THE STEM 49 
for their work ; (3) to conduct to the rest of the plant the 
food materials prepared by the leaves. A stem, together 
with the leaves it bears, is called a shoot. 
How Stems are thickened. The stems of many 
plants have to support an enormous weight of leaves and 
branches as well as to withstand the force of the wind. It 
is necessary therefore that they should grow thick and 
strong ; the strengthening is usually brought about by the 
walls of the water vessels changing into wood. 
In woody stems the growth takes place in a soft layer 
of material between the wood and the bark known as 
cambium. The production of new wood is most rapid in 
summer when the leaves are manufacturing plenty of food. 
As autumn approaches, the activity of growth becomes 
less, and wood of a closer texture is formed. In winter 
the process of growth is suspended ; but with the warmth 
and moisture of spring bringing an abundant supply of 
food it is again renewed. 
The difference in texture between the summer and 
autumn wood is in many trees plainly visible, and gives 
rise to a number of ring-like markings known as annual 
rings, each of which in general represents a year’s growth. 
In forest trees the pith becomes absorbed, and the stem 
practically consists of older and younger wood and bark. 
The old or heart wood, dark in colour, is really dead, i.c., 
except as a mechanical support it is of no use to the tree. 
The younger or sap wood contains the food channels 
through which the salt solutions pass to the leaves... (See 
Figs. 38, 39, 40, p. 50.) 
The Bark. The bark on a young stem is thin, per- 
forated by minute pores (stomata), and contains chlorophyll ; 
hence it is able to absorb and decompose carbon-dioxide. 
As the tree grows older, a layer of cork forms on the inner 
surface of the bark, and cutting off food supplies from the 
D 
