Thermotropism or thermotropic movement is the movement of an organism or a part of an organism in response to heat or changes from the environment's temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures. Mimosa pudica also show thermotropism by the collapsing of leaf petioles leading to the folding of leaflets, when temperature drops. The term "thermotropism" was originated by French botanist Philippe Van Tieghem in his 1884 textbook Traité de botanique. Van Tieghem stated that a plant irradiated with an optimum growth temperature on one side laterally, and a much higher or lower temperature on the opposite side, would exhibit faster growth on the side exposed to optimum temperature. The definition of thermotropism can sometimes be confused with the term, thermotaxis, a mechanism by which temperature gradients can alter the behavior of cells, such as moving toward the cold environment. The difference between them is that thermotropism is more commonly used in botany because it could not only represent the movement in organism level, thermotropism could also represent an organ level of movement, such as movement of leaves and roots toward or away from heat; but thermotaxis can only represent locomotion at the organism level, such as the movement of a mouse away from a warm environment. The precise physiological mechanism enabling plant thermotropism is not yet understood. It has been noted that one of the earliest physiological responses by plants to cooling is an influx of calcium ions from the cell walls into the cytosol, which increases calcium ion concentration in the intracellular space]. This calcium influx is dependent upon mechanical changes in the actin cytoskeleton that alter the fluidity of the cell membrane, which allows calcium ion channels to open. From this information, a hypothesis has formed that the plant cell plasma membrane is an important site of plant temperature perception. Thermotropism in leaves Gardening hobbyists have frequently noted the dramatic change in the shape of Rhododendron or "Rhodie" leaves during warm versus cold weather. In warm weather, the leaf has a flat oblong shape. As the temperature of the leaf drops, the blade curls inward, giving the leaf a tubular, cigar-like shape. Research on Rhododendron leaf thermotropism suggests that the curling response might help prevent damage to cell membranes caused by rapid thawing after a freeze. During the winter months, wild Rhododendrons in the Appalachian Mountains regularly drop to freezing