▸ Auxins and Branching Auxins also regulate cell division in meristems. As a stem grows in length, it produces lateral buds. As you may have observed, the buds near the apex grow more slowly than those near the base of a plant. The reason for this delay is that growth at the lateral buds is inhibited by auxins. Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited. This phenomenon is called apical dominance. If you snip off the tip of a plant, these lateral buds begin to grow more quickly. The plant becomes bushier. This is because the apical meristem—the source of the growth-inhibiting auxins—has been eliminated.

Cytokinins Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds. Cytokinins stimulate cell division, they interact with auxins to help to balance root and shoot growth, and stimulate regeneration of tissues damaged by injury. Cytokinins also delay the aging of leaves and play important roles in the early stages of plant growth.

Cytokinins often produce effects opposite to those of auxins. For example, root tips make cytokinins and send them to shoots; shoot tips make auxins and send them to roots. This exchange of signals can restore lost organs and keep root and shoot growth in balance. Auxins stimulate the initiation of new roots, and they inhibit the initiation and growth of new shoot tips. Cytokinins do just the opposite. So if a tree is cut down, the stump will often make new shoots because auxins have been removed and cytokinins accumulate near the cut.

In Your Notebook Make a 2×2 table labeled Shoot and Root across the top and Auxins and Cytokinins down the side. Then, fill in the effects of these hormones.

FIGURE 24–15 Apical Dominance The basil plant on the right has had its apical meristem pinched off, in contrast to the plant on the left, which hasn't. Observe How are the two plants different?

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