described the rapidity with which forced plants shoot up to the higher temperature, De Candolle to want of light. On the other hand Ray knew perfectly well that the green colour of leaves is not produced by the access of air but by the light, for, as he says, plants become green under glass, and not under an opaque cover; and if they become less green under glass than in the open air, this is because the glass absorbs certain rays of light and reflects others. Ray however, .like almost all later observers till quite recent times, did not keep the elongation and bleaching of etiolated plants sufficiently distinct; his account of this phenomenon is spoilt by the presence of much that is obscure.
It has been justly observed by other writers on botanical subjects that no notice is usually taken of one of the most remarkable of the phenomena of which we are here speaking, because, being a matter of every-day occurrence, it is simply accepted as something obviously in accordance with the nature of things; this is the fact, that the main stems of plants grow vertically upwards and their main roots downwards. To the French academician Dodart, whom we have already encountered in the history of the theory of nutrition, is due the great merit of being the first to find this apparently simple phenomenon really very remarkable; he convinced himself by experiments on germinating plants, that these vertical positions are caused by curvatures, and endeavoured to discover the physical reason why the main roots if placed in an abnormal position escape from it by curving in the downward direction, and the main stems in the upward direction, till they both reach the vertical line. It was a matter of minor importance that his mechanical explanation, which supposed that the fibres of the roots contract on the moister side and those of the stem on the same side lengthen, was quite unsatisfactory; it was much more important that these remarkable phenomena were made the subject of scientific enquiry, and we find that various observers soon after directed their attention to them, and exercised their
