178 
PLANT STUDIES 
whole plant body is exposed to the water supply, and there- 
fore absorption may take place through the whole surface 
rather than at any particular region such as the root. In 
order that this may be done, however, it is necessary for 
the epidermis to have thin walls, which is usually not the 
case in epidermis exposed 
to the air, where a certain 
amount of protection is 
needed in the way of 
thickening. 
(2) Roots much reduced 
or wanting. It must be 
evident that if water is 
being absorbed by the 
whole free surface of the 
plant, there is not so 
much need for a special 
root region for absorp- 
tion. Therefore, in such 
water plants the root sys- 
tem may be much re- 
duced, or may even disap- 
pear entirely. It is often 
retained, however, to act 
as a holdfast, rather than 
as an absorbent organ, for 
most water plants anchor 
themselves to some sup- 
port. 
(3) Reduction of water-conducting tissues. In the ordi- 
nary soil-related plants, not only is an absorbing root sys- 
tem necessary, but also a conducting system, to carry the 
water absorbed from the roots to the leaves and elsewhere. 
It has already been noted that this conducting system takes 
the form of woody strands. It is evident that if water 
is being absorbed by the whole surface of the plant, the 
FIG. 159. Fragment of a common seaweed 
(Fucus), showing the body with forking 
branching and bladder-like air cavities. 
After LUERSSEN. 
