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植物的进化史以一系列适应性变化为标志。

The evolutionary history of plants has been marked by a series of adaptations.

植物的祖先是进行光合作用的单细胞生物，可能类似于今天的藻类。

The ancestors of plants were photosynthetic single-celled organisms probably similar to today’s algae.

像现代的藻类一样，产生植物的有机体可能缺乏像花一样的真正的根、茎、叶和复杂的繁殖结构。

Like modern algae, the organisms that gave rise to plants presumably lacked true roots, stems, leaves, and complex reproductive structures such as flowers.

所有这些特征都出现在植物进化史的后期。

All of these features appeared later in the evolutionary history of plants.

在今天不同种类的藻类中，绿藻可能是与祖先的植物最相似的。

Of today’s different groups of algae, green algae are probably the most similar to ancestral plants.

这种推测源于这两个群体之间密切的系统发育（自然进化）关系。

This supposition stems from the close phylogenetic (natural evolutionary) relationship between the two groups.

DNA比对显示，绿藻是植物现存的近亲。

DNA comparisons have shown that green algae are plants’ closest living relatives.

此外，其他证据也支持了陆生植物是由原始绿藻进化而来的假设：绿藻在光合作用中使用了与陆生植物相同类型的叶绿素和辅助色素。

In addition, other lines of evidence support the hypothesis that land plants evolved from ancestral green algae: green algae used the same type of chlorophyll and accessory pigments in photosynthesis as do land plants.

这种情况不适用于红藻或褐藻。

This would not be true of red or brown algae.

绿藻以淀粉的形式储存食物，陆生植物也是如此，它们的细胞壁由纤维素构成，与陆生植物的细胞壁组成相似。

Green algae store food as starch, as do land plants and have cell walls made of cellulose, similar in composition to those of land plants.

同样，红藻和褐藻的食物储存和细胞壁分子是不同的。

Again, the food storage and cell wall molecules of red and brown algae are different.

今天，绿藻主要生活在淡水中，这表明它们的早期进化史可能发生在淡水栖息地。

Today green algae live mainly in freshwater, suggesting that their early evolutionary history may have occurred in freshwater habitats.

如果是这样的话，绿藻就会受到环境压力的影响，从而产生适应性，增强它们产生陆地生物的潜力。

If so, the green algae would have been subjected to environmental pressures that resulted in adaptations that enhanced their potential to give rise to land-dwelling organisms.

与海洋环境不同，淡水环境的环境条件变化很大。

The environmental conditions of freshwater habitats, unlike those of ocean habitats, are highly variable.

水温会随季节甚至每日波动，而降雨量的变化会导致水中化学物质浓度的波动，甚至会导致水生栖息地的周期性干涸。

Water temperature can fluctuate seasonally or even daily, and changing levels of rainfall can lead to fluctuations in the concentration of chemicals in the water or even to periods in which the aquatic habitat dries up.

古老的淡水绿藻一定进化出了能够承受极端温度和干旱时期的特征。

Ancient freshwater green algae must have evolved features that enabled them to withstand extremes of temperature and periods of dryness.

当他们入侵陆地时，这些适应性很好地服务了他们的后代。

These adaptations served their descendants well as they invaded land.

陆地世界现在是绿色的，但一开始并不是这样的。

The terrestrial world is green now, but it did not start out that way.

当植物在4亿多年前第一次向岸上迁移时，陆地上一片荒芜，不适合生命生存。

When plants first made the transition ashore more than 400 million years ago, the land was barren and desolate, inhospitable to life.

然而，从植物进化的角度来看，它也是一片充满机会的土地，没有竞争者和捕食者，充满了二氧化碳和阳光（光合作用的原材料，在空气中的浓度远远高于在水中）。

From a plant’s evolutionary viewpoint, however, it was also a land of opportunity, free of competitors and predators and full of carbon dioxide and sunlight (the raw materials for photosynthesis, which are present in far higher concentrations in air than in water).

