LETTER X. 



97 



plants above, and growing concurrently with them. 

 And it requires for its evolution equally as these do 

 for theirs, a supply of prepared or elaborated sap. 

 But the sap is elaborated only in the leaves ; and as 

 the woody layer extends from the base of these down- 

 wards to the tips of and even beyond the roots of the 

 previous year, so the sap can be supplied only from 

 above, and must descend in order to reach every part 

 of the layer in question, — its descent, however, in 

 common with its ascent, being immediately due to 

 the attractive force exerted by living and growing 

 tissue. 



7. But the agency now dwelt upon as seated in 

 the growing parts, and thence exerting an attractive 

 influence over the sap, is not the only one con- 

 cerned in the movement of this fluid. The well known 

 experiments of Hales, and those subsequently insti- 

 tuted by Dr Daubeny and others, leave no room to 

 doubt, that at the extreme points of the roots (desig- 

 nated the spongioles of the roots) there resides another 

 and a very efiicient moving power.* Let us consider 



* " Hales cut off the stem of a vine in the spring, when the sap 

 rises with the greatest velocity, and luted a tube to the top of the 

 stump, bent in the manner we have described in the construction of 

 the Endosmometer. As the sap rose into the tube, mercm*y was 

 introduced at the open end ; and a measure of the force of the rising- 

 sap was thus obtained, and found to equal the pressure of an atmos- 

 phere and a half." — Henslow, Descriptive and Physiological Botany ^ 

 p. 181. 



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