852 ECOLOGY 



Features favoring the deposition of pollen on stigmas. Stigmas, as 

 previously noted, secrete sticky substances, and their hairy or papillate 

 surfaces still further favor pollen reception (fig. 1158). In many plants 

 the stigmas at maturity have essentially the same position as that of the 

 mature anthers (as in the figwort, figs. 1178, 1179), so that the part of 

 the insect which is covered by pollen is likely to touch the stigma. In 

 many other cases (as in the violets) the stigma projects beyond the an- 

 thers, so that it is likely to receive pollen from the entering proboscis. 



In Centaurea mechanical irritation (as from a visiting insect) causes the fila- 

 ments to contract, thus exposing the stigma to pollination by the visitor. The most 

 remarkable situation is in the orchids, where the pollinia above noted, after removal 

 from the flower, move into such a position that they are likely to come into contact 

 with the stigma of the next flower visited. The orchid stigmas remain receptive a 

 remarkably long time if potent pollen fails to come in contact with them, though 

 they wither soon after the proper pollen begins to germinate. The corollas also 

 remain fresh on unpollinated flowers some days or even weeks longer than on polli- 

 nated flowers. 



Features which impede close pollination and facilitate cross pollina- 

 tion. Mechanical features impeding close pollination. In a vast 

 number of flowers close pollination is difficult or even impossible. Fre- 

 quently the stigma projects beyond the anthers (as in certain lilies and 

 evening primroses), so that pollen cannot fall upon the stigma of the same 

 flower (fig. 1174); in pendulous flowers, of course, the stamens would 

 have to project beyond the stigma to have a like result. Sometimes, as 

 in Iris, the receptive surface of the stigma is so oriented that the insect 

 rubs against it upon entering, but not upon leaving the flower, thus 

 facilitating cross pollination and preventing close pollination. Close 

 pollination is difficult in flowers with extrorse anthers. In orchids it is 

 almost impossible for the pollinia to come into contact with the stigma 

 of the same flower; in some lady's slippers (as in Cypripedium Cal- 

 ceolus) the insect enters and leaves the flowers by different routes, brush- 

 ing the stigma upon entering and the anthers before leaving. 



Dichogamy. The commonest floral feature that facilitates cross 

 pollination and makes close pollination difficult is dichogamy, or the 

 consecutive maturity of anthers and stigmas, contrasting with simulta- 

 neous maturity or homogamy. Dichogamy may be complete, that is, 

 the pollen may be shed before the stigma matures, or the stigma may 

 wither before the pollen sheds; more commonly it is incomplete, that is, 

 there is a partial overlapping of the periods of stigmatic receptiveness 



