544 History of the Doctrine of [BOOK in. 



undertook to determine whether the moisture, the low tempera- 

 ture, or the absence of light in the earth made the roots curve 

 downwards, and he was obliged by the result of his experiments 

 to deny that they do. But he was unfortunate in his own 

 explanation of the movements which we should now call geo- 

 tropic, heliotropic and periodic, for he came to the conclusion 

 that the ' direction of the vapours ' inside the vessels of the 

 plant and round about the plant has more to do with pro- 

 ducing these movements than any other causes, and that if 

 warmth and light appear to influence them, it is perhaps only 

 because they produce vapours or communicate a definite move- 

 ment to them. As regards the movements of the leaves of 

 Mimosa, Du Hamel repeated the experiment made by Mairan 

 in 1729, in which the periodic movement continued even in 

 constant darkness ; he found that this was so, and concluded 

 that the periodic movements of Mimosa do not essentially 

 depend on temperature and changes of light ; Hill had de- 

 termined in 1757 that the alternation of day and night was the 

 cause of the movements connected with the sleep of plants, 

 because he found that darkness artificially produced in the day- 

 time made the plants assume the nocturnal position ; but Zinn 

 in 1759 came to the same conclusion as Mairan and Du 

 Hamel. It was not till long after that the question was to 

 some extent cleared up by Dutrochet. Du Hamel thought it 

 necessary to give a formal refutation of the opinion expressed 

 by Tournefort, that the movements of plants are produced by 

 muscles, and to show that Tournefort's vegetable muscles are 

 hygroscopic fibres. 



We have to mention in conclusion, that Du Hamel was the 

 first who observed that the two branches of a vine-tendril twine 

 in opposite directions round a support that happens to be 

 between them ; he also appears to have been the first who 

 compared the irritability of the stamens of Opuntia and Ber- 

 beris with that of Mimosa-leaves ; the stamens of Berberis 

 were afterwards examined by Covolo in 1764, by Koelreuter 



