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  3. Cities are often described as being alive. A nice metaphor, but does it mean anything? And, if it does, can town planners and bi

Cities are often described as being alive. A nice metaphor, but does it mean anything? And, if it does, can town planners and bi

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问题     Cities are often described as being alive. A nice metaphor, but does it mean anything? And, if it does, can town planners and biologists learn from one another? Steven Strogatz, a mathematician at Cornell University, wrote last year that Manhattan and a mouse might just be variations on a single structural theme. His point was that both are, in part, composed of networks for transporting stuff from one place to another. Roads, railways, water and gas mains, sewage pipes and electricity cables all move things around. So do the blood vessels of animals and the sap-carrying xylem and phloem of plants. How far can the analogy be pushed?
    Peter Dodds of the University of Vermont draws a particular analogy between the blood system and a suburban railway network. The commuter-rail system of a city ramifies from the centre. The farther out you go, the sparser it is. By analogy, Dr. Dodds predicted, the network of capillaries would not be as dense in large animals as it is in small ones. They, too, branch ultimately from a central source — the heart. Surprisingly, no one had looked for this before, but in a paper published recently in Physical Review Letters Dr. Dodds shows that this does indeed turn out to be the case.
    Dr. Dodds’s calculations overthrow a 70-year-old rule of thumb which is known as the 3/4 law of metabolism. This suggests energy expenditure is proportional to body mass raised to the power of three-quarters. That a mouse expends more energy per gram than an elephant does is well known. But Dr. Dodds’s calculations show that metabolic rates must fall off faster than had previously been believed as animals get bigger because less glucose than thought is being transported by the smaller than predicted capillary network. The law needs to be adjusted to something more like two-thirds.
    Two other studies published in the same volume similarly overthrow conventional wisdom about plants. Traditionally, biologists have celebrated the trunk, branch and twig system of a tree as no accident. Many mathematical formulas have suggested it is the best, least wasteful way to design a distribution network. But the very end of such a network, the leaf, has a different architecture. Unlike the xylem and phloem, the veins in a leaf cross-link and loop. Francis Corson of Rockefeller University in New York used computer models to examine why these loops exist.
    From an evolutionary point of view, loops seem inefficient because of the redundancy inherent in a looped network. Dr. Corson’s models show, however, that this inefficiency is true only if demand for water and the nutrients it contains is constant. By studying fluctuations in demand he discovered one purpose of the loops: They allow for a more nuanced delivery system. Flows can be rerouted through the network in response to local pressures in the environment, such as different evaporation rates in different parts of a leaf.
    The leaf, then, is a resilient distribution network — one whose principles could be applied to, say, electricity grids. Next time your power is cut off because a tree has fallen on the cable, remember that.
Which of the following has NOT pushed Steven Strogatz’s analogy that Manhattan and a mouse might be variations on a single structural theme?
选项 A、An analogy drawn by Dr. Dodds.
B、The 3/4 law of metabolism.
C、Dr. Corson’s computer models.
D、Two other studies published recently in Physical Review Letters.
答案B
解析 推断题。从第三段第一句可以判断Dr．Dodds的计算推翻了传统的新陈代谢法则，说明A和B两者对立。从第二段看A进一步细化研究了Strogatz的类比，则答案选B。第四段和第五段分别说明D和C提到的研究是支持Strogatz的论点的。
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