在2015年1月25日的
解析：
本文属于生物类文章，关注的是植物某一具体特征，在TPO里多动物行为，很少直接讨论植物某一具体特征的，但是从机经回忆来看，虽然文章相关背景知识学员不熟悉，但是文章的结构本身还是很清晰的，属于典型的因果型文章，在理解时，学员需要重点把握原因或者理论是什么，以及针对这个原因直接相关的要点论证。
参考阅读：
There is a speculative hypothesis on how and why a plant induces part of itself to die off. The theory holds that leaves and roots are routinely pruned off during the growing season whether they are annual or perennial. This is done mainly to mature leaves and roots and is for one of two reasons; either both the leaves and roots that are pruned are no longer efficient enough nutrient acquisition-wise or that energy and resources are needed in another part of the plant because that part of the plant is faltering in its resource acquisition.
Poor productivity reasons for plant self pruning - the plant rarely prunes young dividing meristematic cells, but if a fully grown mature cell is no longer acquiring nutrients that it should acquire, then it is pruned.
Shoot efficiency self pruning reasons - for instance, presumably a mature shoot cell must on average produce enough sugar, and acquire enough oxygen and carbon dioxide to support both it and a similar sized root cell. Actually, since plants are obviously interested in growing it is arguable, that the "directive" of the average shoot cell, is to "show a profit" and produce or acquire more than enough sugar and gases than is necessary to support both it and a similar sized root cell. If this "profit" isn't shown, the shoot cell is killed off and resources are redistributed to "promising" other young shoots or leaves in the hope that they will be more productive.
Root efficiency self pruning reasons - similarly a mature root cell must acquire on average, more than enough minerals and water needed to support both it and a similar sized shoot cell that does not acquire water and minerals. If this does not happen, the root is killed off and resources sent to new young root candidates.
Shortage/need-based reason for plant self pruning - this is the other side of efficiency problems.
Shoot shortages - if a shoot is not getting enough root derived minerals and water, the idea is that it will kill part of itself off, and send the resources to the root to make more roots.
Root shortages - the idea here is that if the root is not getting enough shoot derived sugar and gases it will kill part of itself off and send resources to the shoot, to allow more shoot growth.
This is an oversimplification, in that it is arguable that some shoot and root cells serve other functions than to acquire nutrients. In these cases, whether they are pruned or not would be "calculated" by the plant using some other criteria. It is also arguable that, for example, mature nutrient-acquiring shoot cells would have to acquire more than enough shoot nutrients to support both it and its share of both shoot and root cells that do not acquire sugar and gases whether they are of a structural, reproductive, immature, or just plain, root nature.
The idea that a plant does not impose efficiency demands on immature cells is that most immature cells are part of so-called dormant buds in plants. These are kept small and non-dividing until the plant needs them. They are found in buds, for instance in the base of every lateral stem.