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TEST 4 PASSAGE 1 参考译文：
The Impact of Wilderness Tourism
荒野旅游的影响
A
The market for tourism in remote areas is booming as never before. Countries all across the world are actively promoting their ‘wilderness’ regions — such as mountains, Arctic lands, deserts, small islands and wetland — to high-spending tourists. The attraction of these areas is obvious: by definition, wilderness tourism requires little or no initial investment. But that does not mean that there is no cost. As the 1992 United Nations Conference on Environment and Development recognized, these regions are fragile (i.e. highly vulnerable to abnormal pressures) not just in terms of their ecology, but also in terms of the culture of their inhabitants. The three most significant types of fragile environment in these respects, and also in terms of the proportion of the Earth’s surface they cover, are deserts, mountains and Arctic areas. An important characteristic is their marked seasonality, with harsh conditions prevailing for many months each year. Consequently, most human activities, including tourism, are limited to quite clearly defined parts of the year.
A偏远地区的旅游市场从未曾像现在这么火爆。世界各国都积极地向高消费游客推广它们的“荒野”地区——如髙山、极地、沙漠、小岛和湿地。这些地区的吸引力是显而易见的：从定义上看，荒野旅游只需要很少的或者完全不需要初始投资。但是，这并不意味着发展荒野旅游不需要付出任何代价。，这些地区是非常脆弱的(即：在异常压力下是极容易被破坏的)，而这种破坏不仅是就当地的生态学而言，而且还会影响到当地居民的文化。在以上方面以及就其所覆盖的地表面积的比例来说，最重要的三种脆弱环境是沙漠、高山和极地。这些地区的一个重要的特征就是显著的季节性，在每年的很多月份里都以恶劣的环境为主。因此，包括旅游在内的大多数人类活动都被清楚地限定在一年中的某些时候。
Tourists are drawn to these regions by their natural landscape beauty and the unique cultures of their indigenous people. And poor governments in these isolated areas have welcomed the new breed of ‘adventure tourist’, grateful for the hard currency they bring. For several years now, tourism has been the prime source of foreign exchange in Nepal and Bhutan. Tourism is also a key element in the economies of Arctic zones such as Lapland and Alaska and in desert areas such as Ayers Rock in Australia and Arizona’s Monument Valley.
游客来到这些地方是因为他们被优美的自然风景和当地人独一无二的文化所吸引。这些地区与外界隔绝，当地贫穷的政府很欢迎“冒险游客”的到来，这让他们获得了一种新的财政收人来源，尤其是游客带来的硬通货。近几年来，旅游业已经成为尼泊尔和不丹的主要外汇收入来源。对于诸如拉普兰和阿拉斯加等北极地区和包括澳大利亚艾尔斯岩石地区和亚利桑那纪念碑山谷在内的沙漠地区而言，旅游业也是当地经济的主要来源。
B
Once a location is established as a main tourist destination, the effects on the local community are profound. When hill-farmers, for example, can make more money in a few weeks working as porters for foreign trekkers than they can in a year working in their fields, it is not surprising that many of them give up their farm-work, which is thus left to other members of the family. In some hill-regions, this has led to a serious decline in farm output and a change in the local diet, because there is insufficient labour to maintain terraces and irrigation systems and tend to crops. The result has been that many people in these regions have turned to outside supplies of rice and other foods.
B一个地区一旦被确定为主要的旅游目的地，其对当地社区的影响是意义深远的。比如，当农夫在几个星期内帮外国背包客搬运行李挣的钱多于在田里工作一年的收入时，很多农夫放弃种田也就不足为奇了。因此，种田的工作就被转移到家庭其他成员身上。在--些丘陵地区，这种变化导致了粮食产量的严重下降和当地饮食的改变，因为没有足够的劳力去维持梯田和灌溉系统以及照管农作物。因此，这些地区的很多人已经依靠外部供给的米和其他食物为生。
In Arctic and desert societies, year-round survival has traditionally depended on hunting animals and fish and collecting fruit over a relatively short season. However, as some inhabitants become involved in tourism, they no longer have time to collect wild food; this has led to increasing dependence on bought food and stores. Tourism is not always the culprit behind such changes. All kinds of wage labour, or government handouts, tend to undermine traditional survival systems. Whatever the cause, the dilemma is always the same: what happens if these new, external sources of income dry up?
