272
Book IV,
VEGETATION.
tube is diminished beyond a certain size, water cannot
ascend in it, because its particles are now larger than
the bore of the tube. Consequently the rise of water
in capillary tubes must have a limit : if they exceed a
certain length, how small soever their bore may be, water ¬
will either not rise to the top of them, or it will not
enter them at all. We have no method of ascertaining
the precise height to which water would rise in a capillary ¬
tube, whose bore is just large enough tó admit a
single particle of water. Therefore we do not know
the limit of the height to which water may be raised
by capillary attraction. But whenever the bore is diminished ¬
beyond a certain size, the quantity of water
which rises in it is too small to be sensible. We can
easily ascertain the height which water cannot exceed
in capillary tubes before this happens ; and if any person ¬
calculate, he will find that this height is not nearly
equal to the length of the sap vessels of many plants.
But besides all this, we see in many plants very long
sap vessels, of a diameter too large for a liquid to rise
in them a single foot by capillary attraction, and yet the
sap rises in them to very great heights.
If any person says that the sap vessels, of plants gradually ¬
diminish in diameter as they ascend ; and that,
in consequence of this contrivance, they act precisely
as an indefinite number of capillary tubes, one standing
upon another, the inferior serving as a reservoir for
the superior—I answer, that the sap may ascend by
that means to a considerable height ; but certainly not
in any greater quantity than it the whole sap vessel had
been precisely of the bore of its upper extremity; for
the quantity of sap raised must depend upon the bore
of the upper extremity, because it must all pass through
that extremity.
