120 PLANT LIFE 
though only indirectly, by the environment, 
whereby the loss of water is limited when 
the moneywort is flourishing in dry soil. 
Most plants do not share this faculty of 
quickly altering their chemical processes so 
as to become adapted to so wide a range of 
conditions. 
The most striking character common to all 
the higher water plants consists in the enormous 
development of intercellular spaces (Fig. 17). 
These air-spaces, communicating finally with 
the atmosphere by the stomata, represent 
an exaggerated development of an aerating 
system that occurs in every land plant. The 
aquatics have not, in this respect, acquired 
anything new, they have merely enlarged 
and often specialised, what was already an 
ancestral trait. Such an aerating system 
sharply marks off the higher water plants 
from the lower ones. In the larger seaweeds 
it is true that there is often a localised forma- 
tion of air cavities. These, however, serve 
rather as floating organs than for the 
general purposes of respiration and gaseous 
exchange generally. 
The remarkable congeries of trees and 
shrubs that make up a mangrove swamp in 
the inlets and estuaries on tropical coasts 
furnish striking examples of specialised aerat- 
ing systems. It is the roots which run 
through the mud of the swamp that are 
principally affected by the urgent need of 
free oxygen. After the root of a mangrove 
