剑桥雅思5 Test 4阅读Passage 3原文翻译 the effects of light on plant and animal species 光对动植物的影响
剑桥雅思5 Test 4 Passage 3阅读原文翻译
Light is important to organisms for two different reasons. Firstly it is used as a cue for the timing of daily and seasonal rhythms in both plants and animals, and secondly it is used to assist growth in plants.
Breeding in most organisms occurs during a part of the year only, and so a reliable cue is needed to trigger breeding behaviour. Day length is an excellent cue, because it provides a perfectly predictable pattern of change within the year. In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount. The seasonal impact of day length on physiological responses is called photoperiodism, and the amount of experimental evidence for this phenomenon is considerable. For example, some species of birds’ breeding can be induced even in midwinter simply by increasing day length artificially (Wolfson 1964). Other examples of photoperiodism occur in plants. A short-day plant flowers when the day is less than a certain critical length. A long-day plant flowers after a certain critical day length is exceeded. In both cases the critical day length differs from species to species. Plants which flower after a period of vegetative growth, regardless of photoperiod, are known as day-neutral plants.
大多数生物的繁殖仅在一年的部分时间内发生，因此需要可靠的线索来触发繁殖行为。日长是一个很好的提示，因为它提供了一年中变化的可预测规律。在春季的温带地区，每天的温度波动很大，但日长每天稳定增加特定的数量。日长对生理反应的季节性影响称为光周期，这种现象的实验证据数量可观。例如，即使是在冬季，也可以通过人工增加日长来诱导某些鸟类的繁殖（Wolfson 1964）。 光周期的其他例子存在于植物中。当白天短于特定的关键长度时，短日照植物就会开花。超过了某个关键的白天长度时，长日照植物就会开花。在这两种情况下，关键的日照长度因物种而异。在不考虑光周期的情况下，经过一段时间生长后开花的植物被称为日中性植物。
Breeding seasons in animals such as birds have evolved to occupy the part of the year in which offspring have the greatest chances of survival. Before the breeding season begins, food reserves must be built up to support the energy cost of reproduction, and to provide for young birds both when they are in the nest and after fiedging. Thus many temperate-zone birds use the increasing day lengths in spring as a cue to begin the nesting cycle, because this is a point when adequate food resources will be assured.
The adaptive significance of photoperiodism in plants is also clear. Short-day plants that flower in spring in the temperate zone are adapted to maximising seedling growth during the growing season. Long-day plants are adapted for situations that require fertilization by insects, or a long period of seed ripening. Short-day plants that flower in the autumn in the temperate zone are able to build up food reserves over the growing season and over winter as seeds. Day-neutral plants have an evolutionary advantage when the connection between the favourable period for reproduction and day length is much less certain. For example, desert annuals germinate, flower and seed whenever suitable rainfall occurs, regardless of the day length.
The breeding season of some plants can be delayed to extraordinary lengths. Bamboos are perennial grasses that remain in a vegetative state for many years and then suddenly flower, fruit and die (Evans 1976). Every bamboo of the species Chusquea abietifolia on the island of Jamaica flowered, set seed and died during 1884. The next generation of bamboo flowered and died between 1916 and 1918, which suggests a vegetative cycle of about 31 years. The climatic trigger for this flowering cycle is not yet known, but the adaptive significance is clear. The simultaneous production of masses of bamboo seeds (in some cases lying 12 to 15 centimetres deep on the ground) is more than all the seed-eating animals can cope with at the time, so that some seeds escape being eaten and grow up to form the next generation (Evans 1976).
有些植物的繁殖期可能会延迟很长的时间。竹子是多年生草本植物，会处于生长状态许多年，然后突然开花，结实和枯死（Evans 1976）。1884年，牙买加岛上的每一种Chusquea abietifolia竹子都开花，结实并死亡。下一代竹子在1916年至1918年之间开花并死亡，这表明其营养循环约为31年。尚不清楚该开花周期的气候触发因素，但适应性意义很明显。大量同时存在的竹子种子（在某些情况下位于地面12至15 厘米深处）超出了吃种子动物的处理能力，因此一些种子逃脱了被食用的命运并成长为下一代（Evans 1976）。
The second reason light is important to organisms is that it is essential for photosynthesis. This is the process by which plants use energy from the sun to convert carbon from soil or water into organic material for growth. The rate of photosynthesis in a plant can be measured by calculating the rate of its uptake of carbon. There is a wide range of photosynthetic responses of plants to variations in light intensity. Some plants reach maximal photosynthesis at one-quarter full sunlight, and others, like sugarcane, never reach a maximum, but continue to increase photosynthesis rate as light intensity rises.
Plants in general can be divided into two groups: shade-tolerant species and shade-intolerant species. This classification is commonly used in forestry and horticulture. Shade-tolerant plants have lower photosynthetic rates and hence have lower growth rates than those of shade-intolerant species. Plant species become adapted to living in .a certain kind of habitat, and in the process evolve a series of characteristics that prevent them from occupying other habitats. Grime (1966) suggests that light may be one of the major components directing these adaptations. For example, eastern hemlock seedlings are shade-tolerant. They can survive in the forest understorey under very low light levels because they have a low photosynthetic rate.