标题: 北美1月3号，归来机经 [打印本页] 作者: whiskeyzc 时间: 2012-1-4 07:30 标题: 北美1月3号，归来机经 600出头的分数。想想自己前前后后也就复习了半个多月，模考完连错题都没看，也可以接受了。 argument，人们在non－profit的情况下，有更高的motivation，更好的完成工作，建议公司大量的钱用于慈善，提高员工效率。 issue，Financial gain is the most important factor in career choosing。 数学，记住jj的一道，-203到200的和，－606。 5个整数，平均数为14，a<b<c<d<e,e是26，求最小的中间数，答案应该是8。
Brower has perfected a technique for tracking monarchs throughout the rest of the year, based on the butterflies' special relationship with milkweed plants. Monarchs lay their eggs on milkweed and, as larvae, eat the plants not only for nutrition but also for the toxins in the sap, which is poisonous to birds that prey on butterflies. The toxic compounds, known as cardiac glycosides, accumulate in larval tissues and persist after the larvae metamorphose into butterflies, making monarchs unpalatable to birds. "I've tasted the sap of the sandhill milkweed," says Brower, "and it's incredibly nauseating." Brower realized in the mid-1970s that he could use these poisons to understand monarch migration. The specific glycosides present in different species of milkweed vary from place to place, so analysis of a plant's glycosides can be used to determine its site of origin. Likewise, the glycosides in adult butterflies are a fingerprint of the plants on which the insects fed as larvae. "The light bulb went on," says Brower. His idea: If monarchs reflect the different arrays of glycosides in milkweeds, then he should be able to analyze butterfly glycosides and correlate individual insects with particular species of milkweed and with certain regions where the milkweed grew. Traveling around the country, Brower trapped monarchs everywhere from the Great Lakes to Mexico and studied their glycosides. "We collected monarchs in the fall migration and established that over 90 percent fed on the northern milkweed pattern," he says. "Then we sampled the butterflies in Mexico and the ones along the Gulf Coast at the end of March and early April and, lo and behold, all of them had the same pattern. The same butterflies were going to Mexico and coming back in the spring." Brower's work on glycosides helped to reveal the entire pattern of monarch migration. The first generation of monarch butterflies to return from Mexico stops along the Gulf Coast from Texas to Florida, where the insects lay eggs and die. The eggs hatch, and the new monarchs (with their distinctive southern milkweed fingerprint) head for the Great Lakes region. Later, the new generation of monarchs born around the Great Lakes flies to the East Coast, where they breed and die. Their offspring, fed on eastern milkweeds, then head south toward the Gulf Coast and on to Mexico, traveling as far as 90 miles per day and completing the great circle of monarch migration.作者: shaoheng1120 时间: 2012-1-8 12:41
顶，LS作者: suntao0209 时间: 2012-1-8 20:02
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