XIIll 



OPENING MECHANISMS 



251 



sori and sporangia of Ptychocarpus unittts are structurally like those of 



Christeiisenia. In the Ophioglossaceae the outermost layer of the wall is 



uniformly indurated over the whole surface, excepting along the line of 



dehiscence, where the cells are smaller than the rest. The slit thus defined 



runs transversely to the fertile leaf-segment, 



but longitudinally to the sporangium itself 



By drying up of the softer tissue of the wall 



the lips are drawn apart. It thus appears that 



the method of dehiscence is not of a high order 



in any of the Eusporangiate Ferns. The annu- 



lus is either absent, or it is of a non-specialised 



type. 



A particular interest attaches to the spor- 

 angia of the Osmundaceae, with which are to 

 be associated those of the Carboniferous fossils 

 Kidstonia, Boweria, and probably others also. 

 The sporangia are all separate and pear-shaped, 

 with relatively thick stalks (Fig. 246). The an- 

 nulus when ripe consists of a group of polygonal 

 thick-walled cells near to its distal end, but 

 rather to one side of it. The slit passes from 

 the centre of the annulus, over the distal end; 

 it extends downwards and gapes widely owing 

 to the action of the mechanical cells. Here is 

 a type of sporangium which has neither the 

 mechanical equipment of the Eusporangiate 

 nor of the Leptosporangiate Ferns. The family 

 is intermediate in so many other characters 

 that this peculiar mechanism of dehiscence 

 claims all the greater attention. 



In the Leptosporangiate Ferns the mecha- 

 nism is precise, and that precision comes with 

 an increasing definiteness of the structure and 

 position of the annulus, and of the stomium. 

 Instead of the outer cell-layer of the sporangial 

 wall being generally indurated, as in the Ophio- 

 glossaceae, or a broad band of cells being 

 thickened, as in Angiopteris or Etapteris, the mechanical thickening is 

 localised in a single row of cells, which forms in all the more advanced 

 types a very efficient means of dispersal. An example has already been 

 described in Diyopteris (p. 14). It is possible by comparison of fossils 

 with certain primitive living types to see steps which may have led to this 



Fig. 246. Todea hai-hara, Moore. 

 Sporangium A, in side view, 

 closed; B, seen from behind; C, 

 from in front, in both cases after 

 dehiscence : the annulus is darkly 

 shaded. (x8o.) (After Luerssen.) 



