lz， 你的4篇是这个吗，请回复，多谢 GWD5-Q35 to Q37: 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. -- by 会员 pengzq (2010/1/13 23:36:51)
就是这个。。。原来是gwd里的啊。。 | 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2010-1-13 23:09:10
楼主
