FUNDAMENTAL PRINCIPLES 
The fact of a cycle in the life history is brought out 
clearly in the following diagram : 
FIG. 135. Diagram of life-cycle of a fern. 
167. Reduction. Since the sporophyte (descended from 
the diploid ob'sperm) has cells of a double nature, resulting 
from fertilization, and since the spores which give rise to 
the gametophyte are of a single (or haploid) nature, 
there must occur, at some stage in the life of the sporo- 
phyte, a process of reduction, restoring the cells, made 
diploid by fertilization, to the haploid condition. Pains- 
taking studies, of cellular structure and processes has 
disclosed the fact that this reduction takes place during 
the two successive divisions (tetrad-divisions) of the spore- 
mother-cell, resulting in the formation of four spores. 
The diploid condition persists in all the cells of the 
sporophyte, and through every cell-division, up to the two 
divisions preceding spore-formation, just as the single or 
haploid condition persists in all the cells of the gameto- 
phyte, up to the very act of fertilization. 
