巴朗托福听力文本-Test 6(下)

巴朗托福听力文本-Test 6(下) BIOLOGY CLASS
Narrator: Listen to part of a lecture in a biology class. The professor is talking about bacteria.
$ Professor:
$ Bacteria is the common name for a very large group of one-celled microscopic organisms that, we believe, may be the smallest, simplest, and perhaps even the very first forms of cellular life that evolved on Earth. Because they’re so small, bacteria must be measured in microns, with one micron measuring about 0.00004 inches long. Most bacteria range from about 0.1 microns to about 4 microns wide and about 0.2 microns to almost 50 microns long. So how can we observe them? I’ll give you one guess.
$ Under the microscope, of course. As I said, bacteria are very primitive and simple. In fact, they’re uni cellular, which means that they’re made up of a single cell. We think they probably evolved about three and a half billion years ago. Some of the oldest fossils are bacterial organisms. They’ve been found almost everywhere on Earth, including all the continents, seas, and fresh water habitats, and in the tissues of both plants and animals.
$ Well, since they’re so prevalent, you might ask, how do they reproduce? Okay, they grow in colonies and can reproduce, quite rapidly, in fact, by a process called fis sion. In fission, the cell, and remember, there’s only one in bacteria, one cell. So the cell increases in size and then splits in two parts. Fission is also referred to in your text as asexual budding. Now you’ll also read about conjugation, and that’s wh en two separate bacteria exchange pieces of DNA, so there are two ways that reproduction can occur, but we think that fission is more common.
$ Okay. Bacteria were virtually unknown until about 1600 when microscopes were introduced, and at that time, bac teria were observed and classified into three main types according to their shapes, and that classification hasn’t really changed that much over the years. So that’s what I want to talk about today—the main types of bacteria. The slides that I’m going to s how you are enlargements of bacteria that I observed under the microscope in the lab earlier today. Now, this first slide is an example of bacilli.
$ The bacilli are a group of bacteria that occur in the soil and air. As you can see, they’re shaped like rods, and if you were to see them in motion, they’d be rolling or tumbling under the microscope. Of course, you can’t see that because this is a still visual, but later, when you go into the lab, you’ll see that rolling motion in examples of bacilli. These are kind of a greenish blue, but some are yellow. So don’t try to identify them by their color. Look at the shape. These bacilli are largely responsible for food spoilage.
$ Okay, the next slide is a very different shape of bacteria. It’s referred to as the cocci group, and it tends to grow in clusters or chains, like this example. This specimen is one of the common streptococci bacteria that cause strep throat.
$ Finally, let’s look at the spiral-shaped bacteria called the spirilla.
$ This is the sp irilla. They look a little like corkscrews, and they’re responsible for a number of diseases in humans. But I don’t want you to get the wrong idea. It’s true that some species of bacteria do cause diseases, but for the most part, bacteria are benign. There’s a lot of bacteria in this room in fact. We all have it on us. They live harmlessly on the skin, in the mouth, and in the intestines. In fact, bacteria are very helpful to researchers because bacterial cells resemble the cells of other life forms in many ways, and may be studied to give us insights. For example, we have a major research project in genetics here at the university. Since bacteria reproduce very rapidly, we’re using them to determine how certain characteristics are inherited.
$ Okay, now, let me review these three types with you . . . cocci are spheres, bacilli are rods, and spirilli are spirals. One of my students came up with a way to remember them. Just try to visualize the first letter in the name of each of the different types: Cocci starts with C like the shape of half a sphere. Bacilli starts with a straight line on the B, and a rod is straight. Spirilla starts with S, and that’s a spiral shape.
$ If it helps you, use it.
$ In any case, although I want you to know the three major classifications, within these basic groups there are virtually hundreds of variations that make them somewhat more difficult to identify and classify than the rather straightforward specimens that I showed you a minute ago. Because, you see, bacteria can join in chains, clusters, pairs. And sometimes, more than one type of bacteria may be found together in a specimen. I think you get the picture.
$ Okay then, in addition to identifying bacteria by their shape, which we now know isn’t really a very good
me thod for distinguishing them easily, if we really want to identify what type of bacteria we’re dealing with, it’s better to study the biochemistry or genetic structure of the specimen. They have one chromosome of double-stranded DNA in a ring, which we can analyze fairly easily.
HISTORY CLASS
Narrator: Listen to part of a lecture in a history class.
