THE EVOLUTION OF THE PTERIDOPHYTES 343 



a root system. For this reason, Anthoceros (Sec. 290) is gen- 

 erally considered the form among the bryophytes most closely 

 approaching the pteridophytes in its structure and possibilities 

 pf development. 



332. The evolution of the pteridophytes. After the sporophyte 

 became independent of the gametophyte, the next important 

 advance was the development of the lateral spore-bearing and 

 vegetative organs called fronds. Then came the differentiation 

 of the fronds into vegetative leaves, given up entirely to chloro- 

 phyll work (photosynthesis), and spore leaves, or sporophylls, 

 devoted chiefly or wholly to spore production. With this also 

 came the massing of the sporophylls in cones, which was really 

 the beginning of the structures called flowers in the seed plants. 



Finally, the condition of heterospory was attained independ- 

 ently in several groups of the pteridophytes, as the water ferns, 

 Selaginella, and Isoetes. Heterospory soon led to very signifi- 

 cant changes in the structure and behavior of the gametophyte 

 generations. They became differentiated in sex, the microspores 

 producing male prothalli, and the megaspores female ones. Fur- 

 thermore, the gametophytes became greatly reduced, finally de- 

 pending wholly, or almost wholly, on food stored in the spores. 

 The food in the female gametophyte also came to contribute to 

 the development of the embryo sporophyte, which was thus fur- 

 nished with food by the sporophyte of the previous generation. 

 At last, in the highest form, Selaginella, the female gametophyte, 

 begins its development while still retained within the megaspore, 

 a condition approximating very closely to the seed habit. 



SUMMARY OF THE PTERIDOPHYTES AND THEIR 

 ADVANCES OVER THE BRYOPHYTES 



333. Summary of the pteridophytes. The chief characters 

 of the pteridophytes and their advances over the bryophytes are : 



1. The development of a leafy shoot and root system with 

 vascular tissues in the sporophyte generation, rendering it 



