SELF-POLLINATION 395 



when the anthers are dehiscing. Even the presence of a 

 complex floral mechanism may not therefore be much 

 safeguard against geitonogamy, and it is not clear that 

 this differs essentially from autogamy. Rare cases have 

 been described in which the inheritance of the reproductive 

 cells at different levels on the axis is different, as in the 

 rogue peas investigated by Bateson and Pellew (1920). We 

 do not know that such cases have any general application. 

 Without definite proof to the contrary we must regard those 

 plants in which geitonogamy may occur as belonging to the 

 same category as those which are often autogamous. 



More than a chance of autogamy exists in many crucifers, 

 such as the wallflower. The anthers of the long stamens 

 surround the stigma and dehisce directly on to it. Insect 

 visits may result in the deposition of foreign pollen, but, 

 unless this is prepotent, autogamy is certainly favoured. 

 According to Knuth, this condition is particularly common 

 in small annual plants. 



In some flowers, where the anthers originally lie below 

 the stigma, they grow up later so as to come in contact with 

 it, as in Adoxa Moschatellina and many saxifrages. In 

 Hypericum perforatum, Lysimachia nemorum, and Azalea 

 procumhens, the anthers at first stand away from the stigmas 

 and later bend or are bent towards it. Here autogamy is 

 postponed till allogamy has failed. These examples may 

 suffice, but we may note that Knuth distinguishes twenty 

 different ways in which autogamy is secured. 



Cleistogamy.— The most extreme case of autogamy is 

 offered by the cleistogamous flowers, in which the perianth 

 never opens and pollination takes place, as it were, in the 

 bud. About 150 species are known with cleistogamous 

 flowers. The most familiar example is the sweet violet, 

 Viola odorata. The sweet-scented spring flowers are not 

 very conspicuous, but are visited by a variety of insects. 

 Presumably the visits are not frequent, for seed is not often 

 set. Later in the summer the cleistogamous flowers are 

 formed hidden deep among the leaves. They are bud-Uke, 

 never open, but set abundant seed (Fig. 61). 



