MARATTIACEAE 515 
group, to the character of tapetum (Fig. 284 £), 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 
Fic. 284. 
Angiopterts evecta, Hofim. A=part of a young sorus seen in surface view from with- 
out. B=vertical (radial) section of a sporangium such as would be seen on cutting the 
sporangium (4) 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 ; sucha section as along the line -r, -v, in Fig. &. A=vertical section of 
a sporangium with spore-mother-cells ; the tapetum is marked +. F=transverse section 
of an almost mature sporangium. All x 200. . 
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 
