47 



On the Action of detached Leaves of Plants. By Thomas Andrew 

 Knight, Esq. F.R.S. In a Letter addressed to the Right Hon. Sir 

 Joseph Banks, Bart. G.C.B. P.R.S. Read June 13, 1816. {Phil. 

 Trans. 1816,^.289.] 



Mr. Knight having on a former occasion inferred, from his expe- 

 riments and observations, that the true sap of trees, from which the 

 living parts are generated, owes its properties to having circulated 

 through the leaves, now adduces other facts, more directly in point, 

 to show that a fluid similar to the true sap actually descends through 

 the leaf-stalks. 



A transverse section was made through the bark of a vine, at the 

 middle of the insertion of the leaf-stalk, by passing a slender knife 

 through the stalk, so as to split it for about two thirds of an inch 

 above its insertion. 



Another transverse incision of the bark having also been made 

 nearly an inch below, these sections were joined by longitudinal in- 

 cisions at each end, so that a piece of the bark, half an inch broad 

 and nearly one inch long, could be detached from the stem, still re- 

 maining united to the lower half of the split leaf- stalk. Being after- 

 wards protected on all sides from the air by waxed paper, it was 

 found to grow in all its dimensions, and to have thin layers of albur- 

 num deposited upon its interior surface. 



In a second experiment, leaves from the potatoe were taken at the 

 period when the tuberous roots were beginning to form, and were 

 planted in pots, under the expectation that these leaves even alone 

 might have power to form tubers. The effect, however, was not 

 exactly as the author had anticipated ; but the power was manifested 

 by the production of a conical swelling at the lower part, more than 

 two inches in circumference, apparently similar in composition to a 

 tuber, and retaining life to the following spring. 



Leaves of mint, also detached in the same manner, were found to 

 throw out roots, and to continue alive through the winter, assuming 

 the character and hue of those of evergreen trees. 



Since it had appeared, from former experiments, that the growth 

 of immature leaves depends upon matter afforded by those already 

 arrived at a state of maturity, Mr. Knight cut off several shoots of a 

 vine, and laying them over basins of water, immersed portions of the 

 larger leaves ; and he found that under these circumstances the young 

 leaves continued to grow for upwards of a month, during which they 

 necessarily depended on the larger leaves for their supply of nou- 

 rishment. 



The progress of fruit, likewise, is proved to depend upon the ma- 

 ture leaves ; for if these be destroyed, the fruit ceases to grow, and 

 gains nothing in ripeness or flavour : and, accordingly, those trees 

 alone are capable of ripening fruit during winter which retain their 

 leaves at that season, of which the Orange, Lemon, Ivy, and Holly, 

 are familiar examples. 



With regard to the period during which the true sap is accumu- 



