NUTRITION IN VEGETABLES. 78 
257. Capillary attraction his been supposed by many, and 
among others, by the late Sir Humphrey Davy, as the prin- 
cipal, if not the sole cause of this movement in the fluids of 
plants; but while it-is probable, that to a certain extent, this 
peculiar force may promote the motion of the fluids, it is, on 
the other hand, impossible to account for either the height 
to which the sap ascends, and the pressure it overcomes in 
its progress upwards, or the rapidity with which this move- 
ment takes place, according to the theory of capillary attrac- 
tion. From some experiments made by Monsieur Delon, it 
was ascertained that water in tubes full of sand, did not rise 
_ to the height of twenty-nine inches, till after the lapse of 
seven months. Some experiments of Hales established the 
fact, that the movement of the sap, in a vigorous branch of 
_ @ pear tree, is fully one inch per minute. 
258.. Recently, a novel and ingenious explanation of 
the motion of the fluids has been proposed by M. Dutro- 
chet. He has remarked, that when fluids of different den-_ 
sities are separated from each other by a membrane, a move- — 
ment ensues, the direction of which invariably depends on 
the position of the two fluids. If a bladder be filled with a 
solution of sugar in water, or mucilage, and a tube be closely 
connected with its neck, on being immersed in a vessel of 
water, the fluid will be observed shortly to ascend in the tube, 
distinctly showing that some of the water must have passed 
into the bladder. On changing the fluids, that is, put- 
ting the less dense, or the water, into the bladder, and the — 
solution of sugar, or gum in water, outside, the volume — = 
of fluid in the bladder gradually diminishes, the move- Se 
a ee 
the movement of the fluids inwards, the latter, pied 
The words are of Greek derivation, wer (endon, ° 
and 42, without). oa 
An me: facts ® he established the general a 
