Referring to Example 219 Suppose the capacitance of the capacitor is reduced by a factor of 2. The two resistors are and , and the battery has an emf of 3.00 V. (a) Find the nal value of the charge on the capacitor. (b) Does the time for the capacitor to charge to 80.0% of its nal value increase, decrease, or stay the same? Explain. (c) Find the time referred to in part (b).
Chapter 22 Notes – Early Development of the Plant Body -‐ Embryogenesis: first two phases of seed development; establishes body plan of the plant; accompanied by seed development o Apical-‐basal pattern along main axis o Radial pattern concentrically arranged tissue -‐ Formation of embryo begins with division of zygote within embryo sac of ovule o First division is asymmetrical and transverse to long axis; apical-‐basal polarity is established § Chalazal pole has apical cell that gives rise to most of mature embryo § Micropylar pole has basal cell that produces suspensor that anchors embryo at micropyle • Polarity fixes the structural axis of the body • Through orderly divisions, embryo eventually becomes embryo proper and suspensor. • Developing embryo before this is called proembryo -‐ Changes in internal structure of embryo proper result in concentrically arranged tissue systems (first radial polarity) o Protoderm formed by periclinal divisions (divisions parallel to surface) in outermost cells of embryo proper § Vertical divisions within embryo proper result in ground meristem and procambium. • Ground meristem (precursor to ground tissue) surrounds procambium (precursor to xylem and phloem) • Primary meristems (Protoderm, ground meristem, and procambium) extend into other regions of embryo -‐ Globular stage: when embryo is spherical, preceding cotyledon development o Heart shape form as cotyledons develop into eudicots (heart stage) o In monocots, embryos become cylindrical and have only one cotyledon o In both monocots and eudicots, apical-‐basal pattern of embryo proper first becomes discernible prior to emergence of cotyledons § Axis partitioned into shoot apical meristem, cotyledons, hypocotyl, embryonic root, and root apical meristem o Torpedo stage: cotyledons and axis elongate, primary meristems extend along with them o Apical meristems found at tips of all shoots and roots; repeated division -‐ Angiosperm suspensors are metabolically active; support early development of embryo proper by providing nutrients and growth regulators o Plasmodesmata connect cells of suspensor with those of developing embryo proper o Suspensor undergoes programmed cell death o In mutants: embryo proper is presumed to transmit specific inhibitory signals to suspensor that suppress its development into an embryo -‐ Very promising results obtained in identifying genes responsible for events in Arabidopsis embryogenesis -‐ At opposite ends of embryo axis are apical meristems of shoot and root o Some embryos, only apical meristem occurs above attachment of cotyledons o Others, embryonic shoot, consisting of epicotyl with one or more young leaves and apical meristem, occurs above cotyledons. § Plumule: first bud; embryonic shoot § Stem-‐like axis below cotyledons: hypocotyl § Radicle: lower end of embryonic root § If radicle cannot be distinguished in embryo, embryonic axis below cotyledon is called hypocotyl-‐root axis -‐ Most or all of food-‐storing endosperm is absorbed by developing embryo à develop large, fleshy cotyledons that nourish embryo -‐ In monocots, single cotyledon functions as food-‐storing/photosynthetic organ, and absorbs food digested from endosperm by enzymatic activity o When fully formed, grass embryo has massive cotyledon, scutellum. Radicle and Plumule enclosed by protective structures called coleorhiza and coleoptile o Outer covering in grass seed are pericarp (mature ovary wall) and remnants of seed coat o Micropyle associated with scar, hilum (seed separation from stalk) -‐ Seed coat: outer coat of seeds which develops from integuments of ovule and provides protection for embryo (in many seeds, very hard and highly impermeable to water) -‐ Maturation phase: second phase of seed development; buildup of food reserves in endosperm, perisperm, or cotyledons o Seed undergoes desiccation as it loses water o Seed coat hardens o Seeds of some plants then enter quiescent state; others become dormant -‐ Germination affected by external factors like water, oxygen, and temperature o Most seeds really dry; germination doesn’t begin until enzymes already present in seed are activated by water; new enzymes synthesized for digestion of stored foods -‐ During early stages of germination, glucose breakdown may be entirely anaerobic, but as soon as the seed coat is ruptured, the seed switches to an aerobic pathway -‐ Many seeds germinate over fairly wide range of temperatures but most have minimum temp between 0-‐25°C and maximum temp between 45-‐48°C -‐ Dormant seeds fail to germinate even when external conditions are favorable o Coat-‐imposed dormancy: impermeability of seed coat to water or oxygen; rigidity of seed coat; conifers, most cereals, and several eudicots o Embryo dormancy: ratio of abscisic acid and gibberellic acid o After-‐ripening: physiologically immature seeds undergo complex series of enzymatic and biochemical changes before they germinate o Dormancy acquired during seed maturation is primary dormancy; seeds that are no longer dormant but encounter unfavorable conditions may be induced to secondary dormancy o Burls or lignotubers on shrubs and small trees contain dormant buds that sprout after shoots are damaged by fire -‐ When germination occurs, the first structure to emerge from most seeds is the root o Primary root, or taproot, continues to grow and develops branch roots, or lateral roots o Root system of adult plant develops from stem-‐borne roots, which arise at nodes and produce lateral roots -‐ Garden bean: root emerges from seed, hypocotyl elongates and becomes bent, hook reaches soil surface, straightens out, pulls cotyledons into air; epigeous -‐ Pea: epicotyl elongates and forms hook, straightens out; Plumule is raised above soil surface; cotyledons remain underground; hypogeous -‐ Majority of monocot seeds, stored food is found in endosperm -‐ Maize: coleorhiza encloses radicle, first structure to grow through pericarp -‐ Period from germination to time seedling becomes established as independent organism is most crucial phase in life history of plant