These are heavier than the cytoplasm and tend to settle out under gravity. When they touch the endoplasmic reticulum some kind of signal is generated that results in correction of growth. Secale root tip stained with iodine showing starch in columella of root cap Allium root cap, showing columella. Pfr is the physiologically active form of phytochrome and apart from conversion to Pr in far red light, it is lost in the dark either by slow reversion to Pr or by degradation of the protein itself. Many plant developmental processes are regulated by phytochrome, whereas growth responses are often regulated by a blue-light absorbing pigment, sometimes called cryptochrome .
Shoots are usually positively phototropic whereas roots are negatively phototropic.
IAA, coming from the stem apex, accumulates on the side of the organ away from the light or on the same side as the gravity stimulus. Because auxin stimulates shoot elongation, but inhibits root elongation this leads to unequal growth on the two sides of the structure and hence curvature. This is the basis of the Cholodny-Went hypothesis of the mechanism of plant tropic responses, which is not universally accepted although it has been the subject of much research over the past 50 years. Once the rhythm is set up or entrained it will continue for several cycles in total darkness. Mechanical stimulation Some plants such as the senstive plant Mimosa pudica have obvious responses to touch or other mechanical stimulation.