Is this tendency of the coloured parts of plants to turn towards the light, due to an attraction exerted by this agent, or is it produced by a peculiarity of growth determined through its influence? A curious experiment has settled this question: A leaf, attached by its footstalk to a pivot, was so arranged that it could freely turn in every direction: under these circumstances, its under surface was exposed to light. If an attraction existed between the most deeply coloured portion and the light, the leaf might be expected to revolve on its pivot, in obedience to this attraction: but instead, the footstalk took on a spiral or corkscrew growth, by means of which the upper portion became in time presented to the light. Now, this experiment sufficiently showed that the manner in which light acts, is by its influence over vegetable growth. But what is the influence of light over vegetable growth? We have already answered this question in our articles on the Sap: we have found that when light is present, the sap becomes elaborated in the green parts of plants; and the use of this elaborated sap is, by developing vegetable fibre, to increase the thickness of stems, and the length of roots. While the ascending sap, by forming vegetable flesh, lengthens the stems, and makes the root thick, the directions of the different parts of plants, by the agency of light, must be in obedience to these functions. We are now in a condition to comprehend the cause of some phenomena. A geranium (_Pelargonium_) stem, placed at a window, curves towards the light: this takes place, because the portion of stem nearest the window elaborates most sap: consequently, in this portion most vegetable fibre is formed. The portion away from the light, on the contrary, has most ascending sap, which forms fleshy tissue, and lengthens the stem; the half of the stem remote from the light is therefore longer, that next the window is shorter; the former is fleshy and elastic, the latter is rigid and fibrous. Need we be surprised, then, that the short, rigid, and fibrous portion should draw down the long, fleshy, and elastic part, and curve it towards the light?--it is but the bending of a bow, by the agency of its string. But why do roots curve away from the light? Neither is this difficult to understand. Roots do not elaborate the sap, nor form vegetable fibre of their own: what vegetable fibre they contain is pushed down through them from the stem: more of this vegetable fibre will force its way downwards, from the part of the stem nearest the light, than from that which is most remote: two forces of unequal intensity will push downwards, through opposite portions of the root; the greater pressure may be expected to overcome the lesser, and in obedience to this, the root will curve away from the light.