【托福写作机经】2025年3月1日托福考试回忆【上午场】

阅读

听力

　　 主题：关于植物knees功能的假说

Oxygen absorption

First, knees may help the trees' roots absorb or take in oxygen. Plant roots require oxygen for important cellular functions, and the soil in the watery habitats where cypress trees often grow is low in oxygen. Roots of plants growing in low-oxygen soil must often obtain oxygen from another source. For example, mangrove trees, which also grow in soil that is underwater, are known for having above-ground roots that absorb oxygen. Like the roots of mangroves, cypress knees may take in oxygen from the air and deliver it to the deeper roots.

First, it seems reasonable that a structure sticking up into the air would help absorb or take in oxygen. But researchers tested this hypothesis. They place cypress knees in tightly closed containers with oxygen. And measured how the oxygen levels inside the container changed over time. Surprisingly, they found that the oxygen levels did not change much. This indicated that the knees absorbed very little oxygen. If the function of cypress knees were to take in oxygen, you would expect the knees to absorb significant amounts of the oxygen in the containers. That did not happen.

Additional stability

Second, knees may help cypress trees stand firmly in the soil, making them better able to remain securely in place when exposed to high winds or strong water currents. The fact that the cypress has knees in addition to a network of roots may provide additional stability, allowing the tree to anchor itself well in the soil. Cypress trees may need this additional support due to the softness of the wet soil in which they grow.

Second, the idea that knees help cypress trees stay firmly rooted in the soil is problematic, too. You see. Many cypress knees are attached to the cypress's horizontal roots. Those horizontal roots do not extend down deep into the soil. And the knee does not provide any additional stability in that situation. These can help with stability only if they're attached to vertical roots that reach deep into the soil. But many knees are not connected to vertical roots at all. Since the knees are so frequently attached to horizontal roots rather than to vertical ones, it's doubtful that physical support is their main purpose.

Methane release

Finally, knees grow on trees in areas where methane gas exists in the soil. The knees are in direct contact with the soil infused with methane, which is of no use to trees. Therefore, the function of knees may be to release methane gas from the trees' roots into the air. The fact that measurable amounts of methane have been detected in the air above the knees supports this idea.

Finally, about methane release. This idea is unconvincing for a couple of reasons. You see, although methane is not useful to trees, it really does them no harm. It seems unlikely that the trees would develop special features like the knees just to get rid of something that has no negative effect. Besides, lots of microorganisms live on cypress knees. And it's completely possible that these tiny organisms produce the methane detected around the knees. Not the knees themselves.

阅读

听力

　　 主题：金星上灰光的来源

1.闪电活动(Lightning activity)：金星上的灰光可能是由频繁的闪电活动产生的。金星的大气层中含有大量的云层和带电粒子，这些云层在运动过程中会产生强烈的摩擦和碰撞，从而引发闪电。闪电释放出的光亮在金星的夜半球就可能表现为灰光，就像地球上的闪电在特定条件下也会产生明显的光亮一样。

1.目前的观测技术已经能够较为准确地探测到金星上的闪电信号，但实际探测到的闪电频率和强度远远不足以产生我们所看到的灰光。也就是说，即使金星上存在闪电活动，其产生的光亮也太微弱，无法解释持续且明显的灰光现象，所以闪电活动不太可能是灰光的主要来源。

2.磷化氢化学反应(Chemical reaction of phosphine)：金星大气层中存在磷化氢(Phosphine)，它可能与其他化学物质发生化学反应，进而产生发光现象。磷化氢是一种具有特殊化学性质的物质，在金星的大气环境中，可能与氧气、二氧化碳等发生反应，反应过程中释放出能量并以光的形式表现出来，形成我们所观测到的灰光。

2.虽然金星大气层中存在磷化氢，但根据现有的化学模型和实验，磷化氢与金星大气中其他物质发生反应所产生的能量和光亮非常有限，难以达到灰光所呈现出的亮度和持续性。因此，磷化氢的化学反应不太可能是导致灰光出现的原因。

3.太阳反射(Sun reflection)：金星的某些表面特征可能会反射太阳光，从而形成灰光。金星表面有山脉、峡谷等不同的地形，这些地形在特定的角度和光照条件下，可能会像镜子一样反射太阳的光线。当这些反射光在金星的夜半球被观测到时，就可能呈现为灰光。

3.金星的自转速度非常缓慢，而且其表面的地形和物质分布使得太阳反射光很难在金星的夜半球形成稳定且明显的灰光。从理论上来说，太阳反射光在经过金星大气的散射和吸收后，到达夜半球时的强度已经非常微弱，无法形成我们所观测到的较为显著的灰光现象，所以太阳反射也不太可能是灰光的来源。

阅读

听力

　　 主题：动物为什么玩耍

The first theory is called the surplus energy theory. This theory maintains that some animals, mammals for example, are so efficient at using energy that they often do not manage to spend all the energy they receive through food. They end up with unused, surplus energy that they cannot store. According to this theory, animals have to play to get rid of the surplus energy that they have not used.

Let's take the first theory, the surplus energy theory. This theory is contradicted by observations of marine mammals such as seals. Young seals sometimes have to survive alone for days, waiting for their mothers to bring food from far away. During that time, the young seals have no energy intake. As the days pass, they have very little stored up energy remaining, certainly no surplus energy. and yet. They still play, even though they have no energy to spare.

A second theory is called the instinct-practice theory. It states that young animals play as part of their physical education. The physical actions that animals will need to use for survival are instinctual, that is, preprogrammed in their brains. The animals, however, have to train their bodies to be able to perform these actions. So this theory explains why the play of young animals centers on real-life tasks. For example, predatory animals—those that hunt, kill, and consume other animals for food—have often been observed attacking and biting during play, which are precisely the skills they will need in order to hunt for food.

Now about the second theory, the instinct practice theory, the problem is that in some animals, playing and practicing don't seem to make much difference when it comes to hunting skills. In one experiment with cats, for example, researchers permitted one group of young cats to play, but they did not allow cats and another group to play. The researchers thought that the cats that were permitted to play would become better hunters. That didn't happen as the animals grew older, the cats that had played were not more skilled in hunting than the other cats。

A third theory, called the social functions theory, is that animals play to establish social relationships within their group. Basically, play gives animals a chance to bond with the other members in their group. For example, in a study of rats, those that played together made high-pitched squeaks that were accompanied by the release of pleasure-causing chemicals in the brain. This association of play with pleasure may motivate the animals to stay together as a harmonious group even while not playing.

And how about the third theory? The social functions theory. Well, although playing may help rats socialize with one another. Play is probably not necessary for rats to develop social behavior. Rats only play when they're very young. But scientists observe that the region of a rat's brain that's important for social behavior can develop even when rats are older and don't play anymore. In other words, rats can develop the ability for social behavior even after they've passed the age when they play.