CHAPTER XXII 
HYDROPHYTES 
192. Adaptations.—When a plant lives entirely or par- 
tially submerged in water, its structure differs in many 
ways from that of an ordinary land plant, and these ad- 
justments to’ water life are called adaptations. On parts 
under water the epidermis is thin and permits absorption, 
so that in a completely submerged plant its whole surface 
absorbs. When this is the case, the root-system is much 
reduced in extent as compared with a land-plant of the 
same size, for it is not the only organ for water absorption. 
In submerged plants the rigid tissues are less developed 
than in land plants, for the buoyant power of water helps 
to support the plant. This fact may be illustrated by taking 
from the water submerged plants that seem to be upright, 
with all their parts spread out; upon removal they collapse, 
not being able to support themselves. Water-plants are 
also usually provided with air-chambers and passageways 
that the air may be free to reach the working cells. 
A few of the most characteristic hydrophytic associa- 
tions are given as illustrations, some of which occur in 
almost every neighborhood. 
193. Pondweed associations.—Water-lilies and pond- 
weeds are conspicuous members of these associations, the 
former with floating leaves (pads) (Fig. 301), the latter 
often entirely submerged. Associated with them are nu- 
merous other forms with floating or submerged leaves. 
The plants are anchored by their roots and rootstocks in 
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