Section 6: Plant Response and Growth
Any plants growth toward or away from a stimulus is called a tropism. A tropism is called positive if the plant grows toward a stimulus or negative if it grows away from it. The growth of a plant toward light is called phototropism. Stems and leaves grow toward light to obtain more light for photosynthesis. Gravitropism is plant growth in response to gravity. This can be seen in how roots grow downward and stems grow upward. Thigmotropism is a plant’s response to touch. Stems of many vines show positive thigmotropism when they encounter something and wrap themselves around it. Hydrotropism is a plant’s growth in response to water.
Plants are able to respond to light, gravity, and touch because they produce hormones. A hormone is a chemical signal that affects how a plant grows and develops cells, tissues, and organs. For example, the hormone auxin stimulates stem elongation.
Plants also mark the changing seasons by measuring photoperiodism. Photoperiodism refers to the seasonal changes in the length of day and night. A “short-day plant,” such as a Christmas cactus, flowers in late summer, fall, or winter when the light periods shorten. “Long-day plants,” such as lettuce, flower in late spring or early summer when light periods lengthen.
Angiosperms, flowering plants, are classified based on the length of their life cycles. Marigolds, petunias, and cucumbers are annuals, which means they have a life cycle within one growing season. They grow from seed to maturity, flower, produce seeds, and then die. Biennials, like parsley and celery, complete their life cycles in two years. In the first year they sprout and grow very short stems, and in year two they grow new stems and leaves, flower, produce seeds, and then die. Perennials are plants that live for more than two years. Trees and shrubs are perennials. Often seeds do not grow when they mature but rather enter a period of dormancy, which is the period when an organism’s growth or activity stops. The seed embryo is alive but not growing. The length of dormancy depends on temperature and moisture.