AND AMERICAN RURAL SPORTS. 
223 
According to Saussure, the cause of the sap’s ascent is to 
be found in a peculiar species of irritability, inherent in the 
sap vessels themselves, and dependant upon vegetable life; 
in consequence of which they are rendered capable of a 
certain degree of contraction, according as the internal 
surface is affected by the application of stimuli, as well as 
of subsequent dilatation, according as the action of the 
stimulus subsides: thus admitting and propelling the sap 
by alternate dilatation and contraction. In order to give 
elucidation to the subject, let the tube be supposed to con- 
sist of an indefinite number of hollow cylinders, united one 
to another, and let the sap be supposed to enter the first 
cylinder by suction, or by capillary attraction, or by any 
other adequate means ; then the first cylinder, being ex- 
cited by the stimulus of the sap, begins gradually to contract, 
and to propel the contained fluid into the cylinder imme- 
diately above it. But the cylinder immediately above it, 
when acted on in the same manner, is affected in the same 
manner; and thus the fluid is propelled from cylinder to 
cylinder, till it reaches the summit of the plant. So also, 
when the first cylinder has discharged its contents into the 
second, and is no longer acted upon by the stimulus of the 
sap, it begins again to be dilated in its original capacity, 
and prepared for the introsusception of a new portion of 
fluid. Thus a supply is constantly kept up, and the sap 
continues to flow. 
But Mr. Knight has presented us with another, which, 
whatever may be its real value, merits at least our par- 
ticular notice, as coming from an author who stands de- 
servedly high in the list of phytological writers. This 
theory rests upon the principle of the contraction and dila- 
tation, not of the sap vessels themselves, as in the theory of 
Saussure, but of what Mr. Knight denominates the silver 
grain, assisted perhaps by heat and humidity, expanding or 
condensing the fluids. On the transverse section of the 
trunk of woody plants, particularly the oak, they appear in 
the form of the radii of a circle, extending from the pith to 
the bark; and on the longitudinal cleft or fissure of the 
trunk of most trees, but particularly the elm, they appear 
in the form of fragments of thin and vertical laminae, or 
plates, interlacing the ascending tubes in a transverse di- 
rection, and touching them at short intervals, so as to form 
with them a sort of irregular wicker-work, or to exhibit 
the resemblance of a sort of web. Such, then, being the 
close and complicated union of the plates and longitudinal 
tubes, the propulsion of the sap in the latter may be easily 
accounted for, as it is thought, by means of the alternate 
contraction and dilatation of the former, if we will but allow 
them to be susceptible to change of temperature; which 
susceptibility is proved, as it is also thought, from the fol- 
lowing facts: on the surface of an oaken plant that was 
exposed to the influence of the sun’s rays, the transverse 
layers were observed to be so considerably affected by 
change of temperature as to suggest a belief that organs 
which were still so restless, now that the tree was dead, 
could not have been formed to be altogether idle while it 
was alive. Accordingly, on the surface of the trunk of an 
oak deprived of part of its bark, the longitudinal clefts and 
fissures, which were perceptible during the day, were found 
to close during the night. But in the act of dilating they 
must press unavoidably on the longitudinal tubes, and con- 
sequently propel the sap; while in the act of contracting 
they again allow the tubes to expand and take in a new sup- 
ply. This is the substance of the theory. 
But, in drawing this grand and sweeping conclusion, 
Keith has well remarked, that it should have been recol- 
lected, that change of temperature cannot act upon the 
transverse layers of a tree that is covered with its bark, in 
the same manner as it acts upon those of a tree that is 
stripped of its bark ; or upon those of a plank ; and if it 
were even found to act equally upon both, still its action 
would be but of little avail. For, according to what law 
is the machinery of the plates to be contracted and dilated, 
so as to give impulse to the sap? According to the alternate 
succession of heat and humidity? But this is by much too 
precarious an alternation to account for the constant, and 
often rapid, propulsion of the sap, especially at the season 
of bleeding. For there may be too long a continuance of 
heat, or there may be too long a continuance of humidity; 
and what is to become of the plant during this interval of 
alternation? If we are to regard it as happening only once 
in the space of twenty-four hours, as in the case of the oak, 
it can never be of much efficacy in aiding the propulsion of 
the sap. But if we should even grant more, and admit the 
alternate contraction and dilatation of the vessels to be as 
frequent as you please, still their effect would be extremely 
doubtful, owing to a want of unity or co-operation in the 
action of different plates, or of different portions of the same 
plate. If heat, like humidity, entered the plant by the root, 
and proceeded gradually upwards, like the ascending sap, 
perhaps it might be somewhat efficacious in carrying a por- 
tion of sap along with it; but as this is not the case, and as 
the roots of plants are but little affected by change of tem- 
perature, while the trunk and upper parts may be affected 
considerably, it can scarcely be supposed that the action of 
the plates will be uniform throughout the whole plant; or 
rather, it must be supposed, that it will often be directly in 
opposition to that which is necessary to the propulsion of 
the sap. But, admitting that the sap is propelled by the 
agency of the plates in question, and admitting that it has 
been thus raised to the extremity of the woody part of the 
plant, how are we to account for its ascent in such parts 
