Plant Growth And Differentiation
The growth and differentiation of the various plant tissue and organ systems are controlled by various internal and external factors. Hormones Plant hormones, specialized chemical substances produced by plants, are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Thus, auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it. In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves. Gibberellins are other important plant-growth hormones; more than 50 k...