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دانلود اپلیکیشن «زبانشناس»
این درس را میتوانید به بهترین شکل و با امکانات عالی در اپلیکیشن «زبانشناس» بخوانید
متن انگلیسی درس
Listen to part of a lecture from a life sciences class.
Prof: Today… ahem, can I have your attention please? Thank you. Today I want to talk about a pungent plant that is known by such flattering aliases as “Stinking Ninny” and “Mare’s Fart.” It’s called ragwort, and although it is a flower, it’s definitely not the type you want to give your wife on Valentine’s Day. Ragwort is a wildflower with characteristics of a weed. Though it’s indigenous to Europe, it can be found all over the world, primarily in cool climates with high precipitation. Ragwort is a hearty weed that can, is capable of rapid multiplication, and sometimes overruns the places where it grows. Because of this, Ragwort is controversial. It has a reputation – an unmerited reputation, in my opinion – as an invasive and, uh, deadly plant.
In the past, we, er, humans have used it as a type of salve to reduce swelling and relieve pain, and also to make green and yellow dyes. Some of the natural ingredients contained in Ragwort is used in still used in some natur [false start] herbal remedies today. Ragwort’s reputation for deadliness springs from its innate chemical organic compounds, called alkaloids. Alkaloids make the plant toxic to animals such as, uh, horses and cattle, which can get fatal liver damage from eating too much of the plant. This has cast Ragwort into, shall we say, disrepute among ranchers and farmers, and created controversy about the actual degree of danger it poses to livestock. The key question being, “how much is too much?” British horse breeders, for instance, have claimed that Ragwort poisoning causes as many as 6,500 horse deaths a year. Hmm…[pause] Critics are extremely skeptical of this number, because research suggests that the animals would have to eat a prodigious amount of Ragwort to produce such a huge number of deaths.
For example … [pause] in one study, a horse lived after eating Ragwort equal to a quarter of its body weight over a period of 140 days. And in another study, 400-pound cows were fed Ragwort equal to about 12-percent of their body weight over a 20-day period. Even though this was a, well, substantial amount of toxin, 25-percent of the cattle survived. Moreover, statistics from the British government dramatically contradict the figures from horse breeders. These stats indicate that annual confirmed cattle deaths from Ragwort poisoning range from only ten to twenty. Considering that studies indicate Ragwort is more toxic to cattle than to horses, and that both cows and horses detest Ragwort’s bitter taste, critics understandably ridicule the 6,500 figure. Government: 10 to 20 deaths per year. Breeders: sixty-five hundred. That’s a rather large discrepancy, is it not? [clears throat] It’s also pertinent to note that sheep routinely eat small quantities of Ragwort with no apparent harmful effects. In - on the contrary, Ragwort seems to benefit them, because the alkaloids kill worms in their digestive tracts.
Perhaps a more, uh, more valid concern is the cumulative effect of Ragwort ingestion. The plant’s toxin itself doesn’t store in the liver, but a derivative of the toxin damages DNA and can destroy liver cells gradually. The fatal toxicity level for horses has typically been claimed to be 3 percent to 7 percent, but, as I noted, they might be able to assimilate a far greater percentage. Why is this? Two reasons. First, the liver can often metabolize small amounts of toxins before they cause any damage, and second, bacteria in animals’ digestive tracts destroy Ragwort’s original alkaloids before they enter the bloodstream. Also for these reasons, it is safe for humans to eat the meat of animals that have ingested Ragwort. Any damage from Ragwort alkaloids would be confined to the animal’s liver; toxic residue does not leech into the meat. So, while the chances of animals contacting Ragwort poisoning appear to be small, livestock owners remain concerned. And who can blame them? Ragwort poisoning has no known antidote, and it causes animals to suffer a very painful death, characterized by depression and a lack of coordination.
The British government classified Ragwort as one of five “injurious weeds” in its Weed Act of 1959, a classification it later amended in the Ragweed Control Act of 2003. A common mistake, er, misperception is that these legislative orders require landowners to prevent Ragwort from spreading on their property. In reality, all the acts do is to empower government officials to order such prevention if they deem it necessary. The acts themselves seem based on another misconception: that Ragwort is an, um, intensely invasive plant. It’s true that Ragwort seeds are dispersed by wind, but evidence shows that these seeds do not travel far. In a United States study of the dispersal of more than 53,000 Ragwort seeds, it was found that almost 90 percent of them traveled less than six yards, and that no seeds were dispersed more than 15 yards from their producer. It seems safe to conclude, therefore, that Ragwort plants pose little threat of widespread colonization.
Despite such scientific evidence, misconceptions about Ragwort persist in the popular press. Imagine that! [laughter] One of the most persistent rumors is that Ragwort is “spreading like a plague.” Another common myth is that horses and cows can be poisoned by eating or inhaling Ragwort seeds or spores. This is ridiculous because Ragwort in fact has no spores. The more one learns about Ragwort, the more it seems that um, that the most likely way a horse or cow can contract Ragwort poisoning is if its owner allows it to graze for long periods in pastures rife with Ragwort plants. This is an action so irresponsible it defies imagination.
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