Plant Responses and Adaptations Chapter 25: Biology II

Plant Hormones Chemical substances that control: A plant’s patterns of growth and development A plant’s responses to environmental conditions Target Cell: cell affected by a particular hormone

Auxins Stimulate cell elongation Produced in the apical meristem Transported downward into the rest of the plant Responsible for: phototropism and gravitropism

Auxins: Phototropism Tendency of plants to grow toward a source of light

Auxins: Gravitropism Tendency of a plant to grow in a direction in response to the force of gravity

Auxins Lateral Bud: meristematic area on the side of a stem that gives rise to side branches Apical Dominance: phenomenon in which the closer a bud is to the stem’s tip, the more its growth is inhibited Herbicide: auxin-like weed killers; compound that is toxic to plants

Cytokinins Plant hormones that are produced in growing roots and in developing fruits and seeds Stimulate cell division and the growth of lateral buds Cause dormant seeds to sprout

Gibberellins Growth-promoting substance produced by plants Causes dramatic increases in size, particularly in stems and fruits Produced by seed tissue Responsible for the rapid early growth of many plants

Ethylene Plant hormone that stimulates fruits to ripen Released in small amounts as a response to auxins Commercial products can use this to control the ripening process; treated with synthetic ethylene

Tropisms The response of plants to environmental stimuli Gravitropism Phototropism Thigmotropism

Thigmotropism Response of plants to touch Examples: A plant that is touched regularly may be stunted in its growth-sometimes quite dramatically Vines and climbing plants-tips wrap around objects

Rapid Responses Some responses do not involve growth; they are not tropisms Examples: Mimosa pudica: the “sensitive plant” The Venus flytrap

Photoperiodism Responsible for the timing of seasonal activities such as flowering and growth Phytochrome: plant pigment responsible for photoperiodism

Short-Day Plants Plants that flower when daylight is short Also called “long-night plants”

Long-Day Plants Plants that flower when days are long Also called “short-night plants”

Winter Dormancy Period of time during which a plant embryo is alive but not growing As cold weather approaches, deciduous plants: Turn off photosynthetic pathways Transport materials from leaves to roots Seal leaves off from the rest of the plant

Abscission Layer Layer of cells at the petiole that seals off a leaf from the vascular system

Aquatic Plants To take in sufficient oxygen, many aquatic plants have tissues with large air-filled spaces through which oxygen can diffuse

Salt-Tolerant Plants Leaves have specialized cells that pump salt out of the plant tissues and onto the leaf surfaces Example: mangroves

Desert Plants Also called: xerophytes Extensive roots Reduced leaves Thick stems that can store water

Nutritional Specialists Plants that have specialized features for obtaining nutrients Carnivorous plants: obtain nutrients from digested prey Parasites: plants that extract water and nutrients directly from a host plant The dodder plant Cuscuta

Epiphytes Plants that grow directly on the bodies of other plants Gather their own moisture and produce their own food, unlike parasitic plants Spanish moss-not a moss, but a bromeliad!

Chemical Defenses Many plants defend themselves against insect attack by manufacturing compounds that have powerful effects on animals