132 



HEREDITY AND EVOLUTION IN PLANTS 



either as new formations, developed (by enation") on some 

 primitive reproductive axis like a strobilus or cone, or else 



Fig. 66. — Diagram to show the increase in prominence of the sporo- 

 phyte stage of plant life from the algae to the higher seed-plants. Among 

 the thallophytes both the sexual and asexual methods of reproduction 

 are represented. A illustrates the asexual, wherein certain cells of the 

 plant divide into smaller cells, the zoospores, which, without union with 

 other cells, develop directly into new plants. B-E illustrate the sexual 

 method, effected through an alternation of generations, wherein a 

 vegetative stage, the sporophyte, alternates with a reproductive stage, 

 thie gametopyte. (After Shimer.) 



as produced by the sterilization of parts originally fertile, 

 i.e. , modifications of reproductive tissues. The sporophyte 

 has become increasingly well developed and increasingly 

 independent, while the gametophyte has become increas- 

 ingly simple and increasingly dependent. The evolution 

 of plants has proceeded by the progressive development of 

 the sporophyte, and the gradual but steady regression of 

 the gametophyte. This changing relationship is roughly 

 indicated in the following diagram (Fig. 67, and also in 

 Fig. 66). 

 107. Steps in the Evolution of the Sporophyte. — The 



