with leaves, which were cut off from their plants, and having been smeared over with oil or other hurtful substances were laid on water, some on their upper some on their under side, the object being to note the time which they took to perish. It is impossible to imagine worse-devised experiments on vegetation; for if Bonnet wished to test Calandrini's 'sensible' conjecture, he ought certainly to have left the leaves on the living plants and have observed the effect of the supposed absorption of dew on the vegetation. It is to be observed, that by rising dew he evidently meant aqueous vapour, for the real dew descends chiefly on the upper side of the leaf; and what could he have expected to learn by laying cut leaves on water? how could this prove that leaves absorb dew? Nevertheless Bonnet came to the conclusion that the most important function of leaves was to absorb dew, and in order to make this result agree with Hales' investigations on transpiration, he propounded the theory[1], that the sap which rises by day from the roots into the stem is carried by the woody fibres assisted by the air-tubes into the under side of the leaves, where there are many stomata to facilitate its exit (evaporation). At the approach of night, when the leaves and the air in the air-tubes are no longer under the influence of heat, the sap returns to the roots; then the under side of the leaves commences its other function; the dew slowly rising from the earth strikes against it, condenses upon it, and is detained there by the fine hairs and by other contrivances (this really takes place to a much greater extent on the upper side). The fine tubes of the leaves absorb it at once, (this is evidently not so, since the dew increases in quantity till sunrise), and conduct it to the branches, whence it passes into the stem. He thought so highly of this strange theory, that he believed he found in it a teleological explanation of the heliotropic and geotropic curvature of leaves and stems, two things which
- ↑ See p. 35 of the German translation by Arnold, 1762.
