78 ESSENTIALS OF BOTANY 
The medullary rays in the young shoot serve as a chan- 
nel for the transference of water and plant-food in a liquid 
form across the stem, and they often contain much stored 
food. 
The vessels carry water upward and (sometimes) air 
downward through the stem. 
The wood-cells of the heartwood are useful only to give 
stiffness to the stem. Those of the sapwood, in addition 
to this work, have to carry most of the water from the 
roots to the leaves and other distant portions of the plant. 
The cambium layer is the region in which the annual 
growth of the tree takes place. 
The most important portion of the inner bark is that 
which consists of sieve-tubes, for in these digested and 
elaborated plant-food is carried from the leaves toward 
the roots. 
The green layer of the bark in young shoots does much 
toward collecting nutrient substances, or raw materials, 
and preparing the food of the plant from air and water, 
but this work may be best explained in connection with 
the study of the leaf (Chapter XIII). 
93. Movement of Water in the Stem. — The student has 
already learned that large quantities of water are taken 
up by the roots of plants. 
Having become somewhat acquainted with the structure 
of the stem, he is now in a position to investigate the 
question as to how the various fluids, commonly known as 
sap, travel about in it.! It is important to notice that sap 
is by no means the same substance everywhere and at all 
times. As it first makes its way by osmotic action inward 
1See the paper on The So-called Sap of Trees and its Movements, by 
Professor Charles R. Barnes, Science, Vol. XXI, p. 535, 
