MARATTIACEAE 



517 



At maturity the more or less^ indurated superficial layer of cells of 

 the sporangial wall is the most conspicuous part, but the thinner-walled 

 cells lying within, though they may shrink, do not entirely disappear. 



The essential parts of the sporangium of Angiopteris, and especially 

 the archesporium, are thus seen to be referable in typical cases to a 

 single parent cell : this also is the case typically for all the other genera. 



FIG. 285. 



Marattiafraxinea, Smith. A = section transversely through a sorus : the sporogenous 

 cells shaded, the tapetum marked (-r, x) ; the left-hand sporangium is the most usual 

 arrangement of sporogenous' tissue, the other two less frequent. B and C show in similar 

 section irregular groupings not referable to a single parent cell. X 200. 



But in Angiopteris and Marattia, and notably in Danaea, this is not 

 always so, and there is much individual divergence of detail. Not only 

 is the segmentation conformable to no strict plan, but the size of the 

 sporogenous groups varies greatly, while in non-typical cases it is not 

 always possible to refer the sporogenous group of one sporangium to a 

 single parent cell (Fig. 285). These irregularities are usually associated 

 with considerable differences in size of the sporangia. In no genus 

 does this irregularity appear more pronounced than in Danaea : an 

 almost exact uniformity in size and segmentation of sporangia is so 



