346 PHYSIOLOGY 



accompanied by an increase of the conducting tissues (sec Part I, p. 243). 

 Annual thickening. — In trees and shrubs the xylem undergoes sec- 

 ondary thickening in the first season of growth, and this is resumed in 

 the second season, and so on, from the persistent cambium. Thus arises 

 a great cylinder of xylem, which constitutes .the wood of the trunk and 



Fig. 644. — Cross section of stem of Ricinus communis, showing ring of secondary 

 xylem; for description, see fig. 541. — From Part I. 



branches. In many trees the xylem formed in the course of the growing 

 season gradually changes its character. The first formed tissues con- 

 tain many large ducts and less mechanical tissue, while the later formed 

 xylem has small ducts and much mechanical tissue. In these cases the 

 open tissues produced in the spring abut on the denser ones last pro- 

 duced in the summer or autumn, and the shaxp contrast marks visibly 

 the periodicity in growth. As these differences in the tissue depend upon 

 growth, and as this is most affected by the annual seasonal changes, the 

 growth rings are usually annual rings, and make possible an estimate of 



