418 THE ECOLOGY OF PLANTS. 



water or in mud where very little oxygen is present for 

 respiration. 



Water plants are subject to less extremes of heat and cold 

 than are land plants, since, owing to its high specific and 

 latent heats, water takes longer to be heated and longer to 

 cool than soil does. As regards nutrition, water plants are 

 well provided with carbon dioxide, since this gas dissolves 

 very readily in water; water at 15 C. dissolves about its 

 own volume of this gas, which is present in such a small 

 proportion (about 4 parts by volume in 10,000) in the atmo- 

 sphere. 



On the other hand, the proportion of oxygen dissolved 

 in water (it need hardly be pointed out that aquatic 

 plants and animals do not get their oxygen for respiration 

 by the splitting-up of water into its elements) is much 

 smaller than that present in the atmosphere. The lower 

 parts of an ordinary rooted water plant is poorly supplied 

 with oxygen, and the same is true of a marsh plant, i.e. a 

 plant having its leaves in the air but its roots and shoot-bases 

 in water o~ mud. The water or mud at the bottom is poor 

 in oxygen as compared with the surface water, and still or 

 sluggish water contains less oxygen than running water. 

 The air-spaces in the leaves, leaf-stalks, stems, and roots 

 serve to convey air to the badly aerated lower parts, and 

 their primary importance for aeration is shown by their large 

 development in marsh plants. 



Since a submerged plant gets salts and carbon dioxide so 

 easily, and lives in very favourable circumstances generally, 

 it grows rapidly, branches freely (the branches are usually 

 about as thick as the main axis itself), and reproduces itself 

 largely by vegetative means, chiefly by the decay of the older 

 parts and the setting-free of the younger branches. 



The water plants of tropical regions grow continuously all 

 the year round, not being hampered by a cold season, but in the 

 temperate regions, where growth is interrupted by the winter, 

 the water plants usually have some method of perennation 

 (nearly all water plants are perennial). In some cases the 

 plant remains unaltered and sinks to the bottom (e.g. Calli- 

 triche) ; in Water-lilies food is stored in the thick rhizome ; 

 in Arrowhead tubers are formed. A very common method 

 of perennation is the formation of winter buds, which are 



