LIBRARY 



CHAPTER XIV 



WATER 



Water is the most important single factor in the environment of 

 plants. The amount needed by different kinds of plants varies 

 greatly but some is necessary for every living organism. It is of vital 

 importance to the plant in several ways. For example, it is a neces- 

 sary constituent of all protoplasm as well as of cell sap. It is a food 

 material necessary in carbohydrate synthesis. All inorganic food 

 materials are absorbed and transported in a water solution. The 

 resistance of a plant to heat or cold is largely determined by the 

 amount of water present. Finally, water has more influence on the 

 external and internal structure of plants than any other factor. 



Several phases of the water factor have been discussed in pre- 

 ceding chapters. There are a number of other facts about the water 

 relation, however, that it is necessary for us to understand and these 

 will be taken up in the present chapter. 



102. The Classification of Plants on the Basis of Water Relation. 

 —Plants are found growing in all sorts of conditions with respect 

 to water supply. Some live on rocks where the amount of water 

 available is extremely limited, others are immersed in water w^here 

 the supply is inexhaustible, while still others are found in all degrees 

 of gradation between these extremes. Those plants that can get 

 along with small amounts of water are called xerophytes. Those 

 that can endure large amounts of water are called hydrophytes. 

 Those that get along best with medium amounts of water are called 

 mesophytes. The majority of plants are readily classified into one 

 of these three groups. There are, however, large numbers that are 

 intermediate between two groups and rather difficult to classify 

 satisfactorily. For this reason we frequently find it convenient to 

 use such terms as xero-mesophyte, to indicate a plant that is some- 

 what more xeric (characterized by scanty moisture supply) than a 

 typical mesophyte, meso-zerophyte, for one that is somewhat less 

 xeric than a typical xerophyte; hydro-mesophyte, for one that is 

 somewhat more hydric (characterized by abundant moisture supply) 

 than a typical mesophyte, etc. It is also customary to use such 

 combinations as shade-mesophyte, sun-hydrophyte, alpine-xero- 

 phyte, etc., in order to more accurately define the type of plant 

 under discussion. 



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