258 



THE SPORE-PRODUCING ORGANS 



[CH. 



cells, or to the " archesporium " itself (see Land Flora, p. io8). The constant 

 feature for the Ferns, and indeed for Pteridophytes at large, does not lie 

 in the sequence or number 



of the seo-mentations, but in . 



the fact that the sporogenous ... _ 



cells originate ultimately ^'-^ ^ 

 from the division of super- 

 ficial cells of the plant-body. 

 These divide periclinally : 

 the outer product then forms 

 part of the sporangial wall ; 

 the inner gives rise by 

 further divisions to sporo- 

 genous cells. The final result 

 of those divisions is a number 

 of highly protoplasmic, thin- 

 walled cells. These round 

 themselves off as spore- 

 mother-cells, separating more 

 or less completely, and float- 

 ing in a semi-fluid medium 

 which fills the cavity of the 

 rapidly enlarging sporan- 

 gium. Here they undergo 

 the tetrad-division, as already 

 described (Chapter I, p. 22, 

 Fig. 30). The chief points for 

 further discussion will be the 

 development, nutrition, and 

 ripening of the spores : also 

 the number of the spore- 

 mother-cells, and the conse- 

 quent spore-output of the 

 sporangium. An accessory 



interest also attaches to the tapetum, which is variable in its relation to 

 the sporogenous tissues, as well as in its behaviour in the ripening of the 

 spores. 



Fig. 254. Marattia fraxinea. Smith. ^=section trans- 

 versely through a sorus : the sporogenous cells shaded, the 

 tapetum marked {x, x) : the left-hand sporangium shows 

 the most usual arrangement of the sporogenous tissue, the 

 other two less frequent. B, C show in similar section 

 irregular groupings not referable to a single parent cell. 

 ( X 200.) 



The Tapetum 



The tapetum is not a morphological constant. In the Bryophyta it is 

 not represented. In the Pteridophyta, and even within the Filicales, it is not 

 uniformly of the same origin. In the Ophioglossaceae and Marattiaceae it 



