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.