THE EVOLUTION OF PLANTS 



133 



possible steps in the evolution of the sporophyte may, 

 on this theory, be tabulated as follows: 1 



1. Sterilization of fertile tissue. 



2. Localization of spore-production in sporangia. 

 Origination of lateral organs (leaves), and of roots. 

 Development of heterospory. 



Introduction of fertilization by the pollen-tube 

 (si phono gamy). 

 6. Assumption of the seed-habit. 



3- 

 4- 



5- 



FIG. 67. Diagram illustrating the gradual change in the relative promi- 

 nence of the gametophytic and sporophytic phases in the life-cycle of 

 plants during their evolution from the primitive alga? (at the left) to the 

 modern seed-bearing plants (at the right). 



108. Second Hypothesis. In a discussion of Bower's 

 theory, Tansley, 2 considers it "a priori in the highest 

 degree unlikely that so fundamentally important an organ 

 as the foliage leaf of the vascular plant appeared in descent 

 as an 'enation' from the surface of a cylindrical body of 

 different morphological nature," and states that "there 

 is no well established case of any such origin of an organ 

 of the importance and with the potentialities of the leaf in 

 the evolutionary history of the plant kindgom." He also 

 calls attention to the fact that the sporophyte (sporo- 

 gonium) of mosses and liverworts has never been known to 

 produce by enation or otherwise, any structure resembling 



1 Following F. O. Bower. 



2 New Phytologist 7:177-129. April and May, 1908. 



