the} ste}m and THE} branched 
23 
of the rings means a year’s growth. Now, this inside 
layer which the cambium grows is the sapwood, the part 
of the trunk where the sap, or plant food, flows up and 
down to the leaves and roots. At first this new layer is 
tender and spongy so that the sap can move freely; at 
this early time it is called Spring Wood. As the summer 
advances the Spring Wood becomes harder and firmer; 
then it is called Summer Wood. After a long, long time, 
perhaps when the tree is thirty or even seventy-five years 
old, the pores—invisible holes and ducts through which 
the sap flowed—fill up with gums or other solid matter. 
Then, when the sap can no longer flow through it the sap- 
wood is called Heartwood; new sapwood has grown all 
around and outside it, and this old remaining heartwood 
is the pith of the tree. 
We do not entirely understand the use of this tree 
pith, we do not know exactly what part it plays in the 
busy life of the tree. The pith can dry and rot away 
until the inside of the tree is entirely hollow, like a 
hollow pillar, yet the tree lives and grows. When a 
tree is diseased men can cut out great chunks of the rot¬ 
ting wood and fill up the cavity with cement, just as the 
dentist can fill your tooth with cement and not injure 
the living tissue which surrounds the filling. 
If you will look once more at the picture of the sawed 
log you will see some lines going out from the center like 
