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2024年4月17日发(作者：免费视频公开课)

托福考试

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TPO 25—3 The Evolutionary Origin of Plants

原文：

【1】The evolutionary history of plants has been marked by a series ofadaptations.

The ancestors of plants were photosynthetic single-celledorganisms that gave rise

to plants presumably lacked true roots, stems, leaves,and complex reproductive

structures such as flowers. All of these featuresappeared later in the evolutionary

history of plants. Of today’s differentgroups of algae, green algae are probably the

most similar to ancestral supposition stems from the close phylogenetic

(natural evolutionary)relationship between the two groups. DNA comparisons have

shown that greenalgae are plants’ closest living relatives. In addition, other lines

ofevidence support the hypothesis that land plants evolved from ancestral

greenalgae used the same type of chlorophyll and accessory pigments

inphotosynthesis as do land plants. This would not be true of red and brownalgae.

Green algae store food as starch, as do land plants and have cell wallsmade of

cellulose, similar in composition to those of land plants. Again, thegood storage

and cell wall molecules of red and brown algae are different.

【2】Today green algae live mainly in freshwater, suggesting that theirearly

evolutionary history may have occurred in freshwater habitats. If so, thegreen algae

would have been subjected to environmental pressures that resultedin adaptations

that enhanced their potential to give rise to land-dwelling ororganisms.


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