Page:Proceedings of the Royal Society of London Vol 2.djvu/79

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Mr. Knight having on a former occasion inferred, from his experiments 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 incisions 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 remaining united to the lower half of the split leaf- stalk. Being afterwards 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 alburnum 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 nourishment.

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