Thigmotropism is a directional growth movement which occurs as a mechanosensory response to a touch stimulus. Thigmotropism is typically found in twining plants and tendrils, however plant biologists have also found thigmotropic responses in flowering plants and fungi. This behavior occurs due to unilateral growth inhibition. That is, the growth rate on the side of the stem which is being touched is slower than on the side opposite the touch. The resultant growth pattern is to attach and sometimes curl around the object which is touching the plant. However, flowering plants have also been observed to move or grow their sex organs toward a pollinator that lands on the flower, as in Portulaca grandiflora. Physiological factors Since growth is a complex developmental procedure, there are indeed many requirements (both biotic and abiotic) that are needed for both touch perception and a thigmotropic response to occur. One of these is calcium. In a series of experiments in 1995 using the tendril Bryonia dioica, touch-sensing calcium channels were blocked using various antagonists. Responses to touch in treatment plants which received calcium channel inhibitors were diminished compared to control plants, indicating that calcium may be required for thigmotropism. Later in 2001, a membrane depolarization pathway was proposed in which calcium was involved: when a touch occurs, calcium channels open and calcium flows into the cell, shifting the electrochemical potential across the membrane. This triggers voltage-gated chloride and potassium channels to open and leads to an action potential that signals the perception of touch. The plant growth hormone auxin has also been observed to be involved in thigmotropic behavior in plants, but its role is not well understood. Instead of asymmetric auxin distribution influencing other tropisms, it has been shown that a unidirectional thigmotropic response can occur even with a symmetric distribution of auxin. It has been proposed that the action potential arising from a touch stimulus leads to an increase of auxin in the cell, which causes the production of an contractile protein on the side of the touch that allows the plant to grip onto an object. Further, it has been shown that when auxin (typically leading to growth away from the side of its localization) and a touch stimulus (causing a tropic response toward its localization) were applied on the same side of a cucumber hypocotyl, the stem will curve towards the touch. Ethylene, another plant hormone, has also been shown