58 A TEXT-BOOK OF BOTANY 
ascending water having stained its path. Of course the 
stain may spread somewhat into adjacent cells. 
In most trees, as the mass of wood increases in diameter, 
the ascending sap abandons the inner (older) wood and 
moves only through the newer wood. This results in a 
different appearance of the two regions, the old central 
wood, abandoned by the sap, becoming darker and often 
characteristically colored (heart wood); and the younger 
outer wood, used by the sap, being lighter colored (sap 
wood). Trees vary greatly in the relative thickness of the 
sap wood; for example, in the beech it is a thick zone, while 
in the oak it is a narrow one. In successful girdling this 
must be taken into account, since an incision which would 
cut off the water supply of an oak sufficiently to kill it 
would not kill a beech. 
The rate of movement of the ascending sap of course 
varies with different plants and different conditions. In 
the pumpkin-vine, in which the movement is very rapid, 
it has been found to reach about twenty feet an hour. It 
is estimated that in ordinary broad-leaved trees the rate is 
probably three to six feet an hour. 
If certain stems are cut off near the ground, it is ob- 
served that after a short time the sap begins to ooze out— 
a phenomenon that is often called bleeding. In some 
woody plants, as grape-vines and birches, the sap flows 
out with considerable force, indicating some pressure be- 
low, which is called root-pressure. While root-pressure may 
force the sap into the stem, it is entirely inadequate to 
force it to the top of a tree. 
The so-called maple sap obtained from the sugar- 
maple is an interesting illustration of the use of sap that 
accumulates in a woody stem in the spring. At that time 
the water has no opportunity to escape through leaf trans- 
piration; so the wood becomes gorged with sap, which can 
be drawn off by boring into the wood and inserting spiles. 
