HYDKOPHYTE ASSOCIATIONS. 183 



the corresponding change of form ; on the same individual 

 the submerged leaves are very narrow, or divided into very 

 narrow lobes, while the floating ones are broad (see Fig. 

 162). The relation of the plant to the water, therefore, has 

 determined the leaf form. The advantage of the floating 

 habit of leaves is not merely a better relation to light, but 

 the carbon dioxide used in photosynthesis and the oxygen 

 used in respiration may be obtained freely from the air, 

 rather than from the water. It will also be noticed that 

 these water plants usually send their flowers to the surface, 

 indicating that such a position is more favorable for the 

 work of the flower than a submerged position. Any asso- 

 ciation of this type will furnish abundant material for 

 observation, and it is, perhaps, the most valuable type of 

 association for study that has been mentioned so far. 



C. Swamp associations. 



130. Definition. — In swamp associations the plants are 

 rooted in water, or in soils rich in water, but the stems 

 bearing the leaves rise above the surface. Among the 

 hydrophytes, swamp plants are least exposed to water, and 

 as the stem and its leaves are exposed to the air, there is 

 no such reduction of the root system and of conducting 

 and mechanical tissues as in the other hydrophytes. Also 

 the epidermis is not thin, and there is no development of 

 floats to increase the buoyancy. However, the root must 

 be aerated, and hence air chambers and passageways are 

 abundant. In ordinary cases the air is admitted through 

 openings in the epidermis of the stem and leaves, and so 

 enters the air-passageways that are continuous to the roots. 

 It has been claimed that a still more elaborate arrangement 

 for root-aeration exists in the so-called " knees " of cypress 

 swamps, which are special growths from the submerged 

 root system and rise above the surface of the water (see 

 Fig. 91). It has been shown that if such swamps are 

 flooded above the level of the knees many of the trees are 



