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 



