SECT. III. CAUSES OF THE SAP's ASCENT. 133 



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 contractation 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 following facts : On the surface of an oaken 

 plank 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 longitu- 

 dinal 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 consequently propel 

 the sap ; while in the act of contracting they again 

 allow the tubes to expand and take in a new supply. 

 This, as I think, is the substance of the theory.* 



But in drawing this grand and sweeping conclu- 

 sion, it should have been recollected 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 

 * Phil. Trans. 1801. 



