the elevated temperatures typical of cities to mean more bitter clover downtown than toward the outskirts. He was startled to find just the opposite. The results couldn’t be explained by fewer clover-eaters downtown. Grazers, mostly snails, were no more likely to damage clover at the urban edge than toward the city center, the researchers found.
The same unexpected pattern of cyanide defense increasing away from the city center also showed up in Boston and New York, but not Montreal. The explanation, Thompson and his colleagues now propose,
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, lies in what happens during winter. ??
Urban snow cover can insulate plants from the worst of winter’s stinging cold, a special threat for cyanide-carrying clover. Cyanide is toxic to plant tissue as well as to its predators. At temperatures above freezing,
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, clover leaves and stems aren’t at risk because cells contain only the molecules that will eventually form cyanide. Stored separately, the ingredients usually don’t come together in their dangerous form until an animal bites,
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, crushing and ripping cells. Such a defense damages some plant tissue but the sacrifice can save the rest of the plant. Freezing, however, ruptures the safety system and sets the cyanide loose on the clover for no apparent benefit.
In Toronto, Boston and New York, the buildup of urban heat thins the protective snow cover,
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, more so downtown than toward city margins. The closer to the city centers a clover plant grows,
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, the less likely it is set up a cyanide defense and risk poisoning itself during hard freezes,
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, Thompson said. Montreal,
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, however,
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,

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