354 



PART IV. VEGETABLE TAXONOMY. 



groups. In the Polypodiacese, the sub-order to which most of 

 our common ferns belong, it has the structure represented in Fig. 

 561, B. The upper part, or sporangium proper, has a row of 

 thick-walled cells, a, which begins at the upper part of the stalk, 

 s, on one side and passes vertically over to the other side, but 

 terminates at d before it again reaches the stalk. This row of 



FlG. 561. — Polypodium vulgare. A, rhizome and two leaves, one showing the inferior 

 surface and sori; s, is one of the sori. B, a sporangium of one of the Polypodiaceas a, 

 annulus, composed of thiclc-walled cells extending vertically around the sporangium to d; 

 near d, the dehiscence begins : s, stalk of the sporangium. 



cells is called the annulus, and it is at or near where it termi- 

 nates that the sporangium ruptures when ripe. The dehiscence 

 is transverse, and the spores are ejected by the elastic straight- 

 ening of the annulus. In this group, as we have seen, the 

 annulus is incomplete, that is, it does not pass completely around 

 the sporangium ; in the Hymenophyllums and in the Cyathaceae, 

 however, it is complete. In some other species, as in the Osmun- 

 das, it is wanting entirely, and in the Gleicheniaceae the sporan- 



