18 Characteristics, Structure, Life of Trees 
for next year’s growth all the way down to the root tips. 
The stored material is then used the folowing spring to form 
leaves and shoots and new rootlets. 
The whole mechanism of the tree can be compared to a 
sytsem of water works— water being the most important 
factor in the life of the plant — the root tips being the pumps 
or valves taking up water with minerals in solution from the 
soil, The roots and stem and branches are the conducting 
pipes; the leaves are the engines where the power is applied 
which sets the current in motion: namely, heat, light, wind, 
and other causes of transpiration (evaporation). 
In the leaves, which may also be regarded as the stomach 
of the plant, the raw food is digested and assimilated into 
a form in which it may be used by the plant, and thence it 
is carried by osmosis‘ to places where it is needed. 
It stands to reason that the amount of foliage which is 
active under the influence of light, determines the amount 
of assimilated food material which will be at the disposal of 
the tree and hence the amount of growth. The leaves and 
their healthy function are, therefore, of the highest im- 
portance to the tree; but just as important is the number 
of root tips capable of securing water and the necessary 
minerals. 
For best results the amount of active foliage and of active 
rootlets must be in direct proportion; and, indeed, there is 
such a close relation between the two, that if for some rea- 
son the normal amount of active leaf surface is reduced, as 
by insect injury, a corresponding amount of rootlets may 
die because not fed, unless a surplus of stored material is 
available. Even more surely, if the root system is in any 
way curtailed, as for instance when a cut for a street is made, 
1 Osmosis is the name for the process by which liquids are diffused 
through membranes. 
