ANATOMY OF TISSUES. co 
sections, meeting at the centre. These are bounded by the 
medullary rays and contain the remaining elements of the wood. 
This xylem or wood portion also appears divided into as many 
separate zones as the stem has years of growth. These are com- 
monly called year’s rings, as, by a normal development, one is 
formed each year. The year’s rings are a constant feature of 
all woody stems in which there is a cessation of activity during 
Fig. 56. 
Cross-section of oak and elm wood showing arrangement of wood parenchyma 
and other elements. 
the winter, the limits of each summer’s growth being marked 
more or less distinctly by a change in the tissues produced. 
This change may be either in the kind of elements generated 
or in the different character of the same kinds. In the wood of 
most dicotyledons, the cells formed in the fall are thicker-walled 
with shortened radial diameter, while the spring wood is much 
richer in ducts, and the ducts have a greater diameter than those 
produced later in the season. In Conifers, which produce no 
secondary ducts, the years’ rings are distinctly marked by the 
thickened walls and shortened radial diameter of the fall cells. 
