THE STRUGGLE FOR EXISTENCE 195 
HYDROPHYTE SOCIETIES. 
Speaking generally, true water plants have the 
following adaptations suitable to their environment. 
1. The epidermis is thin in all parts of the plant, 
for it is through this that the minerals and gases 
dissolved in the water are absorbed. 
2. The roots, not being indispensable as special 
and exclusive organs of water and mineral absorption, 
are absent, or, when present, form as a rule mere hold- 
fasts serving to anchor the plants to the bottom. Root 
hairs are generally absent. 
3. The vessels of the wood (1.e. the water con- 
ducting tissue), being no longer needed, since every 
part of the plant receives water direct, are much 
reduced or entirely wanting. 
4. The fibres and other hard tissues that give 
strength and firmness are generally lacking, for the 
water itself affords all the support that is needed. 
5. Large air spaces are developed in all parts of 
the organism, in the first place to float the plant, and 
in the next to conduct to the submerged tissues the air 
necessary for respiration. 
6. The submerged leaves are usually long and 
strap-shaped as in the vallisneria, or narrow and 
arranged in whorls as in the water-starworts, or 
divided into numerous fine threads as in the water 
milfoils. In this way, damage by running water is 
obviated. They have chlorophyll in their epidermis, 
and are without stomata. Leaves which float on the 
surface, like those of the water-lily and pond-weed, 
are usually entire and oval or round, and have their 
stomata on the upper surface. 
7. Water plants usually grow rapidly, branch 
freely, and reproduce enormously by vegetative means, 
usually by the decay of the older tissue setting the 
young shoots free, as in the pond-weeds and water-lily. 