因此，一旦自然选择形成了适应性，帮助植物克服了在陆地上生存的障碍，植物就会繁荣和多样化。

So once natural selection had shaped the adaptations that helped plants overcome the obstacles to terrestrial living, plants prospered and diversified.

当植物开辟土地时，它们面临着陆地环境带来的一系列挑战。

When plants pioneered the land, they faced a range of challenges posed by terrestrial environments.

在陆地上，水的支持性浮力消失了，植物不再浸泡在营养液中，空气会使植物变干。

On land, the supportive buoyancy of water is missing, the plant is no longer bathed in a nutrient solution, and the air tends to dry things out.

这些条件有利于支持身体结构的进化，运输水和营养到植物所有部分的导管，以及保存水的结构。

These conditions favored the evolution of structures that support the body, vessels that transport water and nutrients to all parts of the plant, and structures that conserve water.

由此产生的对陆地的适应包括一些在植物进化早期就出现的结构特征；现在几乎所有的陆地植物都具有这些特征。

The resulting adaptations to dry land include some structural features that arose early in plant evolution; now these features are common to virtually all land plants.

它们包括根或类似根的结构，覆盖在叶和茎表面的蜡状角质层，限制水的蒸发，以及叶和茎中称为气孔的气孔，允许气体交换，但在缺水时关闭，从而减少水分损失。

They include roots or rootlike structures, a waxy cuticle that covers the surfaces of leaves and stems and limits the evaporation of water, and pores called stomata in leaves and stems that allow gas exchange but close when water is scarce, thus reducing water loss.

其他的适应性变化发生在向陆地生命过渡的后期，现在广泛存在于植物中，但并不普遍。

Other adaptations occurred later in the transition to terrestrial life and are now widespread but not universal among plants.

这些物质包括将水和矿物质从根部向上运输的导管，以及将光合产物从叶子运送到植物体其他部分的导管，以及支撑植物体的硬化物质木质素，帮助植物体最大限度地暴露在阳光下。

These include conducting vessels that transport water and minerals upward from the roots and that move photosynthetic products from the leaves to the rest of the plant body and the stiffening substance lignin, which supports the plant body, helping it expose maximum surface area to sunlight.

这些适应性使越来越多的植物种类得以利用干旱的土地。

These adaptations allowed an increasing diversity of plant forms to exploit dry land.

然而，陆地上的生命也需要将精子运输到卵子的新方法。

Life on land, however, also required new methods of transporting sperm to eggs.

与水生和海洋生物不同，陆生植物不能总是依靠水流来携带它们的生殖细胞和散播它们的受精卵。

Unlike aquatic and marine forms, land plants cannot always rely on water currents to carry their sex cells and disperse their fertilized eggs.

因此，最成功的陆生植物群是那些进化出不依赖水散布受精性细胞的方法和保护发育中的胚胎，使其免于干燥的结构的植物。

So the most successful groups of land plants are those that evolved methods of fertilized sex cell dispersal that are independent of water and structures that protect developing embryos from drying out.

受保护的胚胎和无水性细胞的散布是通过种子植物的起源和它们引入的关键进化创新实现的：花粉、种子，以及后来的花和果实。

Protected embryos and waterless dispersal of sex cells were achieved with the origin of seed plants and the key evolutionary innovations that they introduced: pollen, seeds, and, later, flowers and fruits.

The word “presumably” in the passage is closest in meaning to

According to paragraph 1, all of the following are true of ancestral plants EXCEPT:

The phrase “subjected to” in the passage is closest in meaning to

What can be inferred from paragraph 3 about ancient green algae?

The word “desolate” in the passage is closest in meaning to

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

According to paragraph 4, which of the following is true about the terrestrial world at the time it was colonized by plants?

The word “posed” in the passage is closest in meaning to

According to paragraph 5, all of the following are problems that early terrestrial plants had to overcome EXCEPT

What purpose does paragraph 5 serve in the larger discussion of the origins of terrestrial plants?

