托福考试 复习
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 featuresappeare
d later in the evolutionary history of plants. Of today’s differentgroups 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 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.
【3】The environmental conditions of freshwater habitats, unlike those ofocean habitats, are highly variable. Water temperature can fluctuate seasonallyor even daily and changing level of rainfall can lead to fluctuations in theconcentration of chemical in the water or even to period in which the aquati
chabitat dries up. Ancient fresh water green algae must have evolved featuresthat enable them to withstand extremes of temperature and periods of dryness.These adaptations served their descendant well asthey invaded land.
【4】The terrestrial world is green now, but it did not start out thatway. When plants first made the transition ashore more than 400 million yearsago, the land was barren and desolate, in hospitable to life. From a plant’sevolutionary view point, however, it was also a land of opportunity, free ofcompetitors and predators and full of carbon dioxide and sunlight (the rawmaterials for photosynthesis, which are present in far higher concentrations inair than in water).So once natural selection had shaped the adaptations thathelped plants overcome the obstacles to terrestrial living, plants prosperedand diversified.
【5】When plants pioneered the land, they faced a range of challengesposed by terrestrial environments. On land, the supportive buoyancy of water ismissing, the plant is no longer bathed in a nutrient solution, and air tends todry things out. These conditions favored the evolution of the structures thatsupport the body, vessels that transport water and nutrients to all parts ofplant, and structures that conserve water.
The resulting adaptations to dryland include some structural features that arose early in plant evolution; nowthese features are common to virtually all land plant. They include roots orroot like structures, a waxy cuticle that covers the surfaces of leaves andstems and limits the evaporation of water, and pores called stomata in leavesand stems that allow gas exchange but close when water is scarce, thus reducingwater loss. Other adaptations occurred later in the transition to terrestriallife and now wide spread but not universal among plants. These includeconducting vessels that transport water and minerals upward from the roots andthat move the photosynthetic products from the leavesto the rest of the plantbody and the stiffening substance lignin, which support the plant body, helpingit expose maximum surface area to sunlight.
【6】These adaptations allowed an increasing diversity of plant forms toexploit dry land. Life on land, however, also required new methods oftransporting sperm to eggs. Unlike aquatic and marine forms, land plants cannotalways rely on water currents to carry their sex cells and disperse theirfertilized eggs. So the most successful groups of land plants are those thatevolved methods of fertilized sex cell dispersal that are independent of waterand structures that protest developing embryos from drying out. Protectedembryos and waterless dispersal of sex cells were achieved with the origin ofseed plants and the key evolutionary innovations that they introduced: pollen,seeds, and later, flowers and fruits.
题目：
1.The word “presumably”(paragraph 1)i n the passage is closest in meaning to
B.supposedly.
C.obviously.
D.usually.
2.According to paragraph 1, all of the following are true of ancestral plants EXCEPT
A.They had cellulose-based cell walls.
B.They were closely related to green algae
C.They were able to store nutrients
D.They had a sophisticated multicellular structure.
3.The phrase “subjected to”(paragraph 1) in the passage is closest in
B.distant from
Dbined with
4.What can be inferred from paragraph 3 about ancient green algae?
comparisons
A. They lived in a generally wet environment that was sometimes dry.
B.They adapted better to changes in water temperature than did to other changes in the environment.
C.They inhabited areas that were close to the ocean.

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