REPRODUCTION AND DISPERSAL 855 



tionj on the stigma of the same flower. Complete impotence is compara- 

 tively rare, well-known cases being found in Corydalis cava, Hemerocallis 

 fulva (day lily), Fagopyrum esculentum (buckwheat), Secale cereale 

 (rye), 1 and also in several of the Leguminosae, but there are many plants 

 in which foreign pollen (i.e. pollen from other flowers) is prepotent 

 (i.e. more or earlier effective) on a given stigma than own pollen 2 (i.e. 

 pollen from the same flower); foreign pollen that is sown on a stigma 

 several hours after own pollen often gains the ascendency in a very short 

 time. The acme of impotence is found in various orchids, in which own 

 pollen actually is prejudicial to the stigmas (or vice versa), appearing 

 to behave like a poison. In the Leguminosae own pollen is much more 

 potent in the annual species than in the perennials. In some legumes 

 (as Cytisus Laburnum) the usual impotence of own pollen is due to the 

 fact that the pollen tube cannot penetrate the cuticle of the stigma; 

 when this is ruptured artificially, own pollen is potent. In Corydalis 

 cava own pollen frequently germinates, but the pollen tube is unable to 

 penetrate to the ovules. 



In nearly every case pollen from a given flower is no more potent on 

 other flowers of the same plant than on the stigma of the flower that pro- 

 duced it, thus showing in a most striking way that geitonogamy is essen- 

 tially the same as autogamy and should not be classed with xenogamy. 

 In a number of cases own pollen appears sometimes to be impotent, and 

 sometimes variously potent (as in Eschscholtzia and in Brassica Rapa), 

 possibly by reason of varying external conditions. From the viewpoint 

 of pollen potency, therefore, there are three classes of plants: (i) those 

 in which own pollen is as potent as foreign pollen, forming a class with 

 numerous representatives (as Oenothera and most crucifers); (2) those 

 in which foreign pollen is prepotent, also forming a class of large size; 

 and (3) those in which own pollen is impotent, forming a comparatively 

 small class. 



Among the most remarkable examples of impotence are those afforded 

 by heterostyled flowers, own pollen being completely impotent in Linum, 

 and slightly potent in Primula? The most extraordinary feature of 

 these plants, however, is that cross pollination between the anthers and 



Even in rye geitonogamy may occur. 



2 However, the pollen must not be too foreign, as from another genus or family. Thus 

 impotence is found at the extremes of relationship, that is, where pollination occurs be- 

 tween anthers and stigma in the same flower or in flowers of distantly related plants. 



2 Some observers report the complete potency of own pollen in some species of Primula. 