传统上，在北极和沙漠地区的社区，全年的生存都依靠打猎捕鱼和在相当短的收获季节采摘果实。然而，当一些居民开始参与旅游业之后，他们不再有时间去捕猎采摘野果，这让他们更加依赖于购买的食物和卖食物的商店。旅游业并不是隐藏于这些变化背后惟一的“罪魁祸首”：所有付薪资的劳动，或者政府的救济物都趋向破坏传统生存系统。不管原因是什么，困境却总是相同的：如果这些新的外来的收入来源枯竭了该怎么办?
The physical impact of visitors is another serious problem associated with the growth in adventure tourism. Much attention has focused on erosion along major trails, but perhaps more important are the deforestation and impacts on water supplies arising from the need to provide tourists with cooked food and hot showers. In both mountains and deserts, slow-growing trees are often the main sources of fuel and water supplies may be limited or vulnerable to degradation through heavy use.
旅游者对环境的影响是另一个与荒野旅游的发展有联系的严重问题。人们在主要的破坏问题上已经投入了很大的注意力，但也许更重要的是滥伐问题和旅游者由于做饭和洗澡对水资源产生的影响。不管是在高山还是沙漠地带，慢生型树木通常是燃料的主要来源，而水资源也有可能由于使用过量而严重紧缺。
C
Stories about the problems of tourism have become legion in the last few years. Yet it does not have to be a problem. Although tourism inevitably affects the region in which it takes place, the costs to these fragile environments and their local cultures can be minimized. Indeed, it can even be a vehicle for reinvigorating local cultures, as has happened with the Sherpas of Nepal’s Khumbu Valley and in some Alpine villages. And a growing number of adventure tourism operators are trying to ensure that their activities benefit the local population and environment over the long term.
C过去几年里，旅游业所带来的问题越来越多。但是这没有必要成为一个问题。虽然旅游业不可避免地影响着旅游地，这些脆弱的环境和当地文化所付出的代价可以降到最低。实际上，旅游业甚至可以成为加速当地文化复兴的快车，就像尼泊尔昆布山谷的雪帕族和一些阿尔卑斯山的村庄一样。越来越多的冒险旅游业的经营者正在努力，以便确保他们的活动从长期来看有益于当地人民和环境。
In the Swiss Alps, communities have decided that their future depends on integrating tourism more effectively with the local economy. Local concern about the rising number of second home developments in the Swiss Pays d’Enhaut resulted in limits being imposed on their growth. There has also been a renaissance in communal cheese production in the area, providing the locals with a reliable source of income that does not depend on outside visitors.
在瑞士阿尔卑斯地区，社区决定他们未来将依靠旅游业与当地经济更有效的整合。瑞士贝伊东地区对于持续增加的别居开发采取了强制措施以限制其发展。这个地区的公共干酪产量也得到了大幅回升，给当地人提供了不依靠外来游客的更可靠的收入来源。
Many of the Arctic tourist destinations have been exploited by outside companies, who employ transient workers and repatriate most of the profits to their home base. But some Arctic communities are now operating tour businesses themselves, thereby ensuring that the benefits accrue locally. For instance, a native corporation in Alaska, employing local people, is running an air tour from Anchorage to Kotzebue, where tourists eat Arctic food, walk on the tundra and watch local musicians and dancers.
