Responsive growth movements toward or away from an external stimulus are called tropisms. If the plant movement is toward the stimulus, it is a positive tropism; away from the stimulus, a negative tropism.
The tropic response to unidirectional light is called phototropism. In general, shoots grow toward light and hence are positively phototropic; roots grow away from light and are negatively phototropic. Well‐known and often‐repeated experiments with oat seedlings have shown that the auxin IAA, which causes elongation of cells, migrates to the shaded side of oat coleoptiles. The subsequent differential growth on the two sides causes the coleoptiles to bend toward the light. Although green stems also bend and grow toward the light, in this case an IAA inhibitor prevents cells from elongating on the lighted side, while those on the shaded side continue to elongate; the stem bends toward the light as a consequence of the differential growth. Different wavelengths of light cause differing growth responses. The blue end of the spectrum—wavelengths less than 500μm—is most effective in producing a growth response.
Gravitropism is the plant response to gravity. The mechanism of how gravity is sensed by plants is as yet unexplained. None of the numerous hypotheses is fully adequate. Over the eons, plants probably developed several methods to cope with this environmental factor. Shoots are negatively gravitropic, because they grow upward; roots are positively gravitropic—they grow downwards. IAA, calcium ions (Ca 2+), and possibly ABA are involved in instigating growth and curvature in many plants. Still to be proven is the long‐held belief that starch grains migrating from upper to lower sides in the root cap of a horizontally held root initiate the growth response.
The growth response of a plant or a plant part to the touch of a solid object is called thigmotropism. Tendrils of climbing plants wrapping around a support is a common thigmotropic response, accomplished by cells on the side touching the support shortening and those on the opposite side elongating. IAA and ethylene are two hormones probably involved in the response.