CIRCULATION OF THE SAP. 75 



Yet, at mid-summer when the whole energies of the ve- 

 getable are exerted, these trees scarcely bleed at all. 



The flowing of the sap is usually regarded as the ef- 

 fect of its accumulation in the vessels, at a season when 

 there are no leaves through which, in the form of va- 

 pour it may escape. Dr. Smith however regards it as 

 the effect of heat, operating on the irritability of the 

 plant, and he considers it as the first step towards the 

 revival of vegetation from the torpor of winter. To 

 this hypothesis there are very serious objections, for 

 many trees bleed in the autumn after the leaves begin to 

 fade and fall, as well as in the spring before the buds 

 are open. 



During the season of vegetation, the motion of the 

 sap is uninterrupted, and the energy with which it 

 moves, is in some cases very great. Dr. Hales found 

 that it ascends in the stem of the Vine, with force suffi- 

 cient to elevate a column of water more than forty feet, 

 By what means is it thus forcibly propelled ? The capilla- 

 ry attraction of the tubes, the influence of temperature 

 and the winds, have all been regarded as the agents 

 by which the sap is elevated, but they all appear inad- 

 equate to produce the effect. Nor is the hypothesis 

 of Mr. Knight in any degree satisfactory. Having as- 

 certained that the silver grain of the Oak is susceptible 

 of quick changes from variations of temperature, he 

 thinks that these changes may alternately contract and 

 enlarge the diameter of the tubes, and thus forcibly 

 propel the fluids which they contain. But the action 

 of heat operating on the silver grain, would as much 

 promote circulation through a dead and leafless branch 

 as any other. The inherent irritability of the vessels, 

 furnishes the only satisfactory explanation, for it is the 

 only adequate cause of the sap's ascent. And of the 



