Page:History of botany (Sachs; Garnsey).djvu/564

544 undertook to determine whether the moisture, the low temperature, 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 geotropic, 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 producing 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 movement 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 determined 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 daytime 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 Berberis with that of Mimosa-leaves; the stamens of Berberis were afterwards examined by Covolo in 1764, by Koelreuter