sweetbox
托福考试 复习
TPO 25—3 The Evolutionary Origin of Plants
licc
原文：
【1】The evolutionary history of plants has been marked by a ries ofadaptations. The ancestors of plants were photosynthetic single-celledorganisms that gave ri to plants presumably lacked true roots, stems, leaves,and complex reproductive structures such as flowers. All of the 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 clo phylogenetic (natural evolutionary)relationship between the two groups. DNA comparisons have shown that greenalgae are plants’ clost living relatives. In addition, other lines ofevidence support the hypothesis that land plants evolved from ancestral greenalgae ud 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 cellulo, similar in composition to tho of land plants. Again, thegood storage and cell wall molecules of red and brown algae are different.
cheap的比较级【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 ri to land-dwelling ororganisms.
【3】The environmental conditions of freshwater habitats, unlike tho ofocean habitats, are highly variable. Water temperature can fluctuate asonallyor 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.The adaptations rved 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’volutionary 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 prent in far higher concentrations inair than in water).So once natural lection 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 challengespod 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. The conditions favored the evolution of the structures thatsupport the body, vesls that transport water and nutrients to all parts ofplant, and structures that conrve water.
韩国身份证号The resulting adaptations to dryland include some structural features that aro early in plant evolution; nowthe 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 clo 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. The includeconducting vesls 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 expo maximum surface area to sunlight.
【6】The 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 x cells and disper theirfertilized eggs. So the most successful groups of land plants are tho thatevolved methods of fertilized x cell dispersal that are independent of waterand structures that protest developing embryos from drying out. Protectedembryos and waterless dispersal of x cells were achieved with the origin ofed plants and the key evolutionary innovations that they introduced: pollen,eds, and later, flowers and fruits.
题目：
1.The word “presumably”(paragraph 1)i n the passage is clost in meaning to
B.suppodly.
C.obviously.
msdn是什么D.usually.
2.According to paragraph 1, all of the following are true of ancestral plants EXCEPT
gelatin>buckA.They had cellulo-bad cell walls.
ithink
B.They were cloly related to green algae
C.They were able to store nutrients
D.They had a sophisticated multicellular structure.
notatall什么意思
3.The phra “subjected to”(paragraph 1) in the passage is clost in
B.distant from
4.What can be inferred from paragraph 3 about ancient green algae?
A. They lived in a generally wet environment that was sometimes dry.
初级管理会计师B.They adapted better to changes in water temperature than did to other changes in the environment.
C.They inhabited areas that were clo to the ocean.
