第1自然段

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.

第1自然段

光对生物来说有两个重要的原因。首先，它被用作植物和动物的日常和季节性节律定时的线索，其次，它被用于辅助植物的生长。

第2自然段

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.

第2自然段

大多数生物的繁殖只在一年中的某个时间发生，因此需要可靠的线索来触发繁殖行为。日长是一个很好的线索，因为它在一年之中提供了完全可预测的变化模式。在温带地区的春季，温度每天都会大幅波动，但日长却稳定地以可预测的幅度增加。日长对生理反应的季节性影响被称为光周期性，对这一现象的实验证据相当充分。例如，某些鸟类的繁殖甚至可以在深冬通过人工增加日长而诱导（Wolfson, 1964）。光周期性还存在于植物中。短日植物在白天少于某一特定长度时开花，长日植物则在超过某一特定日长后开花。在这两种情况下，关键日长因物种而异。不论光周期如何，那些在经过一段营养生长期后才开花的植物被称为日中性植物。

第3自然段

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.

第3自然段

鸟类等动物的繁殖季节的进化是为了在后代具有最大生存机会的时间段内繁殖。在繁殖季节开始之前，食物储备必须积累起来，以支持繁殖的能量成本，并为幼鸟提供在巢中和离巢后的食物。因此，许多温带地区的鸟类利用春季逐渐增长的日长作为开始筑巢周期的线索，因为这时将有足够的食物资源。

第4自然段

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.

第4自然段

植物光周期性的适应意义也很明确。在温带地区春季开花的短日植物适应于在生长季节内最大限度地促进苗木生长。长日植物适应需要昆虫授粉或较长的种子成熟期的情况。在温带地区秋季开花的短日植物能够在生长季节和冬季作为种子积累食物储备。日中性植物在繁殖的有利时期与日长之间的联系非常不确定时具有进化优势。例如，沙漠一年生植物在适宜的降雨时随时发芽、开花和结籽，而不受日长的限制。

第5自然段

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).

第5自然段

某些植物的繁殖季节可能被延迟到非常长的时间。竹子是多年生的禾本科植物，它们在多年保持营养生长状态，然后突然开花、结实和死亡（Evans, 1976）。1884年，牙买加岛上所有Chusquea abietifolia竹种都开花、结籽并死亡。下一代竹子在1916年至1918年之间开花并死亡，这表明其营养周期约为31年。引发这一开花周期的气候因素尚不清楚，但其适应意义很明确。大量竹子种子的同时产生（有时堆积在地面上12到15厘米深处），超过了所有食籽动物的食量，因此一些种子得以避免被食用，并长成下一代（Evans, 1976）。

第6自然段

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.

第6自然段

光对生物的第二个重要作用是它对光合作用至关重要。这是植物利用太阳能将土壤或水中的碳转化为有机物质以供生长的过程。植物的光合作用速率可以通过计算其吸收碳的速率来衡量。植物对光强度变化的光合作用反应范围很大。一些植物在四分之一的全日照下达到最大光合作用，而其他植物（如甘蔗）从不达到最大值，随着光强度的增加，光合作用速率继续增加。

第7自然段

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.

第7自然段

植物大致可以分为耐阴物种和不耐阴物种两组。这一分类在林业和园艺学中常被使用。耐阴植物的光合速率较低，因此生长速度也较低。植物物种适应生活在某种特定的栖息地，并通过进化一系列特性阻止其占领其他栖息地。Grime (1966)提出光可能是指导这些适应特性的主要因素之一。例如，东北云杉苗木是耐阴的。它们可以在林下非常低的光照条件下存活，因为它们的光合速率较低。