Photoperiodism is the response of plants to relative length of light and darkness. Phototaxis is plant movement where the stimulus is light. Phototropism is tropism in which stimulus is light. Vernalization is application of cold treatment to plants to effect flowering. It is a process in which a plant responds to the relative durative duration of daily light and dark periods. Low temperature, 0°C to 10°C or 18°C - 22°C (according to species) treatment of certain species for a specified period for the induction of ability to promote flowering is called vernalisation. It is a tropic movement, which is induced by light, when supplied unidirectionally. The plants, bending towards the source of light are called positively phototropic. Stems in general, are positively phototropic. Bending away from the source of light is called negative phototropism. Some roots show this response as in sunflower.