很多北极地区的旅游目的地是被外来公司开发的。这些公司雇用短期工，将大部分利润转移到自己国家的公司总部。但是，现在一些北极社区正在自主运作旅游生意，因此能确保增加当地的收益。例如，阿拉斯加本地的公司雇用本地人开展从安克雷奇到扣赞伯的空中之旅，在那里游客可以吃到北极的食物，走在苔原上，并观看当地音乐家和舞蹈演员的表演。
Native people in the desert regions of the American Southwest have followed similar strategies, encouraging tourists to visit their pueblos and reservations to purchase high-quality handicrafts and artwork. The Acoma and San lldefonso pueblos have established highly profitable pottery businesses, while the Navajo and Hopi groups have been similarly successful with jewellery.
美国西南部沙漠地区的当地人也采取了相同的策略。他们鼓励游客到印第安村庄和保留地参观并购买质量上乘的手工艺品和艺术品。阿科马和珊以尔德凡索这两个印第安村庄发展了高利润的陶器生意，那瓦侯和候皮这两个印第安地区在珠宝业方面也获得了类似的成功。
Too many people living in fragile environments have lost control over their economies, their culture and their environment when tourism has penetrated their homelands. Merely restricting tourism cannot be the solution to the imbalance, because people’s desire to see new places will not just disappear. Instead, communities in fragile environments must achieve greater control over tourism ventures in their regions, in order to balance their needs and aspirations with the demands of tourism. A growing number of communities are demonstrating that, with firm communal decision-making, this is possible. The critical question now is whether this can become the norm, rather than the exception.
当旅游业进入自己的家乡时，居住在脆弱环境地区的很多人失去了对经济、文化和环境的控制。因为人们想见识新地方的强烈愿望并不会消失，所以仅仅限制旅游业并不能消除这种不平衡。处于脆弱环境的社区必须要取得对旅游业的更大的控制力，以便平衡旅游业的要求和当地人的需求和渴望。越来越多的社区表明，通过强有力的社区决策，实现这种平衡是完全可能的。现在最棘手的问题是这种平衡的实现能否成为规范，而不仅仅是特例。
TEST 4 PASSAGE 2 参考译文：
Flawed Beauty: the problem with toughened glass
残缺美：钢化玻璃的问题
On 2nd August 1999, a particularly hot day in the town of Cirencester in the UK, a large pane of toughened glass in the roof of a shopping centre at Bishops Walk shattered without warning and fell from its frame. When fragments were analysed by experts at the giant glass manufacturer Pilkington, which had made the pane, they found that minute crystals of nickel sulphide trapped inside the glass had almost certainly caused the failure.
1999年8月3号，天气异常炎热，在英国小镇赛伦塞斯特上，位于主教街的一家购物中心屋顶上的一大片钢化玻璃在没有任何征兆的情况下突然裂成碎片并掉了下来。随即，其生产者——大型玻璃制造商Pilkington公司的专家对掉落的碎片进行了分析。经过分析，专家几乎确定这次事故是由玻璃内部硫化镍的微型晶体引起的。
‘The glass industry is aware of the issue,’ says Brian Waldron, chairman of the standards committee at the Glass and Glazing Federation, a British trade association, and standards development officer at Pilkington. But he insists that cases are few and far between. ‘It’s a very rare phenomenon,’ he says.
“玻璃行业已经意识到了这个问题。”英国贸易协会之一——玻璃及玻璃装配协会标准委员会主席，同时也是Pilkingtcm公司标准发展官员的Brian Waldron宣称。但是他坚持认为这种情况只不过是沧海一粟罢了。“这是非常罕见的现象。”他说。
Others disagree. ‘On average I see about one or two buildings a month suffering from nickel sulphide related failures,’ says Barrie Josie, a consultant engineer involved in the Bishops Walk investigation. Other experts tell of similar experiences. Tony Wilmott of London-based consulting engineers Sandberg, and Simon Armstrong at CladTech Associates in Hampshire both say they know of hundreds of cases. ‘What you hear is only the tip of the iceberg,’ says Trevor Ford, a glass expert at Resolve Engineering in Brisbane, Queensland. He believes the reason is simple: ‘No-one wants bad press.’
