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  Evolution of the Flowering Plants

  Many aspects of the history of flowering plants (angiosperms) remainmysterious.Evidence of the earliest angiosperms comes from fossilizedleaves,stems, fruits, pollen, and, very rarely, flowers.In addition,there hasbeen much study of modern plant morphology (structure)and genetics in orderto determine which living species might be most closely related to the ancientancestors of angiosperms. Despite intensive efforts for over 200 years,scientists have still not reached consensus on which type of plant was theancestor to the angiosperms, and when and where the angiosperms firstevolved Indeed, Charles Darwin himself called the origin of the floweringplants an "abominable mystery."

  What type of plant was the ancestor to the angiosperms? Most botanists nowagree that the flowering plants are monophyletic in origin, meaning that theyevolved from a common ancestor.Some paleontologists have suggestedthat the common ancestor may have been a type of cycad (palmlike tropicalplants). Other paleontologists maintain that the angiosperms may have evolved from seed-bearing ferns. Finally, analysis of the morphologicaltraits of some primitive living plants suggests that the ancestor may have beenrelated to the modern pines.The question of angiosperm ancestryremains unresolved.

  The time and place of the first appearance of flowering plants have long beena topic of great interest.There js good fossil evidence that early angiosperms,including a number resembling modern magnolias, were present in the EarlyCretaceous geologic period (more than 100 million years ago). Angiospermsbecame increasingly abundant during this period. Between 100 million and 65million years ago, a period known as the L .ate Cretaceous, angiospermsincreased from less than 1 percent of flora (plant life) to well over 50 percent.Many of the modern plant families appeared during this time period. In theEarly Tertiary period which followed: angiosperms increased to comprise 90percent or more of Earth's total flora. Where did these successful plants firstoriginate and spread from?

  Analysis of the fossil leaf structure and geographic distribution of the earliestCretaceous angiosperms has led many biogeographers to conclude that theyevolved in the tropics and then migrated poleward. It is known thatangiosperms did not become dominant in the high latitudes until the LateCretaceous. P aleontologists have recovered fossil angiosperm leaves, stems,and pollen from Early Cretaceous deposits in eastern South America andwestern Africa. These two continents were joined together as part ofGondwanaland, one of two supercontinents that existed at that time. Thelocations of these early angiosperm finds would have been close to theequator during the Early Cretaceous and are conformable with a model bywhich angiosperms spread from the tropics poleward.

  Not all botanists agree with an African-South American center for the evolutionand dispersal of the angiosperms, pointing out that many of the most primitiveforms of flowering plants are found in the South P acific, including portions ofFiji, New Caledonia, New Guinea, eastern Australia, and the MalayArchipelago. Recent genetic research has identified the rare tropical shrubAmborella as being the living plant most closely related to the ancient ancestorof all the angiosperms. This small shrub, which has tiny yellow- white flowersand red fruit; is found only on New Caledonia, a group of islands in the SouthP acific. Many botanists conclude that the best explanation for the largenumbers of primitive living angiosperms in the South P acific region is that thisis where the flowering plants first evolved and these modern species are relicsof this early evolution. Comparisons of the DNA of Amborella and manyhundreds of species of flowering plants suggest that the first angiosperm arose and the development of separate species occurred about 135 million yearsago.

  Recently discovered fossils complicate our understanding of the origin of theangiosperms even further Paleontologists from China have found beautifullypreserved fossils of an angiosperm plant, including flowers and seeds, inJurassic period deposits from China, The site, which is about 130 million yearsold, is near modern Beijing. The new fossil plant found at the site is now theoldest known angiosperm. The age of the fossils and the very primitivefeatures of the flowers have led the discoverers to suggest that the earliestflowering plants may have evolved in northern Asia.

  15. The word intensive in the passage is closest in meaning to

  A. periodic

  B. various

  C. strong

  D. promising

  Paragraph 1

  16. The phrase not reached consensus on in the passage is closest inmeaning to

  A. not formulated a theory aboutB. not agreed on

  C. not been able to explain

  D. not found evidence for

  Paragraph 1

  17. According to paragraph 1, all of the following types of evidence have beenimportant for studying the history of angiosperms EXCEPT

