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 var. 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 (ar) 

 already opened. 



young- prothallial cells it is not easy to determine exactly when 

 the whole spore cavity hecomes filled up with cellular tissue, 



