THE MODIFICATIONS OF THE CHROMOSOME CYCLE 53 



spores in the female) and the diploid plant which bears them is the 

 sporophyte. They develop into a haploid plant, which, since it bears the 

 gametes, is known as the gametophyte. The factors which affect the 



Fig. 18. Life Cycles of Mosses and Ferns. — A. in a moss the main plant is the 

 haploid gametophyte phase. The gametes are borne on sexual organs among the 

 leaves, sometimes both sexes on the same plant, sometimes on different plants. 

 Fertilization takes place in these organs, and from the female organ the zygote 

 grows into the diploid sporophyte, which remains attached to the gametophyte. 

 The reduction division occurs in the spore capsule, and the haploid spores, when 

 released, grow into new haploid plants. 6. In ferns the main plant is the diploid 

 sporophyte. The reduction division takes place in the "sori" on the under side 

 of the leaves, and the spores, after being set free, grow into an inconspicuous 

 flat plant, C, which is separate from the sporophyte. This is the gametophyte, 

 known as the prothallus; it bears the sexual organs in which the gametes are 

 formed, and after fertilization the new diploid sporophyte grows out from it 

 and absorbs it. In the drawing on the right it is more enlarged than the fern 6, 

 and the first leaf of a sporophyte is just growing out from it. 



characters of the diploid phase are inherited according to Mendel's 

 original laws, and need not be further discussed here. 



I. Inheritance in the Haploid Phase 



The chromosomes of the haploid phase are in the reduced condition; 

 there is only one of each kind. If the chromosomes contain the genes, 

 there can be only one gene of each locus. Heterozygosity should there- 



