532 



FILICALES 



flattened expansion : and developmentally it is found that this arises by 

 transverse growth of the superficial tissues, especially at the wings them- 

 selves. The result in Osmunda is a mere flattened leaf-base ; but in Todea 



superba the development extends also as a 

 transverse commissure across the adaxial 

 face of the leaf-stalk. This development, 

 so exceptional in Ferns, may be compared- 

 with the stipular development in the 

 Marattiaceae, which is also extended as 

 a commissure across the leaf-stalk. The 

 two cases, though differing in detail, appear 

 alike in morphological nature. 



The sporangia of Todea are borne 

 only on the under surface of the leaves, 

 but in Osmunda they are inserted in- 

 differently on both sides of the much- 

 contracted part, so that at maturity it 

 appears to be completely covered by them. 

 There is no protective indusium. The 

 sporangia themselves are relatively large, 

 of pear-like form, and thick-stalked. They 

 consist at maturity of a single layer of 

 cells forming the wall, but with a few 

 tabular cells within : a group of polygonal, 

 thick-walled cells in a lateral position, 

 but nearer the distal end, is recognised 

 as the annulus : it is related to the slit 

 of dehiscence so that the latter passes 

 from the centre of the annulus, over the 

 distal end, and approaches the stalk 

 on the opposite side of the sporangium 

 (Fig. 294). The line of dehiscence, de- 

 fined structurally by narrow thin-walled 

 cells, gapes widely at ripeness ; this 

 arrangement requires elbow-room, which 

 the lax arrangement of the sporangia 

 readily allows. Those sporangia which 

 are in near proximity to one another 

 originate simultaneously : there is no 

 interpolation, nor any marked sequence of their origin. Nor is there 

 any regularity in their orientation : in Osmunda the sporangia face in 

 the most various directions : and even in Todea, where their arrangement 

 has some reference to the nerves of the leaf, the sporangia upon a 

 single nerve show no common rule of orientation. These Ferns are thus 

 non-soral. 



FIG. 294. 

 Todea barbara, Moore. 



Sporangium. 

 B seen from 



A, in side view, closed. 

 behind. C from in front, in both cases after 

 dehiscence ; the annulus is darkly shaded. 

 X 80. (After Luerssen.) 



