THE TRANSPORT OF WATER IN THE PLANT 77 



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 nutri- 

 tive 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 con- 

 centration 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 

 such plants as grow in deserts or in rocky situations, there 

 is but little formation of new substance. 



