SECT. III. CAUSES OF THE SAP's ASCENT. 125 



or not, is at any rate admitting a species of la- 

 teral communication. 



SECTION III. 







Causes of the Sap's Ascent. 



FROM the evidence exhibited in the foregoing 

 section the ascent of the sap is demonstrated, and 

 the channel of its distribution ascertained. But 

 what is the cause of that ascent ; or by what power 

 is the sap propelled ? 



The great and almost impenetrable obscurity in 

 which this subject is unavoidably involved has oc- 

 casioned much diversity of opinion among phyto- 

 logists. Grew states two hypotheses which he Hypmhe- 

 seems to have entertained at different periods, G rew . 

 though it is not quite certain to which of them he 

 finally gave the preference. In one of them he 

 attributes the ascent of the sap to its volatile nature 

 and magnetic tendency, aided by the agency of 

 fermentation.* But this hypothesis is by much 

 too fanciful to bear the test of serious investigation. 

 In the other he attributes the entrance and first 

 stage of the sap's ascent to the agency of capillary 

 attraction, and accounts for its progress as follows : 

 The portion of the tube that is now swelled with 

 sap, being surrounded with the vesiculae of the 



* Anat. of Veg, chap, iii. 



