300 Mr. Knight on the inverted Action 
the true sap of the plant, and as these vessels extend into the 
runners, 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 potatoe, 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 albur- 
nous 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 down- 
wards 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 
* This experiment does not succeed till the leaf has attained its full growth and 
maturity, and the alburnum of the annual shoot its perfect organization. 
