CHEMISTRY AND PHYSICS OF PLANTS. 15 
to this view, the force which raises the water, in some cases 
to the height of a hundred metres or more, is the attraction 
of the surfaces of the cellulose molecules for the layers of 
water which surround them. 
145. The rapidity of the upward movement of water 
varies greatly in different plants and under different con- 
ditions. In a silver-poplar a rate of 23 cm. (9 in.) an hour 
has been observed; in a cherry-laurel 101 cm. (40 in.); and 
in a sunflower 22 metres (72 feet). 
ADDITIONAL Notes on the movement of water in the plant. 
I. Root. Pressure.—tif the root of a vigorously growing plant be cut 
off near the surface of the ground and a glass tube attached to its 
upper end, the water of the root will be forced out, often to a con- 
siderable height. Hales more than a hundred and fifty years ago 
observed a pressure upon a mercurial gauge equal to 11 metres 
(36.5 ft.) of water when attached to the root of a vine (Vitis). Clark 
(1878), in a similar manner, found the pressure from a root of a birch 
(Betula lutea) to be equal to 25.8 metres (84.7 ft.) of water. This root- 
pressure appears to be greatest when the evaporation from the leaves 
is least; in fact, if the experiment is made while evaporation is very 
active, there is always for a while a considerable absorption of water 
by the cut end of the root, due probably to the fact that the cell-walls 
had been to acertain extent robbed of their water by the evaporation 
from above. Root-pressure is probably a purely physical phenom- 
enon, due to a kind of endosmotic action taking place in the root- 
cells. : 
II. The Flow of Water (sap) from the stems and branches of certain 
trees, notably from the sugar-maple, appears to be due to the quick 
alternate expansion and contraction of the air and other gases in the 
tissues from the quick changes of temperature. The water is torced 
out of openings in the stem when the temperature suddenly rises; 
when the temperature suddenly falls, as at night, there is a suction 
of water or air into the stem. When the temperature is nearly uni- 
.form, whether in winter or summer, there is no flow of sap. 
Ill. Wo Circulation of Sap —While there is an upward movement 
of the water in plants because of the evaporation from the leaves, 
there is no downward movement as has been popularly supposed. 
The ‘‘circulation of the sap,” in the seuse that there is an upward 
stream in one portion of the plant and a corresponding downward 
