Light is important to organisms for two different reasons. 光对生命体的重要性来源于两个方面。 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. 首先，对于动植物而言，光代表着一天和一个季节的节奏变 化；其次，光对植物生长有辅助作用。

Breeding in most organisms occurs during a part of the year only, and so a reliable cue is needed to trigger breeding behaviour. 大部分生命体的繁殖只发生在每年当中的某个时段，因此需要有一个准确信号用来启动繁殖行为。 Day length is an excellent cue, because it provides a perfectly predictable pattern of change within the year. 日照时间是一个很好的信号，因为它可以用来准确地预测一年中的各种因素的变化。 In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount. 在温带，春季 温度的日常波动很大，但是日照时间会稳定地增加。 The seasonal impact of day length on physiological responses is called photoperiodism, and the amount of experimental evidence for this phenomenon is considerable. 日照时间的季节性变化对于生理反应的影响被称为 光周现象，有很多实验证据可以证明这一现象。 For example, some species of birds’ breeding can be induced even in midwinter simply by increasing day length artificially (Wolfson 1964). 例如，如果在冬天通过人工方式延长日照时间，有些鸟 类就会开始繁殖行为（Wolfson，。 Other examples of photoperiodism occur in plants. 植物也存在光周现象。 A short-day plant flowers when the day is less than a certain critical length. 当日照时间短暂到一定程度，短日照植物 便会开花。 A long-day plant flowers after a certain critical day length is exceeded. 反之，长日照植物会开花。 In both cases the critical day length differs from species to species. 对于以上两种情况，不同植物对日照时间长度的要求也不同。 Plants which flower after a period of vegetative growth, regardless of photoperiod, are known as day-neutral plants. 生 长一段时间后就会开花，而不受光周影响的植物被称为日中植物。

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. 动物（例如鸟类）的繁殖发生在一年当中的某个时段，这样做事为了使后代拥有最大的生存概率。 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. 在繁殖季节开始前，必须积累一定数量的食物为繁殖行为提供能量，也用来为雏鸟提供食物。 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. 因此，很 多温带鸟类将不断增长的日照时间作为信号，用来启动繁殖周期，因为春季的食物资源丰富。

The adaptive significance of photoperiodism in plants is also clear. 植物的光周作用具有明显的适应性。 Short-day plants that flower in spring in the temperate zone are adapted to maximising seedling growth during the growing season. 在温带，有的短日照植物在春季开花，以便其幼苗在成长期可 以最好地生长。 Long-day plants are adapted for situations that require fertilization by insects, or a long period of seed ripening. 长日照植物需要由昆虫来授粉，因此种子成熟的周期也较长。 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. 在温带，还有一些植物会 在秋季开花，以便积累足够的食物使种子度过生长期和冬季。 Day-neutral plants have an evolutionary advantage when the connection between the favourable period for reproduction and day length is much less certain. 在最佳繁殖期和日照长度的关系并不明 确的条件下，日中植物就比较具有进化优势。 For example, desert annuals germinate, flower and seed whenever suitable rainfall occurs, regardless of the day length. 例如，沙漠中的一年生植物在有降水的时候就会生长，开 花和结果，不管日照长度如何。

The breeding season of some plants can be delayed to extraordinary lengths. 某些植物的繁殖期可以推迟很长一段时间。 Bamboos are perennial grasses that remain in a vegetative state for many years and then suddenly flower, fruit and die (Evans 1976). 竹子是多年生植物，它可以多年处于生长状态，然后突然开花、结果和死亡（Evans，。 Every bamboo of the species Chusquea abietifolia on the island of Jamaica flowered, set seed and died during 1884. 1884 年，牙买加的 Ca 竹子全都完成了开花、结果和死亡的过程。 The next generation of bamboo flowered and died between 1916 and 1918, which suggests a vegetative cycle of about 31 years. 这些竹子的下一代在 1916 年到 1918 年间开花并且死亡，这意味着其生长周期长达 31 年。 The climatic trigger for this flowering cycle is not yet known, but the adaptive significance is dear. 气候对这一开 花周期的诱因尚不明确，但是这种植物的适应能力是非常明显的。 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 chat some seeds escape being eaten and grow up to form the next generation (Evans 1976). 这种竹子会在同一时间产生大量种 子（有时种子堆积的厚度甚至有 12-15 厘米），以种子为食的动物无法吃完全部种子，因此有的种子逃 脱了被吃掉的命运而成长为下一代的竹子。

The second reason light is important to organisms is that it is essential for photosynthesjs. 光对生命体的第二个重要性在于光对光合作用的重要性。 This is the process by which plants use energy from the sun to convert carbon from soil or water into organic material for growth. 光合作用是指植物利用阳光把土壤和水 中的碳转化为生长所需的有机物的过程。 The rate of photosynthesis in a plant can be measured by calculating the rate of its uptake of carbon. 我们可以通过测量植物吸取碳的速度来计算光合作用的速 度。 There is a wide range of photosynthetic responses of plants to variations in light intensity. 光合作用对光的强度会有各种反应。 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. 有的植物在四分之一全日光时达到最大光合作用，而其他的植 物，例如甘蔗，永远不会达到最大光合作用，而是随着光照强度的增加而发生更强的光合作用。

Plants in general can be divided into two groups: shade-tolerant species and shade-intolerant species. 植物大体可以分为两类：喜光植物和喜阴植物。 This classification is commonly used in forestry and horticulture. 这一分类广泛用于林业和园艺业。 Shade-tolerant plants have lower photosynthetic rates and hence have lower growth rates than those of shade-intolerant species. 喜阴植物的光合 作用速度较低，因此它的生长周期长于喜光植物。 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. 植物会适应不同类型的生长环境，并且在适应过程 中，进化出各种特征来防止它们占据其他植物的生长地。 Grime (1966) suggests that light may be one of the major components direrting these adaptations. Grime提出，光可能是导致这些适应性 的主要因素之一。 For example, eastern hemlock seedlings are shade-tolerant. 例如铁杉的幼苗属耐阴型。 They can survive in the forest understorey under very low light levels because they have a low photosynthetic rate. 它们可以在日照级别很低的森林底层地区生存下来，这 是因为它们的光合作用速度很低。