542 



MOSSES AND FERNS 



CHAP. 



vividly the endosperm formation of most Angiosperms. On 

 account of the extremely thin walls and dense contents of the 



Fig. 312. — Isoetes echinospora van Braunii. A, Longitudinal section through the apex 

 of the female prothallium, showing the first cell formation, X300; B, similar sec- 

 tion of a prothallium with the divisions completed and the first archegonium iar) 

 already opened. 



young prothallial cells it is not easy to determine exactly when 

 the whole spore cavity becomes filled up with cellular tissue. 



