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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO-25-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 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 used the same type of chlorophyll and accessory pigments in photosynthesis as do land plants. This would not be true of red and 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 good 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 or 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 level of rainfall can lead to fluctuations in the concentration of chemical in the water or even to period in which the aquatic habitat dries up. Ancient fresh water green algae must have evolved features that enable them to withstand extremes of temperature and periods of dryness. These adaptations served their descendant well asthey 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 view point, 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 air tends to dry things out. These conditions favored the evolution of the structures that support the body, vessels that transport water and nutrients to all parts of 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 plant. They include roots or root like 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 now wide spread but not universal among plants. These include conducting vessels that transport water and minerals upward from the roots and that move the photosynthetic products from the leavesto the rest of the plant body and the stiffening substance lignin, which support 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 protest 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.

植物的进化历史已经有了一系列的改变。植物的祖先是光合单细胞生物体,导致植物大概缺乏真正的根,茎,叶和复杂的繁殖结构如花。所有这些特征都出现在植物的进化史中。在当今不同类型的藻类中,绿藻可能是与祖先植物最相似的。这个假设源于两个群体之间的密切的系统发育(自然进化)关系。 DNA比较显示,绿藻是植物最亲近的活体亲属。另外,其他证据支持这样的假设,即从祖先绿藻演化而来的陆地植物与陆地植物一样,在光合作用中使用相同类型的叶绿素和辅助色素。红藻和褐藻并不是这样。绿藻将食物存储为淀粉,如同陆地植物一样,并且具有由纤维素制成的细胞壁,其组成与陆地植物的组成相似。同样,红色和棕色藻类的良好储存和细胞壁分子是不同的。

 

今天,绿藻主要生活在淡水中,这表明它们早期的进化历史可能发生在淡水栖息地。如果是这样的话,绿藻就会受到环境压力的影响,导致适应,从而增加了它们引起土地栖息或生物体的潜力。

 

与海洋栖息地不同,淡水栖息地的环境条件变化很大。水温可能会季节性变化,甚至是每天变化,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干涸。古代淡水绿藻必须具有进化的特征,使其能够承受极端的温度和干燥时期。这些适应使他们的后代好好地入侵土地。

 

现在的地球世界是绿色的,但并不是这样开始的。当植物在4亿多年前首次进行转换时,这片土地荒芜荒凉,不适宜生活。然而,从植物的进化观点来看,它也是一个机会之地,没有竞争对手和捕食者,充满了二氧化碳和太阳光(光合作用的原料,在空气中的浓度比在水中高得多)。所以一旦自然选择形成了帮助植物克服陆地生物障碍的适应,植物繁荣和多样化。

 

当植物开辟了土地时,他们面临着陆地环境带来的一系列挑战。在陆地上,缺乏水的支撑性浮力,植物不再浸泡在营养液中,空气也容易把东西弄干。这些条件有利于支持身体结构的演变,将水和营养物质输送到植物各个部位的器皿,以及节约用水的结构。由此产生的对旱地的适应包括植物进化早期出现的一些结构特征;现在这些功能对于几乎所有陆地植物都是常见的。它们包括根部或根部结构,覆盖叶和茎表面的蜡质角质层,限制水分蒸发,叶子和茎部有气孔,允许气体交换,但在缺水时关闭,从而减少水分流失。其他适应过程发生在向陆地生活过渡之后,现在在植物中广泛传播但不普遍。这些包括从根部向上运输水和矿物质的导管,并且将来自植物体剩余部分的光合产物和支撑植物体的硬化物质木质素移动,从而帮助其暴露最大表面积至阳光。

 

这些适应使植物形式日益多样化,以开发旱地。然而,陆地上的生活也需要将精子运送到卵子的新方法。与水生和海洋形态不同的是,陆地植物不能总是依靠水流携带其性细胞并驱散受精卵。因此,最成功的陆地植物群体是那些演变出不受水和结构影响的受精卵发育的方法,这种方法可以抗拒发育中的胚胎干燥。受保护的胚胎和性细胞的无水扩散是通过种子植物的起源和他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。


中文翻译

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

  • The ancestors of plants were photosynthetic single-celled organisms that gave rise to plants presumably lacked true roots, A originally. B supposedly. C obviously. D usually. presumably
正确答案: B

网友解析

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    小托君  发表于 2018-08-01 14:46:35

    presumably大概,可能,所以正确答案是B的 supposedly。不认识的同学想想 assume, 叫做假定或者推断,由于前缀pre只表方向或者加强,所以这个词大致意思和assume差不多,代入,植物的祖先是光合作用的单细胞生物,怎么样缺乏根茎等等,由于植物的祖先谁也不知道是啥样,所以单细胞生物形成的植物是没有根茎等等东西这个结论只是个推测,正确答案B。

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