542 History of the Doctrine of [BOOK in. 



here and there single things in his experiments that may be 

 turned to account, but he was himself unable to make any use 

 of them. He set out with a preconception which prevented him 

 from the first from understanding what his experiments showed 

 him it was his object to prove from a multitude of instances, 

 that stalks and leaves so curve, twist and turn in all cases, that 

 the under sides of the leaves are directed towards the ground, 

 in order that they may be able to suck up the dew, which 

 according to his theory is the chief nutriment of plants and 

 rises from the ground. It is no great merit in him, that amid 

 all this confusion a correct observation here and there forced 

 itself upon him, as for instance that organs, chiefly such as are 

 young and ductile, if they are put out of their natural position, 

 endeavour to recover it by bending and twisting. On the 

 other hand his conclusions with regard to the mechanical 

 causes of these movements are utterly inane ; the least skill in 

 judging of the results of his experiments must have led him to 

 very different ideas ; warmth and moisture, he says, appear to 

 be the natural causes of movement, but warmth is more 

 effective than moisture, and the warmth of the sun more 

 effective than that of the air. This explanation is unsuitable to 

 just those cases which he chiefly studied, the geotropic and 

 heliotropic curvatures. In one point only he arrived ultimately 

 at a right judgment, namely that the great lengthening of the 

 stem, the small size attained by the leaves and the want of 

 colour in plants grown under cover, are caused by partial or 

 entire absence of light ; Ray however had shown this before as 

 regards the colour. 



Though Du Hamel, like many later writers, treated Bonnet's 

 investigations, uncritical as they were and without plan, with 

 great respect, he gave himself a much better account of the 

 various movements of plants. In the sixth chapter of the 

 fourth book of his 'Physique des arbres,' 1758, he discussed 

 all the phenomena of the kind that were known to him under 

 the title : ' On the direction of stem and roots, and on the 



