Acid Growth Hypothesis - Explains phototropism by suggesting that increased acidity in the walls of certain cells (stimulated by the hormone auxin) increases their flexibility and expandability, so that more water can diffuse into the cells and cause cell elongation.

Active Transport - Movement of substances across cell membranes that requires energy expenditure on the part of the cell; contrasts with passive diffusion, or osmosis.

Apoplast - The pathway from the root surface to the core by which water moves along cell walls and through intercellular spaces, bypassing the cells themselves.

Auxin - One in a class of plant hormones that stimulates (among other things) cell elongation, secondary tissue growth, and fruit development.

Cytokinin - One in a class of plant hormones that promotes cell division and tissue growth.

Day-Neutral Plant - Plant in which blooming is not affected by photoperiod, so that flowering occurs independently of the duration of day and night.

Ethylene - A plant hormone that controls fruit ripening and promotes senescence (aging).

Florigen - Name given to the hypothetical hormone that might control flowering in plants.

Gibberellin - One of a class of plant hormones that stimulates stem elongation, germination, and conversion of the embryonic food source into usable sugars.

Gravitotropism - Reaction of a plant to gravity; a stem grows against gravity, roots toward gravity.

Hormone - A hormone is a chemical that affects the ways in which an organism functions; it is produced in one part of the plant body but, by traveling to target cells throughout the body, affects many other parts as well.

Inhibitor - One in a class of plant hormones that inhibits growth and prolongs dormancy in buds and seeds.

Leaf Abscission - Hormone-stimulated leaf loss; caused by the formation of a weak, thin-walled abscission layer at the base of the leaf.

Long-Day Plant - Plant in which blooming is affected by photoperiod so that flowering occurs when the hours of darkness in a 24-hour photoperiod fall below a certain level.

Osmosis - The passive diffusion of water across a membrane. Osmotic concentration refers to the concentration of solutes (dissolved substances) in the water; when the osmotic concentrations of two regions differ, water will flow from the area of low concentration to the area of high concentration. In contrast, the solutes themselves will flow from areas of high osmotic concentration to areas of low osmotic concentration.

Phloem - Vascular tissue composed of cells that are living at maturity; transports the products of photosynthesis throughout the plant body.

Photoperiodism - An organism's response to the length of day and night within a 24-hour period (photoperiod); in many plants, this phenomenon determines when flowering will occur.

Photosynthesis - The process by which plants and other autotrophic organisms convert light energy into vital organic materials.

Phototropism - The growth of a plant toward a light source, resulting from the rapid elongation of cells on the dark side of the plant; stimulated by auxin.

Phytochrome - Pigment in leaves that allows them to measure the duration of day and night.

Pressure Flow - The mechanism by which sugars are transported through the phloem, from sources to sinks; dependent upon the high turgor pressure of sources and the low turgor pressure of sinks.

Root Hair - An outgrowth of a plant root that provides an increased surface area for the absorption of water and dissolved minerals from the soil.

Short-Day Plant - Plant in which blooming is affected by photoperiod so that flowering occurs when the hours of darkness in a 24-hour photoperiod rise above a certain level.

Sieve Element - A living conductive cell of phloem.

Sink - Regions of the plant, such as growing tissues, that are in need of nutrients; characterized by low turgor pressure.

Source - Nutrient-rich region, such as a leaf, that supplies sugars for the rest of the plant; characterized by high turgor pressure.

Symplast - The pathway from the root surface to the core by which water enters the root hair membrane and travels through the cytoplasm of adjacent cells, via channels that connect their contents.

Target Cell - A cell that receives hormone signals.

TATC - Transpiration-Adhesion-Ttension-Cohesion; the mechanism by which scientists theorize that fluids are pulled upward through the xylem (driven by transpiration, the evaporation of water from the leaf, and the cohesion between water molecules).

Thigmotropism - Reaction of a plant to touch; results from differential cell elongation.

Transpiration - The process by which a plant loses water to its environment through evaporation.

Tropism - Long-term growth of a plant toward or away from a stimulus as a result of differential cell elongation.

Turgor Movement - Relatively rapid, easily reversible plant movement, occurring in response to a stimulus, that results from changes in turgor pressure in certain plant cells.

Turgor Pressure - The force that the contents of a plant cell exert on the cell wall after the osmotic entry of water into the cell.

Vascular System - Mechanism of internal water and nutrient transport, made up of the vascular tissues xylem and phloem, that is characteristic of tracheophytes.

Vascular Tissue - A conductile component (either xylem or phloem) of the system that transports food and nutrients throughout the plant body.

Water Potential - The pressure that causes water to move across a membrane; water always moves naturally from areas of higher water potential to those of lower water potential.

Xylem - Vascular tissue composed of cells that are dead at maturity; transports water and dissolved minerals upwards from the roots to the shoot.

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