116 VEGETABLE SAP. 



the system, scarcely admits of doubt. The ascending 

 current may be exhibited to the naked eye by the 

 well known expedient of supplying roots with co- 

 loured liquids, as has been proved by Darwin, Knight, 

 <&c., and every florist knows that the colours of 

 flowers may be changed, and the bulk of all the parts 

 increased, by the application of suitable manures. 



Much ingenuity has been shown, and the most 

 laboured philosophical dissertations have been written, 

 to account for the ascent of the sap, one of the most 

 simple processes in nature. For surely, if a bit of 

 sponge, or any tissue of filaceous matter be capable of 

 imbibing water to any height, no doubt need be 

 entertained of the power of vegetable organisation, 

 with its myriads of cellular and intercellular ducts 

 and tubes, to do the same. The elongation, disten- 

 sion, or inflation of any tube, cell, or vessel, in the 

 extended head, must necessarily produce a vacuum 

 which is instantly filled up. The pressure of the 

 atmosphere, capillary attraction, or simple imbibition 

 of areolated substances, are all ordinary agents capable 

 of effecting this result. The like effect takes place in 

 an old oak gate-post; and why should not the 

 hydraulic action of the extending shoots, the 

 perspiring leaves, and the craving fruit, produce a 

 similar result in living plants ? It would be a waste 

 of time to insist farther on such an obvious truth ; for, 

 in fact, the ascent of sap may not inaptly be compared 

 to the common evaporation of all fluids under the 

 action of heat. 



