<P> Auxins are compounds that positively influence cell enlargement, bud formation and root initiation . They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, and fruits, and convert stems into flowers . Auxins were the first class of growth regulators discovered . They affect cell elongation by altering cell wall plasticity . They stimulate cambium, a subtype of meristem cells, to divide and in stems cause secondary xylem to differentiate . Auxins act to inhibit the growth of buds lower down the stems (apical dominance), and also to promote lateral and adventitious root development and growth . Leaf abscission is initiated by the growing point of a plant ceasing to produce auxins . Auxins in seeds regulate specific protein synthesis, as they develop within the flower after pollination, causing the flower to develop a fruit to contain the developing seeds . Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots . Because of this property, synthetic auxin herbicides including 2, 4 - D (2, 4 - dichlorophenoxyacetic) and 2, 4, 5 - T have been developed and used for weed control . Auxins, especially 1 - Naphthaleneacetic acid (NAA) and Indole - 3 - butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants . The most common auxin found in plants is indole - 3 - acetic acid or IAA . </P> <P> Brassinosteroids are a class of polyhydroxysteroids, the only example of steroid based hormones in plants . Brassinosteroids control cell elongation and division, gravitropism, resistance to stress, and xylem differentiation . They inhibit root growth and leaf abscission . Brassinolide was the first identified brassinosteroid and was isolated from extracts of rapeseed (Brassica napus) pollen in 1979 . </P> <P> Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation . They were called kinins in the past when the first cytokinins were isolated from yeast cells . They also help delay senescence of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth . Cytokinins and auxins often work together, and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime . Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts, and fruits . </P> <P> Ethylene is a gas that forms through the breakdown of methionine, which is in all cells . Ethylene has very limited solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant . Its effectiveness as a plant hormone is dependent on its rate of production versus its rate of escaping into the atmosphere . Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness . New growth and newly germinated seedlings produce more ethylene than can escape the plant, which leads to elevated amounts of ethylene, inhibiting leaf expansion (see Hyponastic response). As the new shoot is exposed to light, reactions by phytochrome in the plant's cells produce a signal for ethylene production to decrease, allowing leaf expansion . Ethylene affects cell growth and cell shape; when a growing shoot hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell . The resulting thicker stem can exert more pressure against the object impeding its path to the surface . If the shoot does not reach the surface and the ethylene stimulus becomes prolonged, it affects the stem's natural geotropic response, which is to grow upright, allowing it to grow around an object . Studies seem to indicate that ethylene affects stem diameter and height: When stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker, more sturdy tree trunks and branches . Ethylene affects fruit - ripening: Normally, when the seeds are mature, ethylene production increases and builds - up within the fruit, resulting in a climacteric event just before seed dispersal . The nuclear protein Ethylene Insensitive2 (EIN2) is regulated by ethylene production, and, in turn, regulates other hormones including ABA and stress hormones . </P>

Hormones that stimulate cell elongation and are produced