270 EVOLUTION OF PLANTS 
usually almost circular in outline. Stomata are de- 
veloped upon the exposed surface whose outer cell- 
walls are also cutinized, but are absent from below. 
Many submersed aquatics have the roots imperfectly 
developed, serving merely as organs of attachment, or 
they may be quite absent, as in the common bladder- 
weed, Utricularia. As these plants absorb most of their 
Fie. 59. — A, two leaves of an 
aquatic buttercup (Ranun- 
culus Purshii); L, aerial 
leaf; w, submersed - leaf; 
B, twig of horse-chestnut, 
showing a winter-bud pro- 
tected by thick scales, sc; 
C, plant of pine-sap (Mono- 
tropa), a colorless sapro- 
piste with rudimentary 
eaves, sc. 
food from the substances dis- 
solved in the surrounding 
water, the roots are much less 
important than in plants whose 
upper members are exposed to 
the air. However, the devel- 
opment in rootless forms of 
special contrivances for pro- 
curing nitrogenous food, such 
as the traps of Utricularia 
(Fig. 58), would indicate that 
the roots, even of these sub- 
mersed forms, are still of im- 
portance in absorbing nitrog- 
enous compounds from the 
mud in which they are fast- 
ened. Where plants float 
upon the surface, like the 
duckweed (Lemna), or Sal- 
vinia, there may be either 
true roots developed, or root- 
like organs which replace 
them. 
Most aquatic plants are entirely free from hairs or 
scales, so that the surface is smooth. Exceptions to 
