thin walled cells which constitute the pith rays. 
These function in the lateral conduction of water 
and solutes. And internal to the bundles is the 
pith. Thus the trunk and branches of the tree 
can be thought of as a rigid but growing water- 
proofed system of pipes, some carrying solutions 
upward, some downward. 
The heart wood of the stem, consisting chiefly 
of dead cells, commonly makes up the central 
and by far the greatest portion of older trees. 
Between this and the cambium lies the sap wood, 
through which the water and its solutes absorbed 
by the roots is conducted to the leaves. 
THE LEAVES 
The outer surfaces of the cells comprising the 
upper and lower external layers of leaves, the 
upper and lower epidermis, are commonly cov- 
ered with a thin waxy substance, the cutzn layer, 
which makes the epidermis nearly impermeable 
to water. In both the upper and lower epidermis, 
chiefly in the lower, however, minute pores are 
found. Through these pores gases can enter and 
escape. Carbon dioxide, necessary for photosyn- 
thesis, enters here and diffuses into the chlorophyll 
containing cells of the leaf mesophyll. Oxygen, 
required for respiration, also enters. Water can 
escape as water vapor and oxygen released as a 
by-product of photosynthesis through these pores. 
The solution enters the leaf through the vascu- 
lar system which extends into it to become the 
veins of the leaf. In the millions of cells of the 
many leaves devoted to food manufacture, the 
water in the presence of chlorophyll is combined 
with the carbon dioxide through the energy of 
the sun to produce a kind of sugar called glucose. 
The manufacture of glucose from water and the 
carbon dioxide always present in the air goes on 
4 
