Phototropism Is Caused by Auxins. Auxins Are Plant Hormones That Have Many Functions. Auxins

Phototropism Phototropism is the growth and response to a light stimulus. Phototropism is most often seen in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, while roots usually exhibit negative phototropism, although geotropism may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The shoot is the part of their structure or body which causes the plant to respond to light. When you cut off the shoot of a plant it will not grow towards any sort of light, a once old myth that has been proved correct. Phototropism is caused by auxins. Auxins are plant hormones that have many functions. Auxins are responsible for telling a plant when and how to grow. When they cause one side of the plant’s stem to grow, it starts to bend.

Geotropism Geotropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive geotropism and stems show negative geotropism. The roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with a potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, bending upwards. Most plant stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth in a new direction. Roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. In roots, an increase in the concentration of auxin will stop cell expansion; therefore, the increase of auxin in the root can start growth on one side of the root, resulting in a curved root. A “tropism” is a plant movement triggered by stimuli. The term geotropic refers to a plant whose roots grow down into the soil as a response to gravity. Geotropism will continue even as a plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth's atmosphere, it will still grow roots down and stem up. The reason for this comes from the nature of a plant and its general response to gravity. In both roots and stems auxin builds up. This can cause either the stem or the roots to curve toward or away from gravity.