36 



it develops immediately into the sporophyte. or what we commonly know as 

 the fern. Thns the sexual plant, or gametophyte (female gametophyte) 

 not only produces the sex organs, but serves as the incubator and brooder 

 for the young sporophyte. The life-cycle of the highest plants such as 

 trees and sunflowers consists also of these two generations, but the rela- 

 tive size and mutual relation of sporophyte and gametophyte are different 

 in the higher plants. For example, the beech tree is the sporophyte, the 

 gametophyte being the jiollen tube and the embryosac of the undeveloped 

 seed. Here the reverse condition prevails as regards the mutual relation 

 of sporophjrte and gametophyte to that in the fern, namely, the sporophyte 

 nourishes the young sporophyte as well as both gametophytes. 



Now, we are in the habit of speaking of male and female flowers 

 according as they are wholly staminate or pistillate, and the plant that 

 bears only staminate flowers we call male, while the one bearing only pis- 

 tillate flowers is designated as the female individual. However, in the 

 strict morphological sense the sporophyte is without sex. hence trees can 

 be neither male nor female, and to avoid trouble and useless discussion, 

 it is doubtless better to speak of staminate and pistillate trees ; for we 

 shall see that the sex of any complete life-cycle is determined and fixed in 

 the germ cells. From the foregoing it is quite clear that in the animal 

 kingdom, apart from one or two cell generations, there is nothing in the 

 life-history that is comparable to sporophyte and gametophyte. 



We are now ready to answer the question, upon what does the diffei*- 

 entiation into gametophyte and sporophji;e depend? Our explanation of 

 this doctrine is based upon the theory of the hereditary substance. Doubt- 

 less nearly all biologists concur in the view that the hereditary charac- 

 ters are borne by a substance in the nucleus of the cell called chromatin. 

 When the nucleus divides the chromatin differentiates tnto a definite num- 

 ber of pieces known as chromosomes. The number of chromosomes is 

 always constant for the reproductive cells of any species. In all the cells 

 of the sporophyte of any plant, which lie in the germ tract, there are. let 

 us say. a definite number of chromosomes designated by n. During the for- 

 mation of spores, however, the number is reduced to one-half, or iu. 

 Now each spore has n, chromosomes, and the cells of the gametophyte re- 

 sulting therefrom will possess n., chromosomes: consequently the egg and 

 the sperm will have each ;/, chromosomes. It is apparent that when egg 

 and sperm unite, the fecundated egg and the individual arising from it 

 will contain h, plus n. or n chromosomes. 



