598 



FILICALES 



already noted 1 that the receptacle is derived from the leaf-margin, while 

 the cup-like indusium originates as two flap-like outgrowths of the lower 

 and upper surfaces ; excepting that the indusium is not two-lipped, and 

 that the receptacle is cylindrical instead of flattened, it resembles in its 

 main features the sorus of Dicksonia. The relations of the sporangia within 

 the sorus have been examined in Dennstaedtia apiifolia, Hook., with the 

 result that the basipetal succession has been found to be much more 

 marked than in Dicksonia. The receptacle is cylindrical and elongate 

 and traversed by tracheides for a considerable distance (Fig. 332 bis, A) 

 upon it the sporangia arise, as a rule, in strictly basipetal order. Ttu 

 sporangia themselves undergo the usual segmentation; when mature the] 



FIG. 332 Us. 



A = Dennstaedtia apiifolia, Hook! Sorus showing basipetal succession throughout. 

 C = dehiscent sporangium of the same showing very slightly oblique annulus. B = 

 Dennstaedtia rubiginosa, Kaulf. Sorus in vertical section showing that it has been at 

 first basipetal, but with a mixed character supervening. D = dehiscent sporangium of the 

 same, seen from the base, showing that the annulus stops short on either side of the 

 insertion of the stalk (st~). All X 100. 



have long stalks composed of three rows of cells; the head is not 

 bilaterally symmetrical, but one side is more strongly convex than the other; 

 and it will be seen from the figure that the more convex side is the 

 peripheral one. The annulus is not exactly vertical, but running round the 

 margin of the sporangium to the attachment of the stalk, it is there slightly 

 diverted to one side (Fig. 332 bis, c) ; usually the sequence of cells of the 

 annulus is not wholly interrupted by the insertion of the stalk, but its cells 

 are more or less in contact with one another, and the annulus is in such 

 cases actually continuous at the base, as it is in those sporangia where it is 

 more conspicuously oblique. In fact, the sporangium of D. apiifolia shows, 

 though in a less obvious degree than in Dicksonia, the oblique annulus. 

 The nearer side of the sporangium shown in Fig. 332 bis, c is the less 

 convex one, the more convex side is the peripheral face as regards the 



l Loc. cit., p. 19. 



