Even more than mountainside slides of mud or snow, naturally occurring forest fires promote the survival of aspen trees. Aspens’ need for fire may seem illogical (5) since aspens are particularly vulnerable to fires; whereas the bark of most trees consists of dead cells, the aspen’s bark is a living, functioning tissue that—along with the rest of the tree—succumbs quickly (10) to fire.
The explanation is that each aspen, while appearing to exist separately as a single tree, is in fact only the stem or shoot of a far larger organism. A group of thousands of aspens can actually constitute a single organism, called a clone, that shares an interconnected root system and a unique set of genes. Thus, when one aspen—a single stem—dies, the entire clone is affected. While alive, a stem sends hormones into the root system to suppress formation of further stems. But when the stem dies, its hormone signal also ceases. If a clone (25) loses many stems simultaneously, the resulting hormonal imbalance triggers a huge increase in new, rapidly growing shoots that can outnumber the ones destroyed. An aspen grove needs to (30) experience fire or some other disturbance regularly, or it will fail to regenerate and spread. Instead, coniferous trees will invade the aspen grove’s borders and increasingly block out sunlight needed by the aspens.
The primary purpose of the passage is to explain the
A. qualities that make a particular organism unique
B. evolutionary change undergone by a particular organism
C. reasons that a phenomenon benefits a particular organism
D. way in which two particular organisms compete for a resource
E. means by which a particular organism has been able to survive in a barren region
The explanation is that each aspen, while appearing to exist separately as a single tree, is in fact only the stem or shoot of a far larger organism.
It can be inferred from the passage that when aspen groves experience a “disturbance” (line 30), such a disturbance
A. leads to a hormonal imbalance within an aspen clone
B. provides soil conditions that are favorable for new shoots
C. thins out aspen groves that have become overly dense
D. suppresses the formation of too many new aspen stems
E. protects aspen groves by primarily destroying coniferous trees rather than aspens
Theauthor of the passage refers to “the bark of most trees” (line 6) most likely inorder to emphasize the A. vulnerability of aspens to damage from fire when comparedto other trees B. rapiditywith which trees other than aspens succumb to destruction by fire C. relativelygreat degree of difficulty with which aspens catch on fire when compared toother trees D. differencein appearance between the bark of aspens and that of other trees E. benefitsof fire to the survival of various types of trees
LZ 喜欢大火的 白杨树 是这篇咩 | 
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发表于 2016-9-19 06:24:42
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