CIRCULATION OF THfi SAP. ^J 



I shall now proceed to relate, to be also capable of an inverted 

 action, when that becomes necessary to preserve the existence 

 of the plant. 



As soon as the leaves of the oak were nearly full grown Experiments 

 in the last spring, 1 selected in several instances two poles of ^'^^ young 

 the same age, and springing from the same roots in a coppice, 

 which had been felled about six years preceding ; and making 

 two circular incisions at the distance of three inches from each 

 other through the bark of one of the poles on each stool, I 

 destroyed the bark between the incisions, and thus cut off the 

 stem and roots, through the bark. Much growth, as usual, 

 took place above the space from which the bark had been 

 taken off, and very little below it. 



Examining the state of the experiment in the succeeding 

 winter, I found it had not succeeded according to my hopes : 

 for a portion of the alburnum, in almost every instance, was 

 lifeless, and almost dry, to a considerable distance below the 

 space from which the bark had been removed. In one in- 

 stance the whole of it was, however, perfectly alive; and in 

 this I found the specific gravity of the wood above the decor- 

 ticated space to be 114, and below it 111 ; and the wood of 

 the unmutilated pole at the same distance from the ground to 

 be 112, each being weighed as soon as it was detached from 

 the root. 



Had the true sap in this instance wholly stagnated above the 

 decorticated space, the s])ecific gravity of the wood there 

 ought to have been, according to the result ol former experi- 

 ments,* comparatively much greater: but I do not wish to 

 draw any conclusion from a single experiment ; and indeed 

 I see very considerable difficulty in obtaining any very satis- 

 factory, or decisive facts from any experiments on plants, in 

 this case, in which the same roots and stems collect and 

 convey the sap during the spring and summer, and retain, 

 within themselves, that which is, during the autumn and 

 winter, reserved to form new organs of assimilation in the 

 succeeding spring. In the tuberous-rooted plants, the roots Experiment 

 and stems which collect and convey the sap in one season, '^^'"^ "-^'^ P^''-*" 

 and those in which it is deposited and reserved for the suc- 

 ceeding season, are perfectly distinct organs ; and from one 



* Phil. Trans, for 1805. 



