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. 
