The growth and development of many plants are regulated by the activity of plant hormones. Hormones are biochemical substances produced in one part of a plant and transported to a different part where they exert a particular effect.
An example of a plant hormone is a series of substances called auxins. Auxins increase the length of most plant cells and thereby contribute to the growth and elongation of the plant.
Another plant hormone is abscisic acid, which is produced in mature leaves and inhibits growth in developing leaves and germinating seeds. The inhibition occurs during the winter, contributing to the plant’s dormancy. Another hormone, ethylene, encourages ripening and the dropping of leaves and fruits from the trees. Slight pressure permits the fruit to break loose from the stem.
Two important growth-regulating hormones are the gibberellins and the cytokinins. Gibberellins affect plants by stimulating their growth via rapid stem elongation. Cytokinins induce plant cells to undergo mitosis; therefore, they encourage increased growth in the roots and stems of plants. They also enhance flowering and stimulate some types of seeds to germinate.
The interactions of hormones and stimuli in the environment often result in a bending or turning response in the plant called a tropism. When the plant turns toward a stimulus, the tropism is said to be positive. If the plant turns away from a stimulus, the tropism is negative.
One of the most familiar plant responses is the bending of the stem toward a light source. Light is the stimulus, and the response of the plant is called a phototropism. A geotropism is a turning of the plant away from or toward the earth. A negative geotropism is a turning away from the earth, such as by a plant stem that grows upward. A positive geotropism is a turning toward the earth, such as in a root that grows downward.