52 BRITISH PLANTS 



orbicular floating leaves ; the arrowhead (Sagittaria 

 sagittifolia) has long ribbon - leaves under water, and 

 arrow-shaped leaves standing out above it ; the yellow 

 water-lUy {Nwphar luteum) in deep water forms long 

 ribbon-shaped leaves instead of the ordinary orbicular 

 floating ones. 



Dangers to which Aquatics are exposed. 



Most of the dangers to which xerophytes are exposed 

 may be summed up in two words — physiological drought. 

 Aquatics live in a medium which is all water, but, hke 

 xerophytes, aquatics have their physiological troubles 

 too, though of quite a different kind : 



1. Difficulty of Transpiration, — Being surrounded by 

 water, the submerged aquatic is able to absorb water 

 through the whole of its surface, but it cannot transpire. 

 The cells, however, can get rid of superfluous water by 

 allowing it to escape into the large air-cavities that 

 abound in the tissues. But if these happen to be filled 

 with water instead of air, the aeration of the plant is 

 rendered difficult, physiological disturbances of all kinds 

 are set up, and growth ceases. The aquatic whose air- 

 cavities are waterlogged soon dies. In the case of floating 

 aquatics, the upper surfaces of the leaves, in contact with 

 the air, bear stomata. Interchange of gases can, there- 

 fore, take place directly .between the atmosphere and 

 the plant, but transpiration is still difficult, because the 

 air immediately above the water is always moist. 



2. Scarcity of Air. — Land-plants are never troubled 

 with lack of oxygen for respiration ; it is all round them. 

 With water-plants it is different. Running water is 

 well aerated, and aquatics living in it never suffer from 

 lack of air to breathe. The danger of suffocation, how- 

 ever, is a real one in the case of plants growing in stag- 

 nant water which is strongly charged with carbonic acid 

 gas, but contains little or no dissolved oxygen. There 

 is a good deal of rotting material in stagnant water, and 

 what oxygen is absorbed is at once utilized for the decom- 

 position of this dead matter, and little is left for the 

 living. 



3. Light. — Light is considerably altered in its passage 

 through water. A great deal is lost by reflection upon 



