490 



MOSSES AND FERNS 



CHAP. 



cavity of the spore. The thickenings upon the outer spore- 

 coat are very evident even before the primary nucleus divides, 

 and they increase rapidly in size, as the spore develops. A 

 very casual examination suffices to show that the tapetal cells 

 of the sporangium here play a most important part, not only 

 in the development of the spore-coat, but also in the growth 

 of the prothallium. The rapid increase in the amount of 

 protoplasm in the spore during the growth of the prothallium, 

 as well as the growth of the spore itself, can only be accounted 



A. 



Em. •' 



Fig. 256. — Selaghiella Kraussiana (A. Br.). A, Nearly median section of a fully-developed female 

 prothallium, showing the diaphragm {d), X 180. One of the archegonia has been fertilised, and 

 the suspensor {sus) has penetrated through the diaphragm into the tissue below it ; B-E, develop- 

 ment of the archegonium, X360 ; F, two-celled embryo, belonging to the suspen.sor shown in A, 

 X 360 ; G, end of a suspensor with two-celled embryo (<??«)) X 360. 



for by the activity of these cells, which are in close contact 

 with the spore, and show every evidence of being active cells, 

 through whose agency the materials are conveyed to the spore 

 for its further development. 



The first archegonia begin to form shortly before the spores 

 are shed, and soon after, the exospore splits along the three 

 ventral ridges and exposes the central part of the prothallium. 



