fragmentation experiment show that loss of mechanical strength, even in this apparently resistant species, is considerable.
Most species appear to exhibit a higher rate of degradation in the stream environment than in the lake. This is perhaps most clearly shown in the case of Alnus. Examination of the type of destruction suggests that the cause for the greater loss of material in the stream-processed leaves is a combination of both biological and mechanical degradation. The leaves exhibit an angular fragmentation, which is characteristic of mechanical damage, rather than the rounded holes typical of the attack by large particle feeders or the skeletal vein pattern produced by microbial degradation and small particle feeders. As the leaves become less strong, the fluid forces acting on the stream nylon cages cause successively greater fragmentation.
Mechanical fragmentation, like biological breakdown, is to some extent influenced by leaf structure and form. In some leaves with a strong midrib, the lamina breaks up, but the pieces remain attached by means of the midrib. One type of leaf may break cleanly, whereas another tears off and is easily destroyed after the tissues are weakened by microbial attack.
In most species, the mechanical breakdown will take the form of gradual attrition at the margins. If the energy of the environment is sufficiently high, brittle species may be broken across the midrib, something that rarely happens with more pliable leaves. The result of attrition is that where the areas of the whole leaves follow a normal distribution, a bimodal distribution is produced, one peak composed mainly of the fragmented pieces, the other of the larger remains.
To test the theory that a thin leaf has only half the chance of a thick one for entering the fossil record, all other things being equal, Ferguson (1971) cut discs of fresh leaves from 11 species of leaves, each with a different thickness, and rotated them with sand and water in a revolving drum. Each run lasted 100 hours and was repeated three times, but even after this treatment, all species showed little sign of wear. It therefore seems unlikely that leaf thickness alone, without substantial microbial preconditioning, contributes much to the probability that a leaf will enter a depositional environment in a recognizable form. The results of experiments with whole fresh leaves show that they are more resistant to fragmentation than leaves exposed to microbiological attack. Unless the leaf is exceptionally large or small, leaf size and thickness are not likely to be as critical in determining the preservation potential of a leaf type as the rate of microbiological degradation.
1. The passage is primarily concerned with
The passage reads primarily about leaves, making that its primary concern, so eliminate Choices (C) and (D) right off. Choice (A) is too broad, as other causes of disintegration may exist that the passage doesn’t mention. Choice (B) is too specific: The passage mentions leaf structure, but not as its primary focus. Correct answer: Choice (E).
2. Which of the following is mentioned as a reason for leaf degradation in streams? Consider each of the three choices separately and select all that apply.
The second paragraph of the passage tells you that “loss of material in stream-processed leaves is a combination of biological and mechanical degradation.” Choice (C) is incorrect because the passage specifically states that the pattern of holes is contrary to that of large particle feeders. The correct answers are Choices (A) and (B).
3. The conclusion that the author reached from Ferguson’s revolving drum experiment was that
The middle of the last paragraph tells you that leaf thickness alone is unlikely to affect the final form of the leaf. You probably need to reread that sentence a few times to get past the jargon, but a detail or fact question is the type of question you should be sure to answer correctly. Choice (B) introduces facts not discussed in the passage; the passage doesn’t talk of leaves in mud or silt. Choice (C) is mentioned in the passage but not in Ferguson’s experiments.
Nothing about high temperatures appears in the passage, which eliminates Choice (D). Choice (E) sounds pretentious and pompous — and nice and scientific — but has nothing to do with Ferguson. To answer this question correctly, you need to return to the passage to look up Ferguson specifically, not merely rely on your memory of the passage as a whole. Correct answer: Choice (A).
4. The tone of the passage is
The
