384 



MOSSES AND FERNS 



CHAP. 



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. 198, A, B). Berggren's ^ figures of A. Caro- 

 lihiana^ 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 prothallium, 

 by which it expands and ruptures the exospore, which breaks 

 open by three lobes at the top. 



..ep 



Fig. 198. — Azolla filiculoides (Lam.). A, Longitudinal section through the upper part of the 

 germinating macrospore, X 220 ; b, b, the basal wall of the prothallium ; ar, young archegonium ; 

 n, free nuclei ; B, similar section of a nearly developed female prothallium, X 220 ; C, D, 

 archegonia, X 375 ; h, neck canal cell ; v, ventral canal cell ; 0, egg ; E, two transverse sections 

 of a prothallium with the three first archegonia, X 160 ; F, median section of a macrospore ^^'ith 

 large prothallium {pr), X 65 ; in, indusium ; j/, 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. 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- 

 thallium (Fig. 198, A). This nucleated protoplasm is free 

 from the large albuminous granules in the lower part of the 

 spore cavity, and in stained sections presents a finely granular 



1 Berggren (2), Figs. 7, 9. 



