HYDROPHYTE SOCIETIES. 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 rela- 

 tion 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 fav- 

 orable for the work of the flower than a submerged position. 

 Any society of this type will furnish abundant material for 

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

 society for study that has been mentioned so far. 



C. Swamp societies. 



135. Definition. In swamp societies 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 reduc- 

 tion 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. This 

 aeration of the root system reaches a very high develop- 

 ment in such swamp trees as the cypress. In cypress swamps 

 the so-called " knees" are abundant, and they are found to 

 be special growths from the root system, which rise above 

 the surface of the water, both for bracing and to admit air 

 to the roots (see Fig. 91). It has been shown that if such 

 swamps are flooded above the level of the knees, many of 

 the trees are killed. In ordinary cases the air is admitted 

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