FUNDAMENTAL PRINCIPLES 



183 



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 oosperm) 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 fertiHzation. 