但不同意见也同时存在。参与主教街事件调查的咨询工程师Barrie Josie宣称，“平均每个月我都会看到一两幢建筑物遭遇类似的硫化镍问题。”其他专家也讲述了类似经历。位于伦敦的Sanberg公司的咨询工程师Tony Wilraott以及来自汉普西尔的CladTech协会的Simon Armstrong均声称自己了解大量此类情况。“公众所知道的只不过是冰山一角罢了，”昆士兰州布里斯班市Resolve工程公司的玻璃专家Trevor Ford说道。他认为原因很简单：“没人想要坏新闻。”
Toughened glass is found everywhere, from cars and bus shelters to the windows, walls and roofs of thousands of buildings around the world. It’s easy to see why. This glass has five times the strength of standard glass, and when it does break it shatters into tiny cubes rather than large, razor-sharp shards. Architects love it because large panels can be bolted together to make transparent walls, and turning it into ceilings and floors is almost as easy.
钢化玻璃被广泛应用于汽车、候车亭以及世界各地数以千计的建筑物上的窗子、墙面和屋顶。原因很简单，钢化坡璃的强度是普通玻璃的5倍，破碎时裂成细小颗粒而不是锋利的大块碎片。大块的钢化坡璃可以拼成透明的玻璃墙，用它做屋顶和地面也很方便，所以建筑师对钢化玻璃情有独钟。
It is made by heating a sheet of ordinary glass to about 620°C to soften it slightly, allowing its structure to expand, and then cooling it rapidly with jets of cold air. This causes the outer layer of the pane to contract and solidify before the interior. When the interior finally solidifies and shrinks, it exerts a pull on the outer layer that leaves it in permanent compression and produces a tensile force inside the glass. As cracks propagate best in materials under tension, the compressive force on the surface must be overcome before the pane will break, making it more resistant to cracking.
钢化玻璃是由普通玻璃制成的：把一块普通玻璃加热到大约620摄氏度，使之轻度软化、结构膨胀，然后用冷气流迅速将其冷却。这导致玻璃的外层先于内部收缩和凝固。玻璃内部最终冷却并凝固后对玻璃外层产生拉力作用，使之始终处于压力之下，它还会使玻璃内部产生张力.裂缝最容易在有压力的物体中扩张，所以要防止玻璃破晬就必须消除玻璃表面的压力，使之不容易破碎。
The problem starts when glass contains nickel sulphide impurities. Trace amounts of nickel and sulphur are usually present in the raw materials used to make glass, and nickel can also be introduced by fragments of nickel alloys falling into the molten glass. As the glass is heated, these atoms react to form tiny crystals of nickel sulphide. Just a tenth of a gram of nickel in the furnace can create up to 50,000 crystals.
当玻璃含有硫化镍杂质时，问题就出现了。痕量的镍和硫通常出现在制作玻璃的原材料中，熔化过程中镍合金碎片也会增加玻璃的镍含量。玻璃被加热之后，这些原子相互作用形成了微型的硫化镍晶体。熔炉中0.1克的镍就会产生多达5万个晶体。
These crystals can exist in two forms: a dense form called the alpha phase, which is stable at high temperatures, and a less dense form called the beta phase, which is stable at room temperatures. The high temperatures used in the toughening process convert all the crystals to the dense, compact alpha form. But the subsequent cooling is so rapid that the crystals don’t have time to change back to the beta phase. This leaves unstable alpha crystals in the glass, primed like a coiled spring, ready to revert to the beta phase without warning.
