plants for light, water, nutrients, and space. This strategy will fail when environmental conditions change rapidly. If a certain bug quickly takes advantage and kills all of the nearly identical perennials, then there will be a far lesser chance that a random mutation will slow the bug compared to more diverse annuals. Plant self-pruning 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