Auxin

One of them, auxin, can become unevenly distributed across a growing shoot or root. The two sides then grow at different rates. The faster-growing side becomes longer, making the organ bend; the whole plant has not moved towards or away from the stimulus.

One important group of plant hormones is the auxins. Auxin is produced mainly in the growing tips of shoots and roots and reaches cells just behind the tip. Growth involves cell division in meristematic tissue and the elongation of new cells.

Auxin is one example. It is produced mainly near growing tips and affects cells just behind them as they elongate. If one side of a shoot or root grows faster than the other, that part curves as it grows.