SECT. III. CAUSES OF THE SA?'s ASCENT. 131 



accession of argument; so that none of the hitherto 

 alleged causes can be regarded as adequate to the 

 production of the effect. 



Perhaps the only cause that has ever been sug- Saussure. 

 gested as appearing to be at all adequate to the 

 production of the effect, is that alleged by M 

 Saussure. 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 consist of an in- 

 definite 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 excited by the stimulus of the sap, 

 begins gradually to contract, and to propel the con- 

 tained fluid into the cylinder immediately 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 cy- 

 linder to cylinder till it reaches the summit of the 

 plant. So also when the first cylinder has dis- 



K 2 



