On heliotropism in tendrils of Pisum sativum: A response to infrared irradiation.

In 1876, Darwin observed that tendrils of Pisum sativum oriented the long axis of their circumnutational sweep to keep it approximately normal to the direction of the sun's rays. Using various types of irradiation sources, the present study confirms Darwin's findings. Although blue light slightly increases the rate of circumnutation, only infrared irradiation (i.e. heat), effects the reorientation. The response can therefore be termed Diathermotropic. Diathermotropism in circumnutating tendrils is probably a device to bend the leaves of the elongating weak-stemmed plant away from the shade and toward the sun.