The particular directions of stems and roots are produced by a combination of causes: if an onion plant, exposed to daylight, be laid horizontally on the ground, the extremities of the stem and roots will in the course of a few hours turn themselves in their natural directions, the one upwards, and the other downwards; if a similar plant be placed in a dark cellar, to which no light has access, the same things will take place; but that which happens in a few hours, in the one instance, will require as many days in the other; and thus we learn that in the production of these effects two causes operate: first, the light; and, secondly, some other principle distinct from light. It will occur to the reader that the absorption of water from the earth, by the most depending part of the plant, and its evaporation above, might, by swelling the lower portion and contracting the upper, produce the upward curving of the stem; to obviate this objection, the plant was placed in water, where no evaporation could occur, and absorption must take place equally over the whole surface; and still it was found that the same things happened. Light, therefore, is most powerfully influential in producing the particular directions of the parts of plants; but there is another principle, distinct from light, which acts in effecting the same phenomena in a minor degree, but not the less absolutely and even more generally. Let our readers bear in mind the existence of this principle, which will form the subject of a future article. For the present, we will examine the manner in which light operates in promoting the directions of stems and roots. We have before hinted that the tendency of the organs of vegetables towards the light, bears a direct relation to the depth and brilliancy of their colours; roots which are usually destitute of colouring matter grow away from the light; the upper surfaces of leaves are always the most deeply coloured; and in those erect leaves which are equally exposed to light, both surface are similarly coloured; if the outer surface of a flower be richly tinted, it is pendent; in erect flowers, on the contrary, the internal surface is always the most brilliantly painted; and in some cases the direction of the flower and fruit is different, connected with similar conditions. But in all these instances we have reason to believe that the organ is not directed towards the light, because it is highly coloured; but that it is highly coloured, because it is presented to the light. In plants growing in the dark, all the organs are colourless; it is only when exposed to the light that they acquire their various hues. Even the extremities of the roots have been found in a singular experiment of Dutrochet’s to acquire a green colour by exposure to the influence of light.