CHAP, in.] the Movements of Plants. 547

growth of stems and roots is caused by a polarity of a definite kind in every plant, from which we may argue, he says, ' to higher connections of our planet in the world of space.' He says again, ' that it is natural to conjecture that light is the cause of the sleep of plants,' and then gives the contradictory statements of Hill, Zinn, and De Candolle, all jumbled together into an inex- tricable tangle in a fashion which sets all maxims of reasonable discussion at defiance. He then puts aside all attempts at mechanical explanation with the remark, that plants observe their regular times of sleep even when kept in the dark and at a low temperature, for this evident habituation is one of the most important marks of vitality. He is led to similar results by Desfontaine's observation, that a Mimosa, exposed to the shak- ing of a wheeled vehicle, closes at first but then opens again. Speaking of the rapid oscillations of the leaves of Hedysarum gyrans and similar movements, he rejects Percival's idea of a will in plants, but says that the attempts to derive them from mechanical or chemical causes has only led to solemn trifling.

It is plain that men who could print such remarks as these and still worse than these, could not possibly effect anything in the province of botany which we are considering. The broad and shallow stream of such opinions as these flowed on till later even than 1830, but it ran dry at last when its supplies were cut off by the effect of new discoveries, and scientific investigation again gained the upper hand. Some calmer thinkers, who could not rest content with empty words, had meanwhile been pursuing the path trodden by Ray, Dodart, Hales, and Du Hamel, and by'experiment and earnest reflec- tion had brought new facts to light, which were at least calcu- lated to pave the way for the mechanical explanation of phyto- dynamical phenomena. Senebier in his ' Physiologic vegetale " (1700) had described some minute researches which he had made into the subject of etiolation ; and though he made the great mistake of attributing the want of colour in the leaves and the excessive elongation of the stems to the decom-

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