222 
THE CABINET OF NATURAL HISTORY. 
We have now seen that the growth of plants, and their 
increase in size, depend upon a peculiar internal movement, 
acting between the leaves and the roots. But in what way- 
does it operate? This is a problem which has exercised the 
ingenuity of all students of vegetable physiology, who have 
contrived theories innumerable to explain the phenomenon 
which is called the circulation of the sap. 
The great and almost impenetrable obscurity in which 
this subject is unavoidably involved, has occasioned much 
diversity of opinion among phytologists. Grew states two 
hypothesis, which he seems to have entertained at different 
periods, though it is not quite certain to which of them he 
finally gave the preference. In one of them he attributes 
the ascent of the sap to its volatile and magnetic nature, 
aided by the agency of fermentation ; but this hypothesis 
is by much too fanciful to bear the test of serious investiga- 
tion. In the other he attributes the entrance and first stage 
of the sap’s ascent to the agency of capillary attraction, and 
accounts for its progress as follows: the portion of the tube 
that is now swelled with sap, being surrounded with the 
vesiculse of the parenchyma, swelled also with sap, which 
they have taken up by suction or filtration, is consequently 
so compressed, that the sap therein is forced upwards a 
second stage, and so on till it reaches the summit of the 
plants. But, if the vesiculse of the parenchyma receive 
their moisture only by suction or filtration, it is plain that 
there is a stage of ascent beyond which they cannot be thus 
moistened, and cannot, consequently, act any longer upon 
the longitudinal tubes. The supposed cause, therefore, is 
inadequate to the production of the effect. 
Malpighi was of opinion that the sap ascends by means 
of the contraction and dilatation of the air contained in the 
air vessels. This supposition is perhaps somewhat more 
plausible than either of Grew’s; but, in order to render the 
cause efficient, it was necessary that the tubes should be 
furnished with valves, which were accordingly supposed; 
but of which the existence has been totally disproved by 
succeeding phytologists. If the stem or branch of a plant 
is cut transversely, in the bleeding season, it will bleed a 
little from above as well as from below: and if the stem of 
any species of spurge is cut in two, a milky juice will exude 
from both sections in almost any season of the year. Also 
if a plant is inverted, the stem will become a root, and the 
root a stem and branches, the sap ascending equally well 
in a contrary direction through the same vessels; as may 
readily be proved by planting a willow twig in an inverted 
position. But these facts are totally incompatible with the 
existence of valves ; and the opinion of Malpighi is conse- 
quently proved to be groundless. 
The next hypothesis is that of M. Be la Hire, who seems 
to have attempted to account for the phenomenon by com- 
bining together the theories of Grew and Malpighi. Be- 
lieving that the absorption of the sap was occasioned by the 
spongy parenchyma, which envelopes the longitudinal tubes, 
he tried to illustrate the subject by means of the experiment 
of making water to ascend in coarse paper, which it did 
readily to the height of six inches, and by particular man- 
agement even to the height of eighteen inches. But, in 
order to complete the theory, valves were also found to be 
necessary, and were accordingly summoned to its aid. The 
sap which was thus absorbed by the root, was supposed to 
ascend through the woody fibre, by the force of suction, to 
a certain height ; that is, till it got above the first set of 
valves, which prevented its return backwards; when it was 
again supposed to be attracted as far before, till it got to the 
second set of valves, and so on till it got to the top of the 
plant. 
This theory was afterwards adopted by Borelli, who en- 
deavoured to render it more perfect, by bringing to its aid 
the influence of the condensation and rarefaction of the air 
and juices of the plant, as a cause of the sap’s ascent. And 
on this principle he endeavoured also to account for the 
greater force of vegetation in the spring and autumn; be- 
cause the changes of the atmosphere are then the most fre- 
quent under a moderate temperature; while in the summer 
and winter the changes of the atmosphere are but few, and 
the air and juices either too much rarefied, or too much con- 
densed, so that the movement of the sap is thus at least 
prejudicially retarded, if not perhaps wholly suspended. 
But as this theory, with all its additional modifications, is 
still but a combination of the theories of Grew and Mal- 
pighi, it cannot be regarded as affording a satisfactory solu- 
tion of the phenomenon of the sap’s ascent. 
With this impression upon his mind, and with the best 
qualifications for the undertaking, Du Hamel directed his 
efforts to the solution of the difficulty, by endeavouring to 
account for the phenomenon from the agency of heat, and 
chiefly on the following grounds: because the sap begins to 
flow more copiously as the warmth of spring returns; be- 
cause the sap is sometimes found to flow on the south side 
of a tree before it flows on the north side ; that is, on the 
side exposed to the influence of the sun’s heat sooner than 
on the side deprived of it ; because plants may be made to 
vegetate even in winter, by means of forcing them in a hot- 
house; and because plants raised in a hot-house produce their 
fruit earlier than such as vegetate in the open air. 
On this intricate but important subject, Linnaeus appears 
to have embraced the opinion of Du Hamel, or an opinion 
very nearly allied to it, but does not seem to have strength- 
ened it by any new accession of argument, so that none of 
the hitherto alleged causes can be regarded as adequate to 
the production of the effect. 
