134 PROCESS OF NUTRITION. CHAP. III. 



Insuffi- 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 ? Ac- 

 cording 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 conti- 

 nuance of heat, or there may be too long a con- 

 tinuance 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 four and twenty 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 doubt- 

 ful, 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 up- 

 wards like the ascending sap, perhaps it might be 

 somewhat efficacious in carrying a portion 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 temperature, while the trunk and upper parts 

 may be affected considerably, it can scarcely be 



