Thigmotropism and Hydrotropism

The third most common type of tropism in plants, behind phototropism and gravitropism, is thigmotropism. The prefix "thigmo" comes from the Greek word for "touch." Thigmotropism is a plant's involuntary movement or response to contact or touch. It usually happens when a plant grows around a surface, such as a wall or a trellis.

Another example of thigmotropism is when vines and other climbing plants develop tendrils that coil around supporting objects. The supporting object may be a fence, a tree trunk, or another plant. The touched cells on the tendrils produce auxins, then transport them to nearby untouched cells. As previously discussed, auxins cause cells to elongate faster than normal. As those untouched cells elongate faster, that rapid cell growth bends the tendril around the object. They become touched cells, and the process continues.

Another plant tropism is hydrotropism, or a plant's growth response to the stimulus of water. Hydrotropism is the positive response of plant roots growing toward areas of higher moisture or water concentration.

The separate existence of hydrotropism is difficult to prove. First, underground roots are difficult to observe. Second, gravitropism is probably a stronger influence on root growth than hydrotropism. Third, hydrotropism and gravitropism probably work together. Separating one tropism from the other might be difficult, but recent genetic studies are beginning to identify genes that may be responsible for a separate hydrotropic response in plants. Some plants seem to have a modified gene that supports positive hydrotropism in their roots.