RESPONSE OF THE PLANT TO ITS SURROUNDINGS 281 
Drawing their nourishment from the loose soil in which they 
are anchored, and lacking the support of a liquid medium, 
they develop roots and vascular stems. The roots of plants 
growing in swamps have difficulty in obtaining proper aér- 
ation on account of the water, which shuts off the air from 
them ; hence they are furnished with large air cavities, and 
the bases of the stems are often greatly enlarged, as in the 
Ogeechee lime (Nyssa capitata) and cypress, to give room 
for the formation of air passages. The peculiar hollow pro- 
fp S Lacie Z Pai St 
Fic. 418. — A Southern cypress swamp, showing on the left the peculiar enlarge- 
mentsfor aération, known as “‘ cypress knees.’’ (From Mo. Botanical Garden Rep’t.) 
jections known as “ cypress knees”’ are arrangements for 
aérating the roots of these trees. 
320. Xerophyte societies are adpated to conditions the 
reverse of those affected by hydrophytes. The extreme of 
these conditions is presented by regions of perennial drought, 
like our Western arid plains and the great deserts of the in- 
terior of Asia and Africa. Under these conditions plants 
have two problems to solve, — to collect all the moisture they 
can and to keep it as long as they can. Hence, plants of 
such regions have a diminished evaporating surface, owing 
to the absence of foliage and the compacting of their tissues 
