HYDROPHYTE ASSOCIATIONS. 



171 



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. 



(2l) 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. 153. Fragment of a common seaweed 

 (Fucw), showing the body with forking 

 branching and bladder-like air cavities. — 

 After LujWJMU. 



