Light is important to organisms for two different reasons. Firstly it is used as a cue for the iming of daily and seasonal rhythms in both plants and animals, and secondly it is used to assist growth in plants.
光对于生物体的重要性是由于两个不同的原因。第一，光被动植物用作日常时间和季节节奏的暗示；第二，光有助干植物生长。
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.
大多数生物体的繁殖都只发生在一年中的某些时候，因此，需要一个可靠的提示来引发繁殖行为。日照长度就是非常好的提示，因为它提供了一个极好的可预测的一年之中变化的模武。在温带的春季，一天与一天之间气温波动很大，但是日照长度却按照可预测的数量稳步增加。受季节性影响的日照长度在生理学上的反应叫做光周期现象。这种现象的试验证据的数量相当可观。例如，仅仅通过人工增加日照长度，一些鸟类甚至在冬至的时候都会被诱发繁殖(Wolfson，1964年)。在植物中也有光周期的例子。当日照低于一定的临界长度，短日照植物会开花。而当日照超过一定的临界长度时，长日照植物就会开花。在这两个例子中，临界日照长度因物种的不同而存在差异。那些不受光周期影响、经过一段时间的生长之后便开花的植物叫做日中性植物。
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 fledging. 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.
诸如鸟类等动物的繁殖期占据了一年中其后代最有可能生存下来的时间。在繁殖期开始之前，必须储存足够的食物以支持繁殖过程中的能量消耗，并且需要为还在巢中和刚长羽毛不久的小鸟提供食物。因此，很多温带的鸟类将春季日照长度的增加当作开始筑巢周期的提示，因为在这个时候能保证有足够的食物来源。
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 thefavourable 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.
植物对光周期现象的适应性也很明显。温带地区的短日照植物在春天开化以便在生长季节让幼苗长得最快。长日照植物会选择能够得到昆虫授精或者使种子有较长成熟期的时期。温带的短日照植物秋季开花，这样能够在生长季节建立食物储备，也能以种子的形式度过冬天。日中性植物有进化优势。对于它们而言，繁殖的最好季节和日照长度之间并没有必然的关系。例如，对于沙漠的一年生植物而言，只要有合适的雨水，它们就会生长、开花、结果，不受日照长短的影响。
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 I 5 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).
一些植物的生殖期会拖得特别长。竹子是多年生植物，在很多年的时间里都保持生长状态，然后突然开花、结果、死亡(Evans，1976)。1884年，牙买加岛上的Chusquea abietifolia竹子全部开花、结果、死亡。下一代的这种竹子在1916年到1918年间也都开花、死亡了，这暗示这种竹子的生长期大约为31年。这种花期循环的气候诱因目前还不清楚，但是其适应性却是很清楚的。大量的竹子同时育种(在一些例子中，种子在地面12～15厘米深)。种子的数量超过了这一时期以种子为食的动物所能吃掉的全部数量，这样，一些种子就能存活下来，长成下一代(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.
光对于生物体来说很重要的第二个原因是由于它对光合作用至关重要。光合作用是植物通过太阳能将土壤或水中的碳转化成供生长所需的有机物质的过程。通过计算植物对碳的吸收速率可以计量出光合作用的速率。光合作用对植物的影响因为光照强度的不同有很大的区别。一些植物在1／4全光照的时候能达到光合作用最高值；而另外一些植物，比如甘蔗，从来不能达到最高值，但是随着光照强度的增加，光合作用的速率会持续增高。
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 Iow light levels because they have a Iow photosynthetic rate.
总体而言，植物可以被分为两种：耐阴类和喜光类。这一分类在林业和园艺业中广泛使用。耐阴植物光合作用的速率低，因此生长速度低于喜光植物。植物物种会逐渐适应生活在某一特定的环境中。并在此过程中产生一系列的特性，这些特性会阻止它们去占据别的环境。Grime(1966)指出，光照可能是指导这些适应性的主要因素之一。例如，东方的芹叶钩吻幼苗是耐阴植物，它们可以生存在森林中光照很少的林下叶层，因为它们的光合作用速率低。
