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). 
