866 ECOLOGY 



a few in the closed flowers. Perhaps the most striking case of reduction 

 is where, instead of a stigma, there is an open passageway to the ovules, 

 recalling gymnosperms. Pollination, of course, is by direct contact, but 

 there are some remarkable cases (as in Oxalis and Impatiens) in which, 

 strictly speaking, there is no pollination at all, since the pollen grains ger- 

 minate within the anther, putting forth their tubes which grow out toward 

 the stigma.* In Lamium and in Viola odorata the anthers do not even 

 dehisce, So that the pollen tubes have to penetrate the anther walls; in 

 Viola the anther wall is devoid of the usual thickening, and the pollen 

 tubes pass readily through permeable spots of small plasmatic cells. As 

 might be expected, cleistogamous flowers do not exhibit marked dichog- 

 amy. Allied to cleistogamy is bud pollination (as in Oenothera and in vari- 

 ous orchids), where autogamy occurs in ordinary flowers before they open. 



The fact that the open flowers of plants which possess also cleistogamous flowers 

 usually produce but few seeds has led to the theory that the failure of cross polli- 

 nation probably has resulted in the evolution of cleistogamy. This theory has no 

 evidence in its favor. In Viola biflora there are cleistogamous flowers, although 

 the showy open flowers fruit abundantly. Furthermore, close pollination can take 

 place quite as readily in the open as in the closed flowers of a given species, since own 

 pollen is as potent in one case as in the other. As noted above (see also p. 901), 

 cleistogamy is in part associated with arrested development, and usually is due to 

 definite external conditions, which are unfavorable for chasmogamy; for example, 

 in Lamium amplexicaule the first flowers in spring and the last flowers in autumn 

 are cleistogamous, while the summer flowers are open and showy. Cleistogamy 

 is advantageous in that closed flowers are well protected from rain and from the 

 visits of pollen-gathering insects. Subterranean cleistogamy is advantageous in 

 that the seeds are self-planted and are well-protected from many seed-eating ani- 

 mals, such as birds. 



The comparative advantages of cross pollination and close pollination. 



— Introductory remarks. — Usually it is believed that cross pollination 

 must be highly advantageous because it is so common, and particularly 

 that the diverse and sometimes extraordinary features which impede 

 or even prevent close pollination are prima facie evidence of the value 

 of xenogamy. The usual reason for regarding cross pollination as supe- 

 rior to close pollination is either that it facilitates the merging of diverse 

 racial characters ^ or that it promotes variability or racial vigor. The 



^ The germination of pollen grains within the anther has been reported occasionally 

 even in chasmogamous flowers. 



' If this conception is true, it still further emphasizes the essential difference between 

 true cross pollination and geitonogamy, since there is no such merging in the latter, the 

 flowers of a single individual having a common immediate ancestry. 



