402 



MOSSES AND FERNS 



CHAP. 



Up to this point the exospore remains intact; the central 

 cell of the archegonium is only separated from the spore cavity 

 by a single layer of cells, and the young prothallium agrees 

 closely with Prantl's account of the similar stage of Salvinia 

 (Fig. 235, A, B). Berggren's figures of A, Caroliniana, at a 

 stage presumably the same, are too diagrammatic to allow of a 

 satisfactory comparison. 



Shortly after the first division in the archegonium a rapid 

 increase takes place in the size of all the cells of the prothal- 

 lium, by which it expands and ruptures the exospore, which 

 breaks open by three lobes at the top. 



d-T* 



Fig. 235. — Azolla Uliculoides. A, Luiigituamal section through the upper part of the 

 germinating macrospore, X220; h, b, the basal wall of the prothallium; ar, young 

 archegonium; n, free nuclei; B, similar section of a nearly developed female pro- 

 thallium, X220; C, D, archegonia, X375; h, neck canal cell; v, ventral canal cell; 

 o, egg; E, two transverse sections. of a prothallium with the three first archegonia, 

 X160; F, median section of a macrospore with large prothallium (pr), X65; in, 

 indusium; sp, remains of sporangium wall; ep, perinium. 



The most remarkable difTerence between Azolla and the 

 other Hydropterides is the further development of the lower 

 of the two primary nuclei.^ In Azolla it undergoes repeated 

 divisions, and the resulting nuclei remain embedded in the 

 protoplasm in close proximity to the lower cells of the pro- 



^ Recently Coker (i) has observed a fragmentation of the nucleus in 

 Marsilia. 



