CHAPTER XI 



RECONQUEST OF THE WATER 



When the plant world had finally reached the land and 

 had varied into orders, families, and species, which again 

 had suited themselves by experience and temperament 

 to such places as they found, then a new field of con- 

 quest presented itself. 



This was nothing less than a fresh occupation of the 

 water. 



On the borders of lakes where the ground is moist 

 and often supplied with fresh silt from the floods, the 

 competition and crowding amongst plants is often very 

 severe. 



Some of these bordering plants began to adapt them- 

 selves to a life below water. A great many even of our 

 ordinary herbaceous and shrubby forms are not in the 

 least injured by floods. 



By slow degrees certain natural orders or individual 

 species discovered how to live a purely aquatic life, and 

 became partly or wholly submerged. 



The first difficulty which they met with was the 

 obvious danger of being drowned. Plant cells, like 

 those of animals, require fresh oxygen for respiration, 

 and this was difficult to get when leaves and shoots 

 were plunged many feet below the surface. 



In all such water plants there is a complex system of 

 air-spaces and channels inside the stems and roots, and 

 often within the leaf also. This constitutes an internal 

 atmosphere and is, no doubt, supplied with oxygen from 

 that which is given off in assimilation. The origin of 



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