134? ON THE INVERTED ACTION 



and cortical fibres and vessels as were sufficient to preserve life. In this 

 position I conceived that if their leaves and stems contained any unem- 

 ployed true sap, it could not readily find its way to the tuberous roots, 

 its passage being obstructed by the rupture of the vessels, and by gravita- 

 tion ; and I had soon the pleasure to see, that instead of returning down 

 the principal stem into the ground, it remained and formed small tubers 

 at the base of the leaves of the depending branches. 



The preceding facts are, I think, sufficient to prove that the fluid, 

 from which the tuberous root of the potatoe, when growing beneath the 

 soil, derives its component matter, exists previously either in the stems 

 or leaves ; and that it subsequently descends into the earth : and as the 

 cortical vessels during every period of the growth of the tuber are filled 

 with the true sap of the plant, and as these vessels extend into the run- 

 ners, which carry nutriment to the tuber, and in other instances evidently 

 convey the true sap downwards, there appears little reason to doubt that 

 through these vessels the tuber is naturally fed. 



To ascertain, therefore, whether the tubers would continue to be fed 

 when the passage of the true sap down the cortical vessels was interrupted, 

 I removed a portion of bark of the width of five lines, and extending 

 round the stems of several plants of the potato, close to the surface of 

 the ground, soon after that period when the tubers were first formed. 

 The plants continued some time in health, and during that period the 

 tubers continued to grow, deriving their nutriment, as I conclude, from 

 the leaves by an inverted action of the alburnous vessels. The tubers, 

 however, by no means attained their natural size, partly owing to the 

 declining health of the plant, and partly to the stagnation of a portion of 

 the true sap above the decorticated space. 



The fluid contained in the leaf has not, however, been proved, in any 

 of the preceding experiments, to pass downwards through the decorticated 

 space, and to be subsequently discharged into the bark below it ; but I 

 have proved with amputated branches of different species of trees that the 

 water which their leaves absorb, when immersed in that fluid, will be 

 carried downwards by the alburnum, and conveyed into a portion of bark 

 below the decorticated space ; and that the insulated bark will be 

 preserved alive and moist during several days * ; and if the moisture 

 absorbed by a leaf can be thus transferred, it appears extremely probable 

 that the true sap will pass through the same channel. This power in 

 alburnum to carry fluids in different directions probably answers very 



* This experiment does not succeed till the leaf has attained its full growth and maturity 

 and the alburnum of the annual shoot its perfect organisation. 



