374 
NATURAL HISTORY 
use of the pediluvium, and that singular symptom in diabetes, 
or the patient’s voiding a much greater quantity of urine 
than there is liquor taken in by the mouth, seem to confirm 
this assertion. 
Plants are possessed of a power of forming their different 
parts and this is done by secretion. We may conjecture 
what the agents are which produce this effect, but in respect 
to the manner of their operation we are entirely in the dark. 
In animals, where the vital power is strong, this is the princi- 
pal agent in producing the new arrangement of parts, which 
is made in every secretion ; but in plants, where this power 
is weaker, it would be unequal to perform the function, if it 
were not assisted by absorption and fermentation. Wherever 
any firm matter is to be secreted, the vessels have a convo- 
luted course, to allow the juice to be fermented, and the 
thinner parts to be absorbed. In this manner, the stones 
and kernels of fruits are supplied with nourishment by 
fibres, which are much convoluted. The proper juice seems 
to be formed only when the sap has ascended towards the 
leaves, and is descending towards the roots. 
The pabula, from which vegetables receive the matter of 
secretion, are contained in the surrounding elements. 
Some botanists have conceived, that plants, as well as 
animals, have a regular circulation of their fluids. Others 
think this very improbable. On both sides, recourse has 
been had to experiments; and from these, conclusions per- 
fectly opposite have been deduced. When a ligature has 
been fixed round a tree, in such a manner that no juice could 
be transmitted through the bark, the tree has been found to 
thicken above the ligature; but below it to continue of the 
same circumference. Hence, some have concluded, that the 
sap ascends through the wood, and descends through the 
bark. Those who are of a contrary opinion have found 
that, in certain cases, the juice ascends through the bark only : 
for when a portion of the wood has been cut out, and the bark 
exactly replaced, the growth of the tree has been found to go 
on unchanged: hence it is said, that the juice is transmitted 
equally through all parts of vegetables. The analogy of ani- 
mal nature appears to favour the opinion, that thejuice rises 
through the wood only, and descends only through the bark : 
but this analogy is not complete throughout. The arteries 
are not placed in the internal parts alone, nor the veins in 
the external, but they accompany each other through every 
other part of their distribution. 
In vegetables, the sap rises from the roots but the proper 
juice descends, towards them : in the descent of thejuice, the 
