HOW TREES GROW AND MULTIPLY 13 



give off their excess moisture through the 

 leaves and bark. Otherwise they would be- 

 come waterlogged during periods when the 

 water is rising rapidly from the roots. 



After the first year, trees grow by increasing 

 the thickness of the older buds. Increase in 

 height and density of crown cover is due to the 

 development of the younger twigs. New growth 

 on the tree is spread evenly between the wood 

 and bark over the entire body of the plant. This 

 process of wood production resembles a factory 

 enterprise in which three layers of material are 

 engaged. In the first two of these delicate tissues 

 the wood is actually made. The inner side of the 

 middle layer produces new wood while the outer 

 side grows bark. The third layer is responsible 

 for the production of the tough, outer bark. Year 

 after year new layers of wood are formed around 

 the first layers. This first layer finally develops 

 into heartwood, which, so far as growth is con- 

 cerned, is dead material. Its cells are blocked up 

 and prevent the flow of sap. It aids in support- 

 ing the tree. The living sapwood surrounds the 

 heartwood. Each year one ring of this sapwood 

 develops. This process of growth may continue 

 until the annual layers amount to 50 or 100, or 

 more, according to the life of the tree. 



