HOW BIG TREES GROW 41 



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 cell. 



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 



