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 afiected 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 twent3'-four hours, as in the case of the oak, 

 it can never be of much efiicacy 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 



