What causes a plant to “grow-up,” roots to grow down, or plants to grow towards a light source? The answer is plant hormones or plant growth regulators as they are sometimes called. In this particular case the hormone involved is the naturally occurring plant substance called indole-3-acetic acid or IAA for short.
IAA’s primary function in life is to stimulate cell growth especially near growing points. In stems the more IAA present the more cell elongation occurs. If you place a living, growing stem on its side it will naturally bend to return to growing in an upright position. Here’s how it works in our horizontal stem example. In response to basic gravity IAA accumulates in the bottom portion of the stem. This increased concentration causes the cells at the bottom of the stem to elongate (grow) at a faster rate then those cells at the top causing the stem to naturally curve upwards. In roots IAA has the opposite effect. The more IAA presents the slower the growth rate causing a natural downward growth. Phototropism, or the growth of plants towards a light source, is also under the control of IAA. IAA is degraded by sunlight and thus accumulates in the shady side of stems causing cells on the shady side to elongate at a faster rate then those on the sunny side curving the stem to naturally grow towards the light. Amazing!