35 



With this is view, we are now ready to consider some of the modern 

 phases of our subject. 



Any effort to trace the development of tlie sexual pi-ocess with all cor- 

 related phenomena would lead us into an overwhelming mass of details. 

 Consequently, I shall merely recall that among the lowest plants sexuality 

 does not exist, and tliat, in the simplest plants with a sexual process, the 

 sex cells, or gametes, are scarcely to be distinguished from the non- 

 sexual reproductive cells. The conclusion is that gametes were originally 

 derived from a sexual propagative cells. There is accordingly no ditferen- 

 tiation into male and female. 'I'he life cycle of these simple sexual plants 

 is also simple, and it is reasonable to suppose that a corresponding degree 

 of differentiation obtains in the chromatin or hereditary substances of tae 

 sex cells. As we ascend in the scale of e^'olution toward higher and more 

 complex organisms, we find a corresponding differentiation in all struc- 

 tures and functions, and may we not assume also that the hereditary sub- 

 stance, or germ plasm, is likewise specialized and differentiated? There- 

 fore, sex is the expression of a very fundamental sort of division of labor. 

 I do not mean by that a division of labor which is of a secondary nature 

 such as man has ascribed to the individuals of his own species, but that 

 of a purely hereditary character — or may I say maleness and femaleness 

 in the broadest and most fundamental sense. 



I/ow then did sex come about? And what is it that determines that 

 one individual or member of a life cycle will be male and another female? 

 To ask such questions fifty or even twenty-five years ago might have been 

 regarded as visionary. Not so today. Considered from the botanical stand- 

 point, the problem of sex determination has to deal with a certain category 

 of phenomena that are in many respects tundamentally different from those 

 presented by animals. In plants in which sex differentiation is well defined, 

 there is in every com] ilete life cycle two phases known as the sexual and the 

 asexual, or gametophyte and sporophyte. The .sporophyte springs from the 

 fertilized egg or the union of sex cells. This sporophyte in turn bears 

 spores which give rise to gametophytes. This may be made clear by means 

 of an example such as the fern. The spores borne on the leaves of the fern 

 do not jjroduce directly new ferns, but very .small plants known as pro- 

 thallia, which in the simpler ferns are independent and self-nourishing in- 

 dividuals. The prothallia are the sexual plants. They bear the sex or- 

 gans, which is turn produce eggs and sperms. The prothallia may be 

 either purely male or female or hermaphrodite. When the egg is fertilized 



