ANNUAL-RINGS : KNOTS 



123 



present, and a similar addition is made to the bast on its inside. 

 The amount of wood produced by the cambium is always very 

 much greater than the bast. Moreover, the bast tissue consists 

 chiefly of thin-walled elements which become crushed into very 

 thin sheets by the pressure of the expanding wood and the re- 

 sistent bark, whereas the wood with its thick-walled cells and 

 vessels suffers little in this manner ; in transverse sections of the 



^T 



A— 



Fig. 57. — I. Piece of a stem of an ash tree. A, Portion three 

 years old ; £, portion two years old. 2. Longitudinal and 

 transverse sections of same. 



trunks and branches of trees and shrubs the cambium appears to 

 the naked eye to produce wood only. 



(i) Annual rings : knots. — If a tree is sawn across and the 

 cut surface then smoothed with a chisel a number of ring-like 

 zones are noticeable in the wood (Figs. 57 and 58); these are 



