1. Most plant cells are very reversible in the aspect of differentiation because gene expression can be reactivated, leading to alternate differentiation or growth of the whole plant from an isolated group of cells. The limitation is that the cell must retain the protoplast at maturity. This is termed totipotent. Steward excised secondary phloem cells, and when they were cultured in liquid nutrient medium they grew roots, whereas when they were placed on solid medium, they grew shoots and developed into complete plants.
2. Both plant and animal hormones are chemical substances that are produced in small quantities in one region and transported to another region where they result in physiological responses. Plant hormones are not produced as definite sites solely involved with the production of that hormone, whereas those of animals are. The five major plant hormones are auxin, cytokinins, gibberellins, ethylene, and abscisic acid.
3. The only naturally occurring auxin is indoleacetic acid (IAA); it is synthesized from tryptophan. It is produced in the shoot apex and transported downward, not by vascular tissue, but by an unknown mode of transport.
4. Auxin increases the plasticity of the plant cell wall, allowing greater stretching during active cell growth. Indoleacetic acid oxidase degrades IAA, preventing its accumulation. Auxin acts rapidly in response to light.
5. Synthetic auxins are used to prevent fruit drop, hold berries on holly, promote flowering and fruiting in pineapples, induce the formation of roots on cuttings, and as a herbicide. 2,4-D is a substance related to the auxins and IAA. Commercially it is used as a weed killer; it acts upon dicots to ``grow themselves to death.''
6. Cytokinins stimulate cell division and regulate differentiation. Cytokinins promote lateral bud formation and inhibit lateral root formation, which are opposite to the affects of auxins. Cytokinins are produced by roots and developing fruit, derived from the biomolecule adenine.
7. Gibberellins is usually lacking in genetically dwarfed plants. If it is, the application of gibberellins will restore normal growth. The three secondary effects of this hormone are (1) to stimulate the production of seed amylase, (2) to induce biennial plants to flower, and (3) to hasten seed germination, substituting for the effects of cold or light. The three commercial uses are (1) to increase fruit and cluster size in grapes, (2) to delay ripening of citrus fruits and speed the flowering of strawberries, and (3) to stimulate the digestion of starches in brewing.
8. Ethylene is the main factor in forming the separation layer between the stem and leaf. It hastens fruit ripening. The common gas that counteracts this effect is carbon dioxide. Commercially, ethylene hastens fruit drop, induces flowering in the pineapple family, and stimulates seed germination. In weed control products, ethylene stimulates the germination of weed seeds, and weeds can then be killed.
9. Abscisic acid is produced in mature green leaves and in fruit and root caps. If spotted on a green leaf, this hormone makes the leaf under the spot turn yellow; cytokinins reverse this effect. Abscisic acid induces the formation of winter buds, suppresses the growth and elongation of buds, and promotes senescence.
10. The stems are positively phototropic and grow toward the light. This allows the leaves to capture a greater amount of light energy. The roots are negatively phototropic and grow away from the light. This causes the roots to grow downward toward a potential supply of water and nutrients. Auxin most completely mediates this action.
11. Gravitropism refers to growth in response to gravity. Previously called ``geotropism,'' the name was changed to reflect gravity rather than position relative to the earth (``geo''). Roots are usually positively gravitropic while shoots are negatively gravitropic.
12. A pulvinus is a jointed, multicellular structure at the base of a leaf enabling it to move, such as in orientation toward the sun, sometimes even allowing the flower to ``track'' the sun throughout the day. Motor cells cause the movement to occur based on unequal turgor pressure on either side of the leaf. The sensitive plant further reacts with altered permeability to specific ions that allows the plant to respond very quickly to tactile stimulus.
13. A burst of red light at 660 nanometers inhibits flowering but can be cancelled by exposure to far-red light of 730 nanometers. The compound that mediates these reactions is pigment. One form absorbs red light, while the other form absorbs far-red light. Therefore, a flash of red light will convert to the far-red sensitive form that blocks flowering and a second flash of far-red light converts phytochrome to the red-sensitive form that does not interfere with flowering.