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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