574 STRUCTURE AND LIFE HISTORIES 



manufacture, and of roots for absorption of water and 

 minerals in large quantities. From this point of view, 

 the plant body of the sporophyte is regarded as produced 

 by the progressive sterilization of tissues originally re- 

 productive. After the formation of a vigorous plant 

 body then spores, produced in special regions (sporangia) , 

 could be nourished in enormous numbers. 



498. Origin of Vegetative Organs. — On the basis of 

 the theory just outlined, we are to regard foliage-leaves 

 and branches, either as new formations ^ developed (by 

 "enation^^) on some primitive reproductive axis like a 

 strobilus or cone, or else as produced by the sterilization 

 of parts originally fertile, i.e., modifications of reproductive 

 tissues. The sporophyte, as we have already seen,^ 

 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 regres- 

 sion of the gametophyte. 



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

 possible steps in the evolution of the sporophyte may be 

 tabulated as follows:^ 



1. Sterilization of fertile tissue. 



2. Localization of spore-production in sporangia. 



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



4. Development of heterospory. 



5. Introduction of fertilization by the pollen-tube 

 (siphonogamy) . 



6. Assumption of the seed-habit. 



1 Chapter XXIII. 



2 Following F. O. Bower. 



