52 BRITISH PLANTS 
orbicular floating leaves; the arrowhead (Sagittarva 
sagittifolia) has long ribbon-leaves under water, and 
arrow-shaped leaves standing out above it; the yellow 
water-lily (Nuphar lutewm) in deep water forms long 
ribbon-shaped leaves instead of the ordinary orbicular 
floating ones. 
Dangers to which Aquaties 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, like 
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. Searcity 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 
