Apical dominance.

• The name auxin comes from the Greek auxein, to grow. The first auxin was isolated from human urine, and the name now covers IAA plus other natural and synthetic compounds that regulate growth the same way.
• Auxins are generally made at the growing apices of stems and roots and move from there to where they act.
• Natural auxins found in plants are IAA and IBA (indole butyric acid). The synthetic ones are 2,4-D (2,4-dichlorophenoxyacetic acid) and NAA, naphthalene acetic acid.
• Uses: they start roots on stem cuttings, which is why they are widely used in plant propagation; they promote flowering, for example in pineapple; they stop young fruits and leaves from dropping early, yet speed the shedding of older, mature ones.
• Apical dominance: in most higher plants the growing apical bud suppresses the growth of lateral (axillary) buds. Removing the shoot tip (decapitation) usually lets the lateral buds grow, which is why tea bushes are plucked and hedges are trimmed.
• Auxins induce parthenocarpy, for example in tomato.
• They are widely used as herbicides: 2,4-D kills dicotyledonous weeds but does not affect mature monocotyledonous plants, so gardeners use it for weed-free lawns.
• Auxin also steers the differentiation of xylem and assists cell division.

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