AND ECOLOGY 



sketches of several flowers representing different types, showing the 

 flower in longitudinal section. 



146. Source and destination of pollen. When a stigma receives 

 its pollen from the anthers in the same flower, the flower is self- 

 pollinated. If the pollen comes from a different flower, the flower 

 is cross-pollinated. Self-pollination is also called autogamy, and 

 cross-pollination, allogamy. The latter is the rule among plants, 

 though autogamy is a frequent occurrence. A great number of 

 plants that are modified for allogamy and are regularly pollinated 

 in this way are so arranged that they may be self-pollinated in 

 case cross-pollination does not occur. Darwin was the first to 

 show conclusively that cross-pollination tends to produce stronger 

 and more vigorous plants, thereby furnishing an adequate ex- 

 planation of the preference which plants have for this method. 

 The numerous modifications of diclinism and dichogamy seem to 

 be for the purpose of preventing self-pollination, while the in- 

 numerable devices for dispersing pollen, attracting insects, etc., 

 are to insure cross-pollination. All diclinic plants and many 

 dichogamous ones can be pollinated in this way alone, while allo- 

 autogamous species, i.e., those capable of pollination in either 

 way, are self-pollinated only after the failure of cross-pollination. 

 Many flowers belonging to different varieties, or more rarely to 

 different species, may cross-pollinate each other. Although the 

 crossing of related forms has been an invaluable method in plant- 

 breeding, it does not seem to be a frequent process in nature, 

 apparently being limited to a few genera, such as the willows, 

 verbenas, etc. Cross-pollination of this sort is termed hybridiza- 

 tion. 



Cross-pollination between tw^o flowers of the same plant is 

 called geitonogamy, i.e., pollination by a neighboring flower; between 

 flowers of different plants it is xenogamy, i.e., pollination by a dis- 

 tant flower. Either may occur in species with perfect or monoe- 

 cious flowers, but xenogamy alone is possible in dioecious plants. 

 In single-flowered plants xenogamy is alone possible, but in many- 

 flowered ones an insect will carry strange pollen only to the first 

 few that are visited on each plant. As would be expected, gei- 

 tonogamy is apparently less beneficial to the species than xenogamy, 

 although in plants where both are possible it is usually much more 

 frequent. Geitonogamy is of greater advantage to the plant than 



