52 COMPOUND ORGANS OF PLANTS. 



duramen or heartwood, which ultimately decays and falls away 

 leaving the interior of the stem rotten and hollow. These 

 changes in the external and internal appearance of the stem 

 are the necessary results of the following peculiarities of its 

 growth. 



We have seen that one layer of bark and one layer of wood 

 is annually deposited from the viscid mucilaginous matter 

 called cambium, which makes its appearance between the bark 

 and the wood in spring. It follows, that the number of annual 

 layers or rings of bark ought to correspond to the number of 

 annual layers or rings of wood. Sometimes in the bark of 

 young shoots of two or three years growth these annual 

 deposits may be traced, but in general the successive layers of 

 bark are so amalgamated by the internal growth and conse- 

 quent pressure of new strata of bark, that it is impossible to 

 distinguish them. To the same cause is to be attributed the 

 fissuring and exfoliation of the outer layers of bark. The 

 diameter of the wood is a constantly increasing quantity, 

 because the growth of the wood is exogenous, each new layer 

 of wood being deposited on the outside of the last annual layer, 

 and therefore each ring of the wood remains unaltered in its 

 dimensions and position until it finally decays ; on the other 

 hand, an increase in the diameter of the bark is constantly 

 prevented by the endogenous growth of the bark, each new 

 layer of bark being deposited on the inside of the last annual 

 layer ; and as new layers of bark are deposited internally, the 

 previous annual layers are subjected to gradual but incessant 

 distention, and finally unable to bear the stretch, are fissured 

 and torn into clefts and rents, causing that cracked and rugged 

 appearance of the bark of trees with which all are familiar. 

 Hence it is that on the cross-section the bark bears but a 



