82 VEGETABLE PHYSIOLOGY 



which they contain is largely dependent upon another 

 feature of the water supply to which attention has already 

 been called. A considerable part of the material from 

 which the food of the plant is constructed is absorbed from 

 the soil in solution in the water, and is transported by 

 means of this stream to the regions of cell-formation. The 

 fact that the quantity of the nutritive salts in the water is 

 extremely small is a further reason for the transport of 

 such large quantities of water as pass through the plant ; 

 for by the gradual concentration of the solution in the cells 

 of the leaf enough new material can be obtained by the 

 protoplasts for the construction of the food necessary for 

 their nutrition, growth, and multiplication. Where there 

 is a large flow of water, as in a tree, there is a continuous 

 formation of new cells and of the various mechanisms their 

 life demands ; where the transpiration is but slight, as in a 

 Cactus, or where the supply of water is limited, as is the 

 case with jsuch plants as grow in deserts or in rocky situa- 

 tions, there is but little formation of new substance. 



