growth promoting hormones that cause cell elongation, and are responsible for many developmental responses including phototropism.

A natural or artificial substance controlling the growth of shoots, roots, etc.

Plant growth regulators involved in apical dominance, phototropism, root induction, and numerous other plant physiological functions. Indoleacetic acid (IAA) is the natural form; others include NAA, IBA, and the herbicide 2,4-D.

Axial System The axial system is the vascular transport system in a stem that runs vertically (up and down). This is in contrast to the lateral system that runs right and left, or out two the margin of the stem. The axial system carries water and raw minerals up the stem in the xylem and it carries glucose in the phloem up and down the stem. The axial system is composed of fibers for strength, sieve-tube members and companion cells in the phloem and fibers, vessels and tracheids in the xylem. The fusiform initials are what produces the axial system.