Phototropism

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phototropism [ foh-TO-truh-piz-uhm, foh-toh-TROH-piz-uhm ] noun: growth or movement of an organism stimulated by light either toward or away from the light source. adjective: phototropic

Stems of a potted succulent plant on a sill bending toward the light from the window. Photos © Mary Free

You have observed phototropism if you grow houseplants–they bend toward the window. If you want them to grow vertically, then you have to routinely rotate the pots. That is because plant shoots grow toward blue wavelengths of light. When the sun is overhead, the plant’s growth hormone, auxin, is evenly distributed at the stem tip. When the light source is to one side, auxins move to the shaded side of the stem where they stimulate growth, which elongates cells causing the plant stems to bend toward the light.

Not all plant movement is toward the light (positive phototropism). Some vines can grow toward solid dark objects (negative phototropism) so that they can grab a hold and climb; and roots in the soil grow away from light sources. Roots and shoots are also gravitropic—roots grow downward toward the earth’s gravitational pull while stems resist gravity and grow upward.

Amaryllis on January 5, 2021 at 10:30 a.m.
Photo © Judy Funderburk

Amaryllis on January 8, 2021 at 9:12 a.m.
Photo © Judy Funderburk

Resinous disks or holdfasts of Parthenocissus quinquefolia (Virginia creeper) — Native *Parthenocissus quinquefolia* (Virginia-creeper) grows toward dark objects where it might find a support to which its holdfasts can adhere. Photo © Mary Free

Reference

Kantharaj GR. Physiology of Plant Movements. Plant Growth and Development. Plant Cell Biology for Masters. (accessed December 10, 2020).