According to paragraph 6, the adaptations made by terrestrial plants had which of the following effects?

Which of the following best describes the author’s presentation of the information about land plants?

Look at the four squares  that indicate where the following sentence could be added to the passage. Scientists believe that chemical changes and a thicker exterior, among other things, may have helped ancient algae overcome the conditions in their environment.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

The Evolutionary Origin of Plants The evolutionary history of plants has been marked by a series of adaptations. The ancestors of plants were photosynthetic single-celled organisms probably similar to today’s algae. Like modern algae, the organisms that gave rise to plants presumably lacked true roots, stems, leaves, and complex reproductive structures such as flowers. All of these features appeared later in the evolutionary history of plants. Of today’s different groups of algae, green algae are probably the most similar to ancestral plants. This supposition stems from the close phylogenetic (natural evolutionary) relationship between the two groups. DNA comparisons have shown that green algae are plants’ closest living relatives. In addition, other lines of evidence support the hypothesis that land plants evolved from ancestral green algae: green algae used the same type of chlorophyll and accessory pigments in photosynthesis as do land plants. This would not be true of red or brown algae. Green algae store food as starch, as do land plants and have cell walls made of cellulose, similar in composition to those of land plants. Again, the food storage and cell wall molecules of red and brown algae are different. Today green algae live mainly in freshwater, suggesting that their early evolutionary history may have occurred in freshwater habitats. If so, the green algae would have been subjected to environmental pressures that resulted in adaptations that enhanced their potential to give rise to land-dwelling organisms. The environmental conditions of freshwater habitats, unlike those of ocean habitats, are highly variable.  Water temperature can fluctuate seasonally or even daily, and changing levels of rainfall can lead to fluctuations in the concentration of chemicals in the water or even to periods in which the aquatic habitat dries up.  Ancient freshwater green algae must have evolved features that enabled them to withstand extremes of temperature and periods of dryness.  These adaptations served their descendants well as they invaded land. The terrestrial world is green now, but it did not start out that way. When plants first made the transition ashore more than 400 million years ago, the land was barren and desolate, inhospitable to life. From a plant’s evolutionary viewpoint, however, it was also a land of opportunity, free of competitors and predators and full of carbon dioxide and sunlight (the raw materials for photosynthesis, which are present in far higher concentrations in air than in water). So once natural selection had shaped the adaptations that helped plants overcome the obstacles to terrestrial living, plants prospered and diversified. When plants pioneered the land, they faced a range of challenges posed by terrestrial environments. On land, the supportive buoyancy of water is missing, the plant is no longer bathed in a nutrient solution, and the air tends to dry things out. These conditions favored the evolution of structures that support the body, vessels that transport water and nutrients to all parts of the plant, and structures that conserve water. The resulting adaptations to dry land include some structural features that arose early in plant evolution; now these features are common to virtually all land plants. They include roots or rootlike structures, a waxy cuticle that covers the surfaces of leaves and stems and limits the evaporation of water, and pores called stomata in leaves and stems that allow gas exchange but close when water is scarce, thus reducing water loss. Other adaptations occurred later in the transition to terrestrial life and are now widespread but not universal among plants. These include conducting vessels that transport water and minerals upward from the roots and that move photosynthetic products from the leaves to the rest of the plant body and the stiffening substance lignin, which supports the plant body, helping it expose maximum surface area to sunlight. These adaptations allowed an increasing diversity of plant forms to exploit dry land. Life on land, however, also required new methods of transporting sperm to eggs. Unlike aquatic and marine forms, land plants cannot always rely on water currents to carry their sex cells and disperse their fertilized eggs. So the most successful groups of land plants are those that evolved methods of fertilized sex cell dispersal that are independent of water and structures that protect developing embryos from drying out. Protected embryos and waterless dispersal of sex cells were achieved with the origin of seed plants and the key evolutionary innovations that they introduced: pollen, seeds, and, later, flowers and fruits.