68 ELEMENTS OF BOTANY. 
The heartwood of a full-grown tree is hardly living, unless 
some of the medullary rays may retain their vitality, and so 
wood of this kind is useful to the tree mainly by the stiffness 
which it gives to the trunk and larger branches, thus prevent- 
ing them from being easily broken by storms. 
91. Movement of Water in the Stem.—The student has 
already learned ($ 50) that large quantities of water are taken 
up by the roots. 
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.t 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 water with a 
little mucilaginous or slimy material added. The sap which 
is obtained from maple trees in late winter or early spring, 
and is boiled down for syrup or sugar, is still richer in 
nutritious material than the water of the grapevine, while 
the elaborated sap which is sent so abundantly into the ear of 
corn, at its period of filling out, or into the growing pods of 
beans and peas, or into the rapidly forming acorn or the 
chestnut, contain great stores of food, suited to sustain plant 
or animal life. 
92. Experiment 18. Rise of Water in Exogenous Stems. —Cut 
some short branches from a grapevine and stand the lower end of each 
in red ink; try the same experiment with twigs of oak, ash, or other 
porous wood, and after some hours examine with the magnifying glass 
and with the microscope, using the two-inch objective, successive cross- 
sections of one or more twigs of each kind. Note exactly the portions 
1 See the paper on The so-called Sap of Trees and its Movements, by Prof. Chas 
Rk. Barnes, Science, X XI, 535. 
