216 HISTORY OP BRITISH FERNS. 



diating lines, which appear to have been produced by the 

 pressure of the three other spores that originally helped to 

 constitute the quaternary union. The spores of some of 

 the other tribes being apparently developed in similar unions, 

 it is probable that similar lines, indicating a mode of 

 opening by valves, also exist on them, and this is certainly 

 the case in some instances, as in Isoetes, Lycopodium, and 

 Osmunda ; and in those cases in which such a structure is 

 not visible, it is probably owing to a thickening of the mem- 

 brane, or a deposition of opake matter on its surface, as in 

 Pilularia, in the mature spores of which they can only be 

 discovered by dissection, and in the abortive ones they can- 

 not be discovered at all after the earlier stages of growth. 

 It, however, does appear that in these plants, after the pro- 

 trusion of the germ, it is immaterial from what part of that 

 body the first leaves, root, or stem shall arise. 



The Pillwort is widely distributed throughout the United 

 Kingdom, but is apparently more abundant in England and 

 Wales, than in Scotland and Ireland. It usually grows on 

 the margins of lakes or pools, where it is covered by the 

 water in winter, and more or less exposed during the sum- 

 mer; but it is also sometimes, though rarely, met with 

 entirely submerged. 



