INFLUENCE OF ENVIRONMENT 269 
like supports. The base of the stem of the common 
Indian corn shows the same thing on a small scale. 
While the mechanical tissues are of course best de- 
veloped in the stem and roots, the leaves, too, as we 
have intimated, may also be used to illustrate the for- 
mation of such tissues, and in Dicotyledons especially, 
the framework for supporting in the air the widely 
expanded leaves is very perfect. In Monocotyledons 
the leaves stand, as a rule, more nearly upright, and 
are commonly linear in form with a much less perfect 
skeleton than that found in most Dicotyledons. 
Aquatic PLants 
The .contrast between the development of the me- 
chanical tissues in closely related forms of aquatic and 
terrestrial plants is very instructive. Aquatic plants 
-are of two kinds, those which are entirely submerged, 
and those with floating leaves. These two differ greatly 
in the character of the leaves, which in the former class 
are either narrowly linear or very much dissected so as to 
expose a maximum surface for the absorption of carbon 
dioxide. This is taken in directly by the superficial 
cells which, unlike those of most land plants, contain 
chlorophyll and have thin outer walls which allow of 
free interchange of fluids and gases. No cuticle is 
developed upon the epidermis, and the result of this is 
seen when these submersed aquatics are exposed to the 
air, where they wither up almost instantly, owing to 
the rapid evaporation of the water from them. Where 
the leaves float upon the surface, as in the various 
water-lilies, they are always broadly expanded and 
