NUTRITION IN VEGETABLES. 75 
262. Decandolle is disposed to consider that this upward 
ovement of the sap takes place chiefly through the inter- 
cellular passages of the plant; a proposition which he con- 
-eeives in a great measure supported by the generally admitted 
fact of the lateral diffusion of the sap, which must necessa- 
_rily be independent of the vascular tissue. 
263. Experiments have been made by Bonnet and Hales 
determine the velocity and force with which the sap moves 
pwards. The former etiolated some plants, by growing 
them in a dark cellar, in order that the movement of coloured 
fluids might beeasily seen, and he noticed the coloured fluid 
to ascend in some of the experiments at the rate of two inches 
in the hour, and in others at the rate of three inches in the 
same time. When a fresh cut branch of a healthy pear 
_tree was immersed in a tube full of water, Hales found that 
the sap rose in it eight inches in six minutes. 
264. The force with which the sap ascends is considerable. 
This was determined by a very ingenious experiment of 
Hales: he cut off the upper portion of a vine branch, and 
enclosed the wounded surface of the lower portion in a tube. 
The force with which the sap ascended enabled it to rise to 
the height of forty-three feet, which is equivalent to a force 
eapable of supporting the weight of an atmosphere and a 
half. 
265. The greater part of the sap ascends through the al- 
burnum to the full grown leaves, which afford it an outlet 
evaporating part of it, and sending the rest through the 
proper vessels to the bark, thus consuming rapidly what they 
receive. The amount of the ascending fluid which is thrown 
varies considerably under different conditions of the plant 
well as of the atmosphere. The usual calculation is, that — 
are thrown off by the leaves, while the rest descends to nou-_ 
h the plant, and of that a proportion is also rejected by — 
Pr sar Lala Gos oe 
average quantity of water exhaled is to that which is ab- g 
bed, in the ratio of two to three ; that isto say, two-thirds _ ae 
