LIVING PARTS OF THE STEM 107 
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 (Figs. 69, 71). 
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 XI). 
117. 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 how the various fluids, commonly known as sap, 
travel about in it.1 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 
through the root-hairs of the growing plant it differs but 
little from ordinary spring water or well water. The 
liquid which flows from the cut stem of a “bleeding” 
grapevine which has been pruned just before the buds 
have begun to burst in the spring, is mainly water with a 
little dissolved mucilaginous material. The sap which is 
1 See the paper on “‘ The So-called Sap of Trees and its Movements,” by 
Professor Charles R. Barnes, Science, Vol. XXI, p. 535. : 