$ Professor:
$ Frontier home design in the United States was greatly influenced by the provisions of the Homestead Act of 1862. The legislation gave settlers the right to open land but mandated that homesteaders build a structure that was at least ten by twelve feet and included at least one glass window, and they had to live on their homestead and improve the land for five years before their claim was recognized. Of course, when they first arrived, most homesteaders lived in their wagons or pitched tents until they filed claims and planted crops. And even then, knowing that fully half of the homesteaders wouldn’t make it through the five years required to complete their claims, homesteaders tended to view the construction of their homes as semi-permanent dwellings . . . more likely they’d build something better later or try to improve on what they’d built initially if they made it through the first five ye ars. So, in addition to the requirements in the Homestead Act, the settlers needed a home that was easy to build, cheap, and maybe even disposable.
$ Well, the log cabin is the construction that comes to mind when we think of Western settlements, but the plains and the prairies had so few trees that log construction was almost impossible. So the sod house was a practical solution for homesteaders on flat, treeless land. So how do you build a sod house?
$ Well, first you wait for a rain that makes the earth soft, then you use a sod cutter to form sod bricks about two or three feet square and a few inches thick. Then, you stack the bricks to form walls, and weave branches or twigs and grass into a roof that’s finally covered with sod as well. Now, there we re tremendous advantages to this type of construction. In the first place, it was very cheap . . . there are journals from the 1800s that document construction prices at about $2.50, and most of that was for the glass window. And, it took very little time to build, probably a day or two. And the thick walls actually kept the house quite cool in the summer and fairly warm in the winter. If a better home could be built later, the sod house would simply dissolve into the soil.
$ But there were serious disadvantages as well. Even well-built roofs leaked onto the dirt floors, forming mud puddles, and sometimes the roof even collapsed from the water weight. Or, in dry spells, the dirt crumbled from the roof into the home. Not to mention the infestations of insects and even snakes that inhabited the dirt walls.
$ So, those settlers who arrived in wooded areas opted to build log cabins instead of sod homes. Like the sod construction, the log cabin could be built in a few days, using simple tools, often only an axe. But it was much more comfortable. There’s evidence that the first log cabins were introduced by Swedish settlers as early as the 1700s but other immigrant settlers quickly adopted the construction. First, you build a foundation of rocks to keep the logs away from dampness that might cause them to rot. Then, you cut down the trees and square off the logs, cutting notches in the top and bottom of each end so they could fit together when they were stacked at the corners and it also had the advantage of assuring structural integrity. And there were several types of notching techniques that were used, depending on the skill of the builder. In any case, with notching, no nails were required and that was good since nails had to be shipped into towns and then transported out to the new settlements. But there were gaps in the walls so these had to be filled by a technique called chinking. In chinking, grass, hay, moss and mud were worked into rolls about a foot long and maybe four inches wide and then they were inserted into the cracks between the logs. $ These rolls were commonly referred to as mud cats and were very effective in keeping out the cold and keeping in the heat. Of course, the tighter the logs, the fewer chinks were required, and that’s important becau se the chinks were the weakest part of the cabin, and with the expansion and contraction that resulted from freezing and thawing, well, chinking tended to deteriorate and needed constant maintenance and repair.
$ Okay, there was usually a stone or brick fireplace along one wall. And the roof was usually made of wood shingles. So you can imagine, this was quite an improvement over the sod house. The advantages were that the home could be kept clean. Even though the floor was usually dirt or gravel because flat boards were difficult to obtain, it was still an effective shelter to keep out the rain and dust.
$ Later, at the end of the 1800s, when the railroads brought materials such as asphalt shingles, tar paper, and finished boards to the frontier, the sod house was abandoned for one-room board shanties, covered with tar paper. Whether this was an improvement is subject to debate. For one thing, since they were often built without foundations, the harsh winds of the prairies literally blew the shanties away. $ Still, many settlers considered the shacks preferable to the old soddies even though they weren’t as easy to heat and cool. To go back to the log cabin for a minute, the effect of new construction materials on the log cabin was . . . aesthetic . . . as well as practical. The logs were often covered on the outside by finished boards and on the inside with plaster, which gave the cabins a more finished look and improved insulation. And by this time the old one-room ten-by-twelve was also being replaced with larger homes with several rooms. The frontier settlers had weathered the hardships of their first five years, they’d received their claims, and they and their homes were a permanent part of the great western expansion.。