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, ag is the 
case with such plants ag grow in deserts or in rocky situa- 
tions, there is but little formation of new substance. 
