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. 



FIG. 235. Azolla filiculoides. A, Longitudinal section through the upper part of the 

 germinating macrospore, X22o; b, b, the basal wall of the prothallium; ar, young 

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

 thallium, X22o; 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, 

 Xi6o; F, median section of a macrospore with large prothallium (pr), X6s; in, 

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



The most remarkable difference between Azolla and the 

 other Hydropterides is the further development of the lower 

 of the two primary nuclei. 1 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- 



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

 Marsilia. 



