54 



COLLEGE BOTANY 



Heartwood and Sapwood. — As tree trunks increase in age 

 and thickness, the cells in the center become less and less active 

 and gradually die. Successive layers of cells die until we have 

 the large cylindrical core of dark-colored cells, which are kno^vn 

 as the heartwood. The fact that hollow trees live ajid thrive 

 proves that the heartwood is of little or no value to the tree, 

 except for support. The zone of lighter-colored wood j ust outside 



Pi 



Fig. 32.— Tendril 

 type of stem. 



Fig. 31. — Longitudinal section of stem showing 

 origin of branches; proper method of pruning (o) 

 and improper method of pruning (&). 



the heartwood is known as the sapwood. Both the sapwood aud 

 the heartwood contain a considerable amount of water. 



Branching of the Stem. — The new lateral growth, whether 

 shoot or leaf, originates in the primaiy meristem. The cells 

 divide rapidly and cause a slight elevation on the surface. 

 This is known as the endogenous origin and is the characteristic 

 method of forming new shoots (Fig. 31). It is quite different 

 from the origin of branches of the roots (page 68). Many of 

 these exogenous growing points (buds) become dormant and 

 completely overgrown bv the other tissues. They are the causes 



