Plant

As a plant grows, it undergoes developmental changes, known as morphogenesis, which include the formation of specialized tissues and organs. Most plants continually produce new sets of organs, such as leaves, flowers, and fruits, as they grow. In contrast, animals typically develop their organs only once, and these organs merely increase in size as the animal grows. The meristematic tissues of plants (see below) have the capacity for cell division and development of new and complex tissues and organs, even in older plants. Most of the developmental changes of plants are mediated by hormonal and other chemical changes, which selectively alter the levels of expression of specific genes.

A plant begins its life as a seed, a quiescent stage in which the metabolic rate is greatly reduced. Various environmental cues such as light, temperature changes, or nutrient availability, signal a seed to germinate. During early germination, the young seedling depends upon nutrients stored within the seed itself for growth.

As the seedling grows, it begins to synthesize chlorophyll and turn green. Most plants become green only when exposed to sunlight, because chlorophyll synthesis is light-induced. As plants grow larger, new organs develop according to certain environmental cues and genetic programs of the individual.

In contrast to animals, whose bodies grow all over as they develop, plants generally grow in specific regions, referred to as meristems. A meristem is a special tissue containing undifferentiated, actively growing, and dividing cells. Apical meristems are at the tips of shoots and roots, and are responsible for elongation of a plant. Lateral meristems are parallel to the elongation axis of the shoots and roots, and are responsible for thickening of the plant. Differences in apical meristems give different species their unique leaf arrangements; differences in lateral meristems give different species their unique stems and bark.

Many of the morphogenetic changes of developing plants are mediated by hormones—chemical messengers that are active in very small concentrations. The major plant hormones are auxins, gibberellins, cytokinins, abscissic acid, and ethylene. Auxins control cell expansion, apical dominance, and fruit growth. Gibberellins control cell expansion, seed germination, and fruit development. Cytokinins promote cell division and organ development, but impede senescence. Abscissic acid can induce dormancy of seeds and buds, and accelerate plant senescence. Ethylene accelerates senescence and fruit ripening, and inhibits stem growth.