36.1 Plant embryo development establishes a basic body plan.
Establishing the Root-Shoot Axis
• As plant development proceeds, cells with multiple potentials are mainly restricted to meristem regions. (p. 756)
• During embryogenesis, the root-shoot axis develops, cell differentiation occurs, and three basic tissue systems are established. Two other critical events also occur: establishment of an embryological food supply, and differentiation of ovule tissue to form a hard, protective embryo covering. (p. 756)
Establishing Three Tissue Systems
• Three basic tissues differentiate while the plant embryo is in the globular stage. (p. 758)
• The root and shoot both arise from apical meristems, but their formation is independently controlled. (p. 758)
• Morphogenesis results from changes in planes and rates of cell division. (p. 759)
• Seed storage molecules such as starch, lipids, and proteins were the first cloning targets because they are so abundant throughout embryogenesis. (p. 759)
36.2 The seed protects the dormant embryo from water loss.
How Seeds Form
• Early in development, the angiosperm embryo stops developing, and the integuments develop into a relatively impermeable seed coat. (p. 760)
• Seeds are adaptively important because they maintain dormancy under unfavorable conditions, protect young plants during vulnerable stages, store food for young plants, and facilitate dispersal. (p. 760)
• Other specific adaptations help ensure that seeds will only germinate under specific conditions. Such adaptations include tough cones and the presence of inhibitory chemicals in the seed coats. (p. 760)
36.3 Fruit formation enhances the dispersal of seeds.
How Fruits Form
• Fruits are mature ovaries (carpels). (p. 761)
• Fruits contain the genotypes of the prior sporophyte generation (fruit and seed coat), the prior gametophyte generation (parts of the seed), and the next sporophyte generation (embryo). (pp. 761-762)
• Fruits can exhibit a wide variety of specialized dispersal methods, including hooked spines, wings, and light feathery appendages. These forms allow dispersal by such methods as animals, wind, and water. (p. 762)
36.4 Germination initiates post-seed development.
Mechanisms of Germination
• Germination begins when a seed absorbs water and resumes metabolism. (p. 763)
• A dormant seed that has imbibed water and is carrying on normal metabolism may still not germinate without an additional environmental signal such as low temperature. (p. 763)
• Gibberellic acid and abscisic acid mediate the utilization of food reserves in the embryo or endosperm. (p. 763)