这些晶体以两种形式存在：髙温下稳定的密度较大的α相和室温下稳定的密度小一些的β相。强化过程中的高温把所有的硫化镍晶体都转化成高密度的α相。但是接下来的冷却过程如此迅速，以至于硫化镍晶体没有足够的时间重新转化成β相。这在玻璃中遗留下不稳定的α相晶体，它就像被压缩的弹簧一样随时可能毫无征兆地重新转化为β相。
When this happens, the crystals expand by up to 4%. And if they are within the central, tensile region of the pane, the stresses this unleashes can shatter the whole sheet. The time that elapses before failure occurs is unpredictable. It could happen just months after manufacture, or decades later, although if the glass is heated — by sunlight, for example — the process is speeded up. Ironically, says Graham Dodd, of consulting engineers Arup in London, the oldest pane of toughened glass known to have failed due to nickel sulphide inclusions was in Pilkington’s glass research building in Lathom, Lancashire. The pane was 27 years old.
当硫化镍晶体由α相转化成β相时，体积膨胀4%。，膨胀产生的压力可以使整块玻璃破裂。破裂时间无法预测，可能是生产出来的几个月后也可能是几十年后，尽管玻璃被日光加热会加快晶体的转化速度。Graham Dodd，伦敦Arup顾问建筑师说：“具有讽刺意味的是，因为含有硫化镍而导致破裂的‘历史最悠久’的钢化玻璃是兰开夏郡拉苏的Pilkington公司的玻璃研究大厦的玻璃，使用时间是27 年。”
Data showing the scale of the nickel sulphide problem is almost impossible to find. The picture is made more complicated by the fact that these crystals occur in batches. So even if, on average, there is only one inclusion in 7 tonnes of glass, if you experience one nickel sulphide failure in your building, that probably means you’ve got a problem in more than one pane. Josie says that in the last decade he has worked on over 15 buildings with the number of failures into double figures.
能够表明硫化镍问题的规模的数据几乎无法找到。硫化镍晶体总是成批出现，使问题更加复杂。所以，即使平均每7吨的玻璃里只有一个杂质，但是，只要有一个硫化镍晶体发生破裂，这就意味着这个建筑物中不止一块钢化玻璃存在着问题。Josie说，在过去的十年间，他参加建造的建筑物超过15个，因为硫化镍产生问题的建筑物数量达到两位数。
One of the worst examples of this is Waterfront Place, which was completed in 1990. Over the following decade the 40-storey Brisbane block suffered a rash of failures. Eighty panes of its toughened glass shattered due to inclusions before experts were finally called in. John Barry, an expert in nickel sulphide contamination at the University of Queensland, analysed every glass pane in the building. Using a studio camera, a photographer went up in a cradle to take photos of every pane. These were scanned under a modified microfiche reader for signs of nickel sulphide crystals. ‘We discovered at least another 120 panes with potentially dangerous inclusions which were then replaced,’ says Barry. ‘It was a very expensive and time-consuming process that took around six months to complete.’ Though the project cost A$1.6 million (nearly ￡700,000), the alternative — re-cladding the entire building — would have cost ten times as much.
一个最糟糕的例子就是建于1990年的Waterfront Place。在建成后的10年间，这个位于布里斯班的40层的大楼经历了一系列的毁坏。在专家最终到来之前，因为硫化镍杂质的问题，有80块钢化玻璃破碎。昆士兰大学的硫化镍专家Barry分析了大厦所有的钢化玻璃。摄影师乘吊车用室内摄像机拍下了毎一块钢化玻璃，之后这些照片被放在改良缩微胶片阅读机上扫描以检测硫化镍的痕迹。“我们发现至少还有120块钢化玻璃存在可能导致危险的杂质，这些玻璃都需要更换，”Bairy说：“这是一个昂贵的、非常耗费时间的过程，大约需要6个月的时间才能完成。”虽然这项工程花费了160万澳币(相当于70万英镑)，但重新修复整个大厦所需的花费将会是这个费用的10倍。
TEST 4 PASSAGE 3 参考译文：
The effects of light on plant and animal species
光对动植物的影响
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.
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，光有助于植物生长。
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式。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 at photoperiodism in plants is also clear. Short-day plants that flower in spring in the temperate zone are adapted to maximizing 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.
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花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 abietifolio 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).
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，这喑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，种子在地下12到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，从来不能达到最髙值，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 understory under very low light levels because they have a low 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。