STEM STRUCTURE 113 
vascular strands are arranged, not marginally, but in 
a central bundle, where they can best meet stresses of 
the kind. In most trees the stems are solid; here 
economy of material is less urgent, as a long period 
of years is available for their building up; the great 
amount of cell-space thus made available for food- 
storage is a valuable asset to the plant, as is evident 
from a consideration of the vast amount of fresh 
tissue produced in a brief period by a deciduous tree 
when it bursts into leaf. As this material, stored in 
the stems and roots, has to be sent up to the twigs 
dissolved in water, and as during the whole period of 
growth vast amounts of water are transpired, an 
elaborate and complete pipe-system is intercalated 
with the reinforced-concrete structure of the tree 
trunk. Pumped up by the roots, and sucked up by the 
leaves, water and food pass rapidly from the ground 
to the topmost twig of the loftiest tree. 
To explain the massiveness of a tree trunk we 
have to remember that, while the cross-section of any 
structure varies as the square of its linear dimension, 
the volume varies as the cube of the same. If we 
double the dimensions of a tree, we increase its weight 
eight times, but the strength of the trunk is increased 
only four times. If a tree 100 feet high is supported 
on a stem 6 feet in diameter, a tree 200 feet high of 
the same proportions would need a stem not 12 feet, 
but over 17 feet in diameter, to be supported equally 
efficiently. This proportion increases rapidly: a 
similar tree 300 feet high would need a stem 30 feet in 
diameter; a tree 1,000 feet high would require a stem 
180 feet in diameter, or 32,400 square feet in cross- 
section. We see, then, why a limit of tree growth is 
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