1. Auxin - indole acetic acid (IAA) - causes stem elongation and
growth - formation of adventitious and lateral roots, inhibits leaf loss, promotes cell division (with cytokinins), increases ethylene production, enforces dormancy of lateral buds produced by shoot apical meristems and other immature parts.
Responsible for phototropism Charles and Francis Darwin wondered what caused plants to
bend toward light. They demonstrated that growing tips of plants sense light. The ability to sense light is not present in areas behind the shoot apex.
Went demonstrated that a chemical produced in the shoot tip is responsible for the shoot bending - he called it “auxin” An agar block can absorb chemicals below a growing shoot tip. When the block is applied to an immature shoot, the shoot elongates more on the side where the agar block is applied.