Hormones and Plant Growth • Unlike most animals, plants do not have a rigidly set organization to their bodies. • But the plant growth is not determined precisely, it still follows general patterns that differ among species. • Biologists have discovered that plant cells send signals to one another that indicate when to divide and when not to divide, and when to develop into a new kind of cell.
Hormones and Plant Growth • Meristems are found at places where plants grow rapidly-the tips of growing stems and roots, and along the outer edges of woody tissues that produce new growth every year. (causes the plant to stay “forever young”) • Plants grow in response to environmental factors such as light, moisture, temperature, and gravity.
Hormones and Plant Growth • Hormone – a substance that is produced in one part of an organism and affects another part of the same individual • Plant hormones are chemical substances that control a plant’s patterns of growth and development, and the plant’s responses to environmental conditions. • Hormone moves through the plant from the place where it is produced to the place where it triggers its response, known as target cell or target tissue.
Hormones and Plant Growth • Charles Darwin led to the discovery of the first plant hormone. • Phototropism- the tendency of a plant to grow toward a source of light. • Darwins suspected the tip of each seedling produced substance that regulated cell growth, now named auxins. • Auxins are produced in the apical meristem and are transported downward into the rest of the plant. They stimulate cell elongation.
Hormones and Plant Growth • Auxins are also responsible for gravitropism, which is the response of a plant to the force of gravity. • Auxins also regulate cell division in meristems. • As a stem grows in length, it produces lateral buds. • A lateral bud is a meristematic area on the sides of a stem that gibes rise to side branches. • Because auxins move out from the apical meristems. The closer a bug is to the stem’s tip, the more it is inhibited. The phenomenon is called apical dominace.
Hormones and Plant Growth • When the tip is removed from a plant, the apical meristem-the source of the growth-inhibiting auxins-goes with it. • Without the influence of auxins, meristems in the side branched grow more rapidly, changing the overall shape of the plant.
Hormones and Plant Growth • Chemists have produced many compounds that mimic the effects of auxins. Because high concentrations of aucins inhibit growth, many of these compunds are used as herbicides, which are compunds that are toxic to plants.
Hormones and Plant Growth • Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds. • In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.
Hormones and Plant Growth • Gibberellin - fungus produced a growth-promoting substance • Gibberellins produce dramatic increases in size, particularly in stems and fruits. • Ethylene – one of the minor components of natural gas. • In reponse to auxins, fruits tissues release small amounts of hormone ethylene. Ethylene then stimulates fruits to ripen.
Plant Responses • Tropisms – reponses of plants to external stmuli • Plant tropisms include gracitropism, phototropism, and the thigmotropism. Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch.
Plant Responses • Thigmotropism – response of plants to touch • Short-day plants – plants such as chrysanthemums and poinsettias flower when days are short • Long-day plants – plants such as spinach and irises flower when days are long
Plant Responses • Photoperiodism – plants response to periods of light and darkness • Phytochrome – plant pigment • Dormancy – period during which an organism’s growth and activity decrease or stop • As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plants.
Plant Responses • Auxin production drops, but the production of ethulene increases when days become shorter and nights become longer. • Abscission layers of cells at the petiole seals the leaf off from the plant’s vascular system.
Plant Adaptations • To take in sufficient oxygen, many aquatic plants have tissues with large air filled spaces through which oxygen can diffuse. • Xerophytes –plants that live in the desert • Plant adaptation to a deset climate include extensive roots, reduced leaves, and thick stems that can store water.
Plant Adaptations • Plants that have specialized features for obtaining nutrients include carnivorous plant and parasites. • Carnivorous plants – plants live in bogs, wet and acidic environments where there is very little or no nitrogen present • Parasites- extract water and nutrients directly from a host plant
Plant Adaptations • Epiphytes – plants that are not rooted in soil but instead grow directly on the bodies of other plants • Many plants defend themselves against insect attack by manufacturing compounds that have powerful effects on animals.