Phototropism/Photoperiodism Essay :)

   There are two main plant responses that involve light they are called phototropism and photoperiodism. Phototropism is the growth and response to a light stimulus. The plant has a hormone called auxin that reacts when phototropism occurs. The auxin causes the plant to have elongated cells on the farthest side from the light. There are two types of phototropism. The first type is called positive phototropism which is when the plant grows toward the light. The second type is called negative phototropism which is when the plant grows away from the light. For example plant shoots show positive phototropism and the plant roots show negative phototropism. The mechanism of control for phototropism is the tip of the shoot which is where the direction of light is detected. This is supported by an experiment done by Darwin. He placed tin foil on the tip of one shoot of a plant and didnt place tin foil on the other. After a couple days the plant with the tin foil grew directly up and the other one without grew towards the light. Photoperiodism is the developmental responses of plants to the relative lengths of the light and dark periods. The mechanism of control for photoperiodism is the circadian system. Night break experiments show that measuring night length is one of the mechanisms used for photoperiod assessment.
   In conclusion, phototropism and photoperiodism are two main plant responses to light. Phototropism is the growth to the light and photoperiodism is the developmental responses of plants to the relative lengths of the light and dark periods.