Originally (with J. v. SACHS, for example) was phototropism called heliotropism, because the plant grows towards the sun. The name was altered when it became clear that plants react also towards artificial sources of light (W. PFEFFER). Before proceeding further shall another problem be outlined shortly: Does the plant react to light or to the air? Here is a citation from A. de CANDOLLE ( 1834/38).
"Gardeners and farmers tell usually that plants are attracted by the air and only TESSIER proved this to be false by a simple experiment: He placed living plants in a basement with two openings, on one side spent a glass window light but no air, on the other brought an airhole leading through a spacious and dark room air but not light. All plants grew towards the glass window."
A.de CANDOLLE noticed as soon as 1809 that the growth towards light is caused by an unequal growth of the opposite parts of an organ. The part exposed to light grows slower than the one that is not exposed. J. v. SACHS discovered the importance of the light quality (the dependence on the wave length) for the phototropic reaction. Blue, violet and ultraviolet light together have the same effect as very strong white light. The effect remains even after the UV part is taken away. Red, yellow , and green light has no effect on most plants. Red light causes a phototropic reaction in some fern prothalliums, though.
The amount of light and the phototropic reaction are linked. This connection is known in literature under the terms Bunsen-Roscoe law, product law or reciprocity theorem (FRÖSCHEL and BLAAUW, 1908, BLAAUW, 1909). The Bunsen-Roscoe law states that the product of time and intensity, and thus the energy amount of the...