56 LIFE OF A TREE. 
There appear to be two principal causes of this 
singular phenomenon—the circulation of the sap. 
One of them is, that the plant loses, by perspiring 
through its leaves, an immense quantity of fluid, 
which it can only obtain by sucking it up from 
the cells next to it; and these becoming empty, 
suck fluid from those below them; and so on, all 
through the stem, down to the very roots. The 
other is the singular phenomenon called Endos- 
mose. Suppose on one side of the tube shewn 
in the figure, we poured a measured quantity of 
water, and on the other side an equal quantity of 
milk, and separated the two fluids by stretching 
a thin piece of bladder across the tube, just in 
the bend at +. At first 
thought we should say 
that they would remain 
without mixing toge- 
ther... Butothe gone 
trary is the case; 
although there is an 
imperforate piece of 
animal membrane to separate them, yet, strangely 
enough, the water goes through and mixes with 
the milk, while little or no milk comes through 
ENDOSMOSE-TUBES. 
