822 FORCE OF THE ASCENT OF THE SAP. 
The force of the ascent of the sap was measured by Hales 
in the stem of the Vine by an apparatus, a modified and im- 
proved form of which is represented in fig. 1150, where is shown 
a vine stock, to the transverse section of which is attached a 
glass tube R, and the tube r fixed into it by the cork k. Fis 
completely filled with water, the upper cork k’ then fixed firmly 
into it, and mercury poured into the tube 
r, so as to stand from the first higher at q’ 
than atq. The bent tube being filled with 
mercury to the level q’ at the commence- 
ment of the experiment, the force of the 
sap was readily calculated by the fall of 
the mercury in one leg of the tube q, and 
its corresponding rise above q in the other 
leg. In this way he found, that in one 
experiment the force of the ascent was 
suticient to support a column of mercury 
324 inches in height. He also calculated 
from his experiments on the Vine, that 
the force with which it rises in this plant 
is nearly five times greater than that of 
the blood in the crural artery of a horse, 
and seven times greater than that of the 
blood in the same artery of a dog. In 
some experiments of Brucke on the force 
of the ascent of the sap in the spring in 
the Vine, he found that it was equal to the 
support of a column of mercury 173 inches 
high. Hales’ experiment is, however, a 
measure of the force of absorption by the 
root (root-pressiie), rather than of ascent 
of the sap (see pages 791 and 821). 
Fig. 1150. Apparatus for the As the fluid rises in the stem it is of 
ree ara ic a a a watery nature, and contains dissolved 
in it- the various inorganic matters in the 
same state nearly in which they were absorbed by the roots. It 
also contains some organic substances which it has dissolved 
in its course upwards. Thus an analysis by Attfield of the 
spring sap from a ‘bleeding’ white birch tree, showed that it 
‘consisted of 99 parts of pure water with 1 part of dissolved 
solid matter; eleven-twelfths of the latter being sugar. But 
although the sap in its passage upwards thus becomes more and 
more altered from the state in which it was absorbed by the roots, 
when it reaches the leaves it is still quite unfitted for the require- 
ments of the plant, and is hence called Crude Sap. It undergoes 
certain changes in the leaves and other green parts, by which it 
becomes altered in several particulars, and is then adapted for 
the uses of the plant. In this state it is termed Elaborated Sap. 
B. Changes of the Crude Sap in the Leaves.—The changes 
Fic. 1150. 
