PHYSIOLOGY 
accompanied by an increase of the conducting tissues (see 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 sharp 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 
