GAMETOPHYTE AND SPOPOPHYTE. 



243 



When fertilization takes place the number of chromosomes 

 is doubled in the embryo. — In the spermatozoid of osmunda 

 then, as well as in the egg, since these are developed on the game- 

 tophyte, there are twelve chromosomes each. The same is true 

 in the sperm-cell (generative cell) of lilium, and also in the egg- 

 cell. When these nuclei unite, as they do in fertilization, the 

 paternal nucleus with the maternal nucleus, the number of chro- 

 mosomes in the fertilized egg, if we take lilium as an example, 

 is twenty-four instead of twelve; the number is doubled. The 

 fertilized egg is the beginning of the sporophyte, as we have seen. 

 Curiously throughout all the divisions of the nucleus in the em- 

 bryonic tissues of the sporophyte, so far as has been determined, 

 up to the formation of the mother cells of the spores, the number 

 of chromosomes is usually the same 



475. Reduction of the number of chromosomes in the nu- 

 cleus. — If there were no reduction in the number of chromosomes 



Fig. 322. 

 Karyokinesis in sporophyte cells of podophyllum (twice the number of chromosomes 

 here that are found in the dividing spore mother cells). 



at any point in the life cycle of plants, the number would thus 

 become infinitely large. A reduction, however, does take place. 



