CHAP, ii.] of Plants. Bonnet. 487 



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 trans- 

 piration, 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 com- 

 mences 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 



1 See p. 35 of the German translation by Arnold, 1762. 



