4 



Water 



Water plays several different roles in the ecological relations of plants 

 and animals. In Chapter 2 we considered the physical characteristics 

 of water in its mechanical action as a medium. In this chapter we 

 shall discuss water as a substance taking part in the living complex, 

 and in subsequent chapters we shall see how water may modify the 

 action of other factors. 



As a material entering the organism water is important as a neces- 

 sary and abundant constituent of protoplasm, and the plant or animal 

 body as a whole generally contains a large percentage of water- 

 sometimes 90 per cent or more. Water is essential also as one mate- 

 rial taking part in the photosynthetic reaction, through which energy 

 becomes available either directly or indirectly to all living beings. 

 Water is necessary as a solvent for food and as an agent for the chem- 

 ical transformation of the materials within the body. Plants obtain 

 mineral nutrients from the soil after they are in aqueous solution. 

 Although most animals take in their food in solid form, this material 

 must be dissolved before it can be absorbed by the blood and tissues. 

 The intestines could be stuffed full of solid food and yet the animal 

 would starve if no water were available for its digestion. Further- 

 more, water serves as a vehicle for transport or circulation within the 

 bodies of organisms. Minerals are carried up the stem of the plant 

 by the transpiration stream. Water is the principal constituent of 

 the circulatory, excretory, and reproductive fluids of animals, and it 

 is necessary as a transfer agent at respiratory and olfactory surfaces. 

 Water also acts as a regulator of temperature for plants and animals. 



For these essential purposes the proper concentration of water must 

 be maintained inside the organism, and, at the same time, the transfer 

 of water must be suitably regulated. If the organism could live 

 hermetically sealed in a capsul, the retention of the necessary amount 

 of water must be suitably regulated. If the organism could live 



90 



