898 ECOLOGY 



believed that external factors act as sex determinants in daphnids, grape lice, aphids, 

 and rotifers, but it is now realized that such factors determine only whether a 

 given generation is to be sexual or parthenogenetic (p. 881) ; in the case of the sexual 

 generation, the maleness or the femaleness of the different individuals is predeter- 

 mined. In some cases at least, the sex of the progeny is determined before egg 

 formation and possibly as a result of external factors ; in one of the rotifers {Hyda- 

 tina) , the better nourished females lay large eggs, which develop parthenogeneti- 

 cally into females, and the more poorly nourished females lay small eggs, which 

 develop parthenogenetically into males. That the sex potentialities of animal 

 gametes may differ from those of plant gametes is shown by the fact that in most 

 cases, eggs which develop parthenogenetically grow into male animals (as in ants, 

 bees, and wasps) ; in those cases in which certain eggs develop parthenogenetically 

 into females (eis in rotifers and grape lice), there are other and smaller eggs, which 

 develop parthenogenetically into males. In bees, in rotifers, and in grape lice, fer- 

 tilized eggs develop with equal certainty into females. 



If it is to be concluded from the above data that sexuality but not sex is deter- 

 mined by external conditions, some further explanation is needed to account for 

 the change in sex noted above for such plants as Zea, Carica, Pulicaria, and 

 Lychnis. These cases seem best explained by assuming that all of these forms are 

 potentially bisexual and that external factors either may cause the suppression of 

 one of the sexes (as in Zea and Pulicaria, and also in most homosporous ferns) or 

 may stimulate to development a sex that commonly is suppressed (as in Carica and 

 Lychnis, and also in Onoclea) , in the latter case acting as releasing stimuli. This 

 view is supported by the fact that Carica and Equisetum are known sometimes to 

 be monoecious, and also by the fact that the staminate and pistillate flowers of 

 Piper Betel may under proper conditions become monoclinous. To what extent 

 other supposedly dioecious species are thus potentially bisexual is unknown ; it may 

 be noted that even the willows, which commonly are thought to be strictly dioecious, 

 occasionally have monoecious individuals and, still more rarely, monoclinous flowers. 



Variations in flower color. — The most variable character of flowers 

 is that of color. Many cases of color variation in flowers of the same 

 species clearly are due to external factors, particularly in those flowers 

 in which colors are due to anthocyan '; such variations may be quanti- 

 tative, involving differences in intensity only, or they may be qualitative, 

 involving differences in wave length. Light seems to be the most im- 

 portant factor determining variations in color intensity. It was dis- 

 covered long ago that when bulbs (as in the tulip or hyacinth) are grown 

 in the dark, they develop colored flowers much as in the light, though 

 the color intensity is less, sometimes being much less, as in blue hya- 



^ However, there are some striking cases of color variation in flowers whose color is 

 due to chromoplasts, as in Tropaeolum and in Castilleja coccinea ; the latter is more likely 

 to have scarlet flowers in rich soil, where the plants are vigorous, and lemon-yellow 

 flowers in peaty soil, where the plants are impoverished. 



