382 THE BIOLOGY OF FLOWERING PLANTS 



§ 13. Cross-Pollination 



Dicliny and Dichogamy. — In strictly dioecious plants 

 only xenogamy is possible. This is seen, for example, in the 

 hydrophilous Vallisneria and Elodea, in the anemophilous 

 poplars and junipers, in the entomophilous willows. In 

 monoecious plants, on the other hand, autogamy is impossible, 

 but geitonogamy may take place. This state is seen in the 

 majority of the conifers, in the beech and the hazel, most 

 sedges and many palms among anemophilous plants, in the 

 arrow-head, and the arums among entomophilous, and in 

 the grass- wracks among hydrophilous plants. In monoecious 

 plants, however, a separation of the sexes in another way 

 almost invariably occurs ; the flowers of the one sex mature 

 before those of the other, and, in fact, the pistillate flowers 

 ripen several days before the stamens shed their pollen. 

 This separation of the two sexes in time, which is also 

 frequent in the individual hermaphrodite flower, is termed 

 dichogamy. We distinguish between proterogyny , the con- 

 dition in all monoecious plants where the stigmas are ripe 

 first, and proterandry where the stamens are ripe first. 



The interval between the receptiveness of the stigmas 

 and the dehiscence of the stamens is, in general, two or three 

 days, but may be longer, e.g. nine days in Alnus viridis, and 

 perhaps in the hazel. Proterogyny would seem to be an 

 absolute safeguard against self-pollination. We have not 

 exact information, however, as to the length of time during 

 which the stigmas remain receptive in absence of cross- 

 pollination. Further, different shoots of the same tree may 

 be ripe at different times. So that there is a possibility of 

 occasional geitonogamy, though it must be unusual. 



The various more complicated schemes of sex distribu- 

 tion are, on the whole, less suited to secure cross-pollination 

 than is dioecism, or even monoecism. They need not be 

 discussed in detail, for their effectiveness can be readily 

 estimated by considering the different types already 

 described. In such cases, however, dichogamy is usually 



