438 STRUCTURE AND LIFE HISTORIES 



perianth can be understood only in connection with 

 pollination. It will be recalled that in the gymnosperms 

 the pollen is transferred by wind, but in Erythronium 

 this transfer is accomplished by means of insects. The 

 perianth, conspicuous to us by its petals, appears to at- 

 tract certain insects. Whether this is accomplished by 

 color, or by odor, or by some other means not clearly 

 demonstrated, is not absolutely known. It is generally 

 believed to be by color, but certain experiments seem to 

 disprove this theory. Be that as it may, we know that 

 the development of a conspicuous perianth appeared in 

 the same geological age (Cretaceous) as did the more 

 highly developed, winged insects, such as the butterflies 

 and moths. In fact the insects probably appeared some- 

 what earlier than the " flowers." 



Attracted to the flowers by whatever means, the insect 

 finds, at the bases of the petals, nectar secreted by glands. 

 While feeding on the nectar the back of the insect becomes 

 dusted over with pollen from the anthers. When he 

 flies to another flower some of this pollen is rubbed off 

 on to its stigma, thus accomplishing pollination. 



391. The Male Gametophyte. The young pollen-grain 

 has already been recognized as a microspore. In some 

 species it develops into a male gametophyte before pollina- 

 tion, in other cases not until afterward. In either case 

 the gametophyte is very greatly reduced. The mature 

 pollen-grain of the milkweed, for example, is a mature 

 gametophyte, having the sperm-cells formed at about the 

 time the flower-buds open. In Erythronium the genera- 

 tive cell, formed between December i and April i, does 

 not divide to form the sperm-cells until after pollination, 

 and after the pollen- tube has begun to form (Fig. 324). 



