TOEFL Reading: ETS-TOEFL阅读机经 - UF0EWP33293533CCL$

For trees to remain healthy as they grow and as circumstances change, tree branches must change in various ways. A. Trees can benefit from shedding inefficient branches that consume more carbohydrates than they produce or that are a major source of water loss. B. Branches can be lost as a result of damage from whether, animals, or disease, but they can also be shed when they are no longer useful, much as leaves are shed in autumn. C. While branch shedding is common and may be necessary for a tree's survival, the corky layer that forms at sites where branches have been shed prevents additional growth in those areas for several years. D. Shedding large branches is such a desperate act for survival that the creosote bush is one of the few plants to use this mechanism for removing wood. E. Larger trees can self-propagate when water stress and heat expansion break off branches, as long as the shed branches fall on or can be transported to a location favorable for taking root. F. Trees prevent branch overcrowding in part by varying branch length through internal mechanisms such as having shoots that can switch from long to short or from short to long, as needed.