208 HISTORY OF BRITISH FERNS. 



observed, the one having thicker, shorter, and more spreading 

 leaves than the other ; in the latter they are more slender and 

 erect. These have been thought distinct varieties or even 

 distinct species by some botanists, but are more probably 

 mere conditions of the plants brought about by external cir- 

 cumstances. Several theories have been propounded as ex- 

 planatory of the way in which this occurs. Sir J. E. Smith 

 says, the taller, more slender variety may, perhaps, be caused 

 by those sudden risings of the waters so frequent in moun- 

 tainous countries, which will account for all its peculiar 

 characters. Mr. Wilson believes the solitary plants with 

 short spreading leaves to be the first full development after 

 the seedling state, and before any lateral extension of the 

 rhizome has taken place ; and argues, that when the plants 

 become crowded, either by lateral increase or offsets, or by a 

 multitude of individuals in close contact, the fronds can only 

 grow erect. Mr. Newman thinks, that when the seeds arrive 

 at maturity, the leaf in whose base the spore-case is situated 

 decays and becomes torn or broken off, allowing free egress 

 to some of the spores, which become dispersed, and produce 

 the isolated dwarf spreading plants. Others and by far the 

 larger number of the spores do not become thus disengaged, 

 and are compelled to germinate in the capsule, throwing up 



