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Dendrochronology, the study of tree-ring records to glean information about the past, is possible because each year a tree adds a new layer of wood between the existing wood and the bark. In temperate and subpolar climates, cells added at the growing season's start are large and thin-walled, but later the new cells that develop are smaller and thick-walled; the growing season is followed by a period of dormancy. When a tree trunk is viewed in cross section, a boundary line is normally visible between the small-celled wood added at the end of the growing season in the previous year and the large-celled spring wood of the following year's growing season. The annual growth pattern appears as a series of larger and larger rings. In wet years rings are broad; during drought years they are narrow, since the trees grow less. Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions. However, trees that grew in areas with a steady supply of groundwater show little variation in ring width from year to year; these "complacent" rings tell nothing about changes in climate. And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count. Certain species sometimes add more than one ring in a single year, when growth halts temporarily and then starts again.
The passage suggests which of the following about the ring patterns of two trees that grew in the same area and that were of different, but overlapping, ages? (A) The rings corresponding to the overlapping years would often exhibit similar patterns. (B) The rings corresponding to the years in which only one of the trees was alive would not reliably indicate the climate conditions of those years. (C) The rings corresponding to the overlapping years would exhibit similar patterns only if the trees were of the same species. (D) The rings corresponding to the overlapping years could not be complacent rings. (E) The rings corresponding to the overlapping years would provide a more reliable index of dry climate conditions than of wet conditions.
In the highlighted text, "uncertainties" refers to (A) dendrochronologists' failure to consider the prevalence of erratic weather patterns (B) inconsistencies introduced because of changes in methodology (C) some tree species' tendency to deviate from the norm (D) the lack of detectable variation in trees with complacent rings (E) the lack of perfect correlation between the number of a tree's rings and its age
The passage is primarily concerned with (A) evaluating the effect of climate on the growth of trees of different species (B) questioning the validity of a method used to study tree-ring records (C) explaining how climatic conditions can be deduced from tree-ring patterns (D) outlining the relation between tree size and cell structure within the tree (E) tracing the development of a scientific method of analyzing tree-ring patterns | 
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发表于 2008-7-10 02:31:00
