TREE PHYSIOLOGY 43 



unfold their miniature shoots. Sometimes the 

 scales grow along with the buds to protect them for 

 a while. Eventually they always fall. 



A cross-section of the trunk of a mature tree 

 is very simple in arrangement. At the very centre 

 is a very small amount of pith which radiates out 

 in all directions as the medullary rays. These pith- 

 rays are serviceable in the movement of sap, both 

 longitudinally and laterally. Surrounding the cen- 

 tral pith are concentric rays of heartwood, corre- 

 sponding in number to the tree's age and compos- 

 ing the larger part of its mass. Life is extinct in 

 its cells. For a tree to be "rotten at the core" is 

 really not a vital thing. The central wood is dead 

 already. Only when the decay approaches the liv- 

 ing cells near the circumference does it become 

 alarming. 



Surrounding the heartwood is a region of sap- 

 wood, comparatively new cells which are gradually 

 solidifying into heartwood. Next is the active cam- 

 bium layer, which we have already discussed at 

 length. Surrounding everything is the bark. 



This stem formation extends throughout the tree 

 except in the smallest twigs where the area of pith 

 is greatly increased. 



In one aspect, a tree is a great community of 

 little plants. Instead of the polypi of the coral, 



