HOW BIG TREES GROW 4] 
ually going on in the leaves. None of these answers is 
clearly proven, so it can only be said of the power of 
sap transmission that it is a function of the living eell. 
A plant or tree grows in height and diameter; and 
increase in these dimensions, of course, means increase 
in volume. Diameter growth is accomplished by the 
repeated division of the cambium layer (this layer has 
the property of adding a new ring each year) which 
lies between the sapwood and the inner bark, and as a 
consequence the tree has a ring for each year of its 
life. Height growth is dependent upon the vigor of 
the terminal bud which extends itself a certain amount 
each year depending upon the amount of energy 
stored up from the previous season. Thus growth is 
largely a matter of plant food and water with suf- 
ficient energy—light and heat—-to work over the raw 
food into starches and sugars. 
In a tree growth is from the outside and from the tip 
so that a nail driven into the trunk four feet from the 
ground will always remain the same distance from the 
ground, and the head will always be the same distance 
from the center of the tree. To picture how growth 
occurs, imagine a hollow cone (the annual ring) with a 
long tapering point (the latter being the height growth 
for the year) being placed on top of a series of such 
cones each year. Thus by sawing across a tree at any 
point we can count the rings and learn the age of the 
tree at that point. 
If severe drought occurs in midsummer the tree may 
cease to grow but start up under the influence of later 
rains. Such conditions give rise to “false rings,” but as 
a rule such rings are irregular and easily distinguished 
from the regular annual rings. Tropical trees do not 
grow by annual rings like the trees in the temperate 