  A. fossils of angiosperm stems and leaves

  B. knowledge of the structure of modern angiosperms

  C. large collections of well-preserved angiosperm flowers

  D. genetic studies of modern angiosperms

  Paragraph 1

  18. The word maintain in the passage is closest in meaning to

  A. reply

  B. assume

  C. suspect

  D. claim

  Paragraph 2

  19. According to paragraph 2, most botanists now believe which of thefollowing about angiosperms.

  A. The ancestor of today's angiosperms was most likely a palmlike tropicalplant.

  B. Today's angiosperms can be traced back to a single ancestor.

  C. Angiosperms likely evolved from three very different plants.

  D. Angiosperms share a common ancestor with modern pines.

  Paragraph 2

  20. Which of the following can be inferred from paragraph 3 aboutangiosperms during the Early Cretaceous?

  A. They represented less than 1 percent of plant life.

  B. Their numbers remained stable during this period.

  C. They existed in forms that are very unlike plants living today.

  D. They only rarely left traces in the fossil record.Paragraph 3

  21. Why does the author include the information that fossil angiosperm leaves,stems, and pollen have been found in places that were once joined together aspart of Gondwanaland?

  A. To provide support for the claim that angiosperms first appeared in thetropics

  B. To explain how scientists have determined the geographical location ofsupercontinents during Earth's early history

  C. To provide evidence that angiosperms evolved in similar ways in differentparts of the world

  D. To challenge the idea that angiosperms spread toward the poles during theCretaceous

  P araqraph 4

  22. According to paragraph 4, researchers believe which of the following aboutangiosperms during the Early Cretaceous?

  A. They reached the poles and began spreading there.

  B. They spread from east to west on the Gondwanaland supercontinent.

  C. They were the main type of plant in high-latitude ecosystems.

  D. They were growing in some areas with a tropical climate.

  Paragraph 4

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

  A. Some botanists who agree with the idea of an African-South Americancenter for the dispersal of angiosperms point to primitive South P acificangiosperms as evidence.

  B. Some botanists argue that the very primitive forms of angiosperms found inthe South Pacific call into question the theory that angiosperm evolution beganin Africa and South America.

  C. Angiosperms in Fiji: New Caledonia, New Guinea, eastern Australia, andthe Malay Archipelago are more primitive than angiosperms in other parts ofthe world.

  D. Botanists disagree over whether the most primitive forms of living plants arelocated in parts of Africa and South America or in the South P acific.

  Paragraph 5

  24. What can be inferred from paragraph 5 about primitive living angiospermsin the South P acific?

  A. They more closely resemble the earliest angiosperms than do most otherliving angiosperms.

  B. They are very similar to angiosperm plants in South America and Africa.

  C. They are only found on the group of islands called New Caledonia.

  D.They provide evidence that angiosperms evolved in several sitessimultaneously.

  Paragraph 5

  25. In describing Jurassic-age fossils found in China as beautifully preservedthe author means that these fossils

  A. seem much younger than they actually are

  B. were found as a result of unusually good luck

  C. have suffered little damage over time

  D. were comparatively easy to dig up

  Paragraph 6

  26. According to paragraph 6, which of the following is true of angiospermfossils discovered in China?

  A. They have made it even more challenging to understand the origin ofangiosperms.

  B. They contain flowers that appear modern for the age of the fossils.

  C. They convinced most botanists that angiosperms did not originate innorthern Asia.

  D. They are not as old as angiosperm fossils discovered in the South Pacific.

  Paragraph 6

  27. Look at the four squarest that indicate where the following sentencecan be added to the passage.But their origin and evolution have been under considerable dispute.Where would the sentence best fit?Click on a square[ ] to insert the sentence in the passage

  Paragraph 2

  28. Directions: An introductory sentence for a brief summary of the passage isprovided below. Complete the, summary by selecting the THREE answerchoices that express the most important ideas in the passage. Some answerchoices do not belong in the summary because they express ideas that are notpresented in the passage or are minor ideas in the passage. This questions isworth 2 points.

  Despite many years of research, some basic questions about the history ofangiosperms remain unanswered.

  Answer Choices

  A. One of the major diffiulties of studying angiosperm evolution is that fossilsfrom angiosperms are very rarely discovered.

  B. Fossil evidence led some botanists to conclude that angiosperms firstevolved in the tropics and spread outward toward the poles.

  C. Emerging research from fossils in South America, the South Pacitic, andAsia suggests that the first angiosperms appeared more recently than wasonce believed.

  D. Though the original ancestor of angiosperms is unknown, they existed atleast by the Early Cretaceous and eventually made up most of the plants onEarth.

  E. Early Cretaceous fossil deposits have confirmed that the ancestor to theangiosperms was a plant resembling the modern magnolia.

  F. The study of living angiosperms in the South Pacific and new fossildiscoveries in China have led to new theories about where angiosperms firstevolved.

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