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 



