Scientists BELIEVE that a tree is like a machine, 
the laws for whose operation can be explained by 
known or knowable laws. Many non-scientists 
believe that there is more to a tree than an operat- 
ing machine and that its activities, directed by 
some unexplained principle of life, cannot ever 
be completely known or explained. Whatever the 
viewpoint, all agree that a tree is an extraordinarily 
efficient organism, manufacturing its own food 
from raw materials acquired by itself and thus 
with this food being able to supply the heat and 
substance necessary for its life, for growth, for 
the replacement of dead cells, and the healing of 
injuries caused by forces external to it. What can 
be said in a short space to describe this organism? 
To say as much as is known about a tree would 
take perhaps a hundred large volumes. And those 
hundred volumes would not describe it fully, for 
much about the processes of a tree are unknown. 
A general description, however, of an idealized 
tree can be attempted. The best procedure is to 
follow the fluids in the circulation system to and 
from the various specialized parts of the tree, 
describing in turn the function of each part. 
Disease of a tree represents disorder and de- 
ranged procedure so there is no attempt here to 
prove disease as resulting from an entity or from 
a causal agent. 
LIFE PROCESSES OF THE TREE 
THE THREE PARTS OF A TREE 
A tree has three main parts: the roots; the stems 
(i.e, trunk, branches, twigs); and the leaves. 
(Most trees also produce flowers and fruits.) 
Going through all three of these parts is a system 
of elongated cells called the vascular bundles 
which connect and carry materials to and from 
the parts. The vascular system is composed of 
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