MARATTIACEAE 515 



group, to the character of tapetum (Fig. 284 K), and the rule appears 

 to be that the whole tapetum is extra-archesporial in its origin. The 

 figure represents in radial section a sporangium which has arrived at the 



Tu;. 284. 



Angiopteris evecta, Hoffm. A =part of a young sorus seen in surface view from with- 

 out. />= vertical (radial) section of a sporangium such as would be seen on cutting tli<- 

 sporangium (6) in Fig. A along the line indicated. C = vertical section of an older 

 sporangium, showing genetic grouping of cells. /? = apex of an almost mature sporangium 

 seen from above ; such a section as along the Mm- i , r, in Fig. E. E vertical section of 

 a sporangium with spore-mother-cells ; the tapetum is marked .r. F= transverse section 

 of an almost mature sporangium. All X2oo. 



stage of complete division of the sporogenous mass, and in which the 

 spore-mother-cells are about to separate and round themselves off, prior 

 to the tetrad-division. 



From such sporangia it is possible to compute the average number of 

 spore-mother-cells, and for this purpose a number of countings have been 

 made. In radial sections the average of the countings gave 59, say in 



