NATURE OF PLANTS 



93 



drought, transplanting, and other factors that limit the annual 

 growth. They also show that the development of the plant is 

 subject to the same rhythm of growth as is seen in the animal. 



Fig. 58. Diagram of a three-j'ear-old stem of basswood cut so as to show 

 the structure in cross-section, C, in radial section, R, and in tangential section, 

 X; c, cortex; ck, cork; ph, phloem with darker bands of stereome; mc, medullary 

 ray in cro^-eection; mr, ray in radial section; mt, ray in tangential section; 

 I, lenticels; cam, cambium cylinder; p, pith. — H. O. Hanson. 



Namely the thickness of the rings increases yearlj' up to a certain 

 age, after which time there is a gradual retardation. So the 

 tree has its youth, maturity and old age. 



