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
