— io8— 



Besides the excretion of a cuticle sufficiently waxy to 

 be more or less unwettable, many ferns have outer walls 

 the convexity of which is a strong factor in the same 

 direction. These convex walls are sometimes confined to 

 the nether surface, sometimes they are more convex in 

 such situations. Such an adaptation is naturally to be 

 found chiefly in ferns growing in places where they are 

 likely often to be wet; as a matter of fact, it is confined 

 entirely to ferns growing in such places. It has already 

 been remarked that half the depth of the cell in Adiantum 

 diaphanum is made of the projections. Between walls 

 which are convex to this extent and trichomes, no line 

 can be drawn. Dry trichomes, unicellular or pluri- 

 cellular, occur over the mesophyll of various species of 

 Nephrodium and others. Very many species have them 

 on the veins, the reason for this restriction being the 

 mechanical one that their bases can be more firmly an- 

 chored in the more solid walls to be found there. A 

 better protection against wetting than is produced by 

 dry hairs is furnished by glandular ones such as are to 

 be found in many species of Nephrodium. These are of 

 characteristic form, size and color for each species. 



Stomata occur on the upper surfaces of the fertile 

 fronds of Acrostichum and C heir o pleura and on all four 

 faces of those of Mono gramma; otherwise they are en- 

 tirely confined to the nether surfaces of all our ferns. 

 Here they may be equally distributed or they may be in 

 streaks or, to a limited measure, in groups. They occur 

 only over the parenchyma. The number varies from 7 

 to 400 per square millimeter. As a general rule, the 

 number and size vary in opposite directions. In most 

 ferns, the outer walls of the guard cells are in the same 

 plane as the outer walls of the epidermal cells. 



The specialization of the epidermal cells of ferns is 

 what the environment demands. In terrestrial species, 

 with very few exceptions, they are not extremely differen- 

 tiated from the parenchyma, but that this difference be- 



