AccelePrep for the ACT Test 2nd Edition Student Text

288 • P OWER U P ! T HE E XCLUSIVE C AMBRIDGE S AMPLE E XAM

The growth of the coleoptiles entirely immersed in water is attributable to a residuum of growth substance that remained after their removal from the plants. Those whose tops extended into the air produced additional growth substance. Experiment 2 Coleoptile segments were submerged in growth substance solutions of varying concentrations and the growth was measured after 2, 4, and 24 hours. The cumulative growth of each coleoptile segment as a percentage of the original length was calculated after 2, 4, and 24 hours. The results are summarized in Table 2. Table 2: Growth of Coleoptile Segments in Growth Substance Solutions Cumulative Growth Growth Substance Concentration (standard units per cm 3 ) End of Hour n 2 4 24 80 3.3 2.1 0.4 40 4.3 8.0 7.2 20 7.4 10.8 15.4 10 11.7 19.9 31.0 1 8.4 15.7 27.0 0.1 6.5 12.1 17.5 0.01 4.5 7.0 15.5 0 3.3 5.6 11.9 Coleoptiles in solutions of high concentrations of growth substance showed a shrinkage after 4 hours and, at the end of 24 hours, lost their turgidity due to a toxic effect of the high concentration of growth substance. Experiment 3 The researchers theorized that either the action of the growth substance is a simple physical change of the cell wall or it depends on processes of a metabolic nature. If the action of the growth substance were a simple physical change, then it would not be affected by the presence of cyanide. If, however, it depends on metabolic processes, cyanide should inhibit the action of the growth substance. A series of tests were conducted to determine the impact of potassium cyanide (KCN) on the growth of coleoptile segments. Table 3 summarizes the results.

PASSAGE II The protective sheath that covers the emerging shoot tip of monocotyledons, such as grasses, is called the coleoptile. Coleoptiles consist of specialized cells that do not divide but increase in size as they accumulate water. When the coleoptile pushes above the soil surface, it stops growing as the ϐŽƒ‰ Ž‡ƒ˜‡• ‡‡”‰‡ ˆ”‘ ‹– ƒ†…‘–‹—‡ –‘ ‰”‘™Ǥ

Figure 1

To study the impact of conditions on the growth rate of coleoptiles, Avena (oat) plants were grown in sand in the dark at 25°C and 85–90% relative humidity and harvested when 4 days old. Shoot tips 3 to 5 millimeters long were removed from the plants 2 hours before being immersed in water or growth solutions prepared from the fungus Rhizopus suinus . Growth substance concentrations were measured in standard units per cubic centimeter. Experiment 1 Coleoptile segments were immersed in pure water and the growth measured at the end of each hour for 7 hours. The test was repeated with the tops of the segments above the water surface. The growth of each coleoptile segment per hour as a percent of the original length was calculated. The averages across all segments for both tests are summarized in Table 1. Table 1: Growth of Coleoptile Segments in Water Average Growth per Hour as a Percentage of Original Length Test End of Hour n 1 2 3 4 5 6 7 Total 1 (tops in water) 1.8 1.5 1.2 0.7 0.6 0.4 0.4 6.6 2 (tops in air) 2.5 2.5 4.0 4.3 4.7 3.3 – 21.3

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