【正确答案】 B

【答案解析】Listen to part of a lecture in a botany class. Professor: Hi, everyone. Our fascinating subject today is all about trees! Now, can anyone tell me what the two main types of trees are? Student: Ah, I’m not sure, but I think they are broadleaf trees and, like, Christmas trees, with needles and cones. Professor: Close enough. Broadleaf trees are called deciduous, and trees with needles and cones are called coniferous. The deciduous is a form of what we call angiosperm, which just means that it is a flowering plant with seeds. Coniferous trees are gymnosperms, which is just another fancy word to say they don’t have flowers but have cones. In fact, you could say that the cones are their flowers and seeds. Now, I will discuss the main differences between the two, and then I want to talk about the structure of trees in general. First, let’s look at the deciduous family of trees. They are our maples, elms, oaks, and so on, the ones on your front lawn and your street. Their leaves are wider and broader than coniferous trees and change color in fall in temperate climates and drop off for the winter. Now they do this because they are really too fragile to survive winter. Coniferous trees, however, keep their needles all year long, oh, except for the oldest ones, which they shed. Examples of coniferous trees are pines, spruces, and firs. As I mentioned already, they have cones instead of flowers and seeds, and these cones fall and may turn into new conifers. Okay, so those are the main differences. Deciduous trees have flowers, seeds, and broad leaves, which they drop in the fall, and coniferous trees have needles and cones and keep almost all their needles while sometimes dropping their cones. And they smell much better, don’t they? Oh, I had a wonderful pine for Christmas this year. The smell is still fresh in my mind. Okay, anyway, now on to the structure of trees. Besides flowers, seeds, and cones, trees have three main parts that enable them to survive: their roots, bark, and their leaves or needles. Roots go deep in the ground and absorb water up through the tree’s system, which is very porous. Now, not all root systems are the same. There are two main types: shallow roots and deep roots. The shallower ones spread not far underground, are numerous, and are generally small in size. They sometimes even stick up through the surface of the ground. Some trees, like pines, have many roots but also a single main root, called a taproot, which goes very deep underground. This taproot can cause problems if someone tries to move the tree because. If you cut it, the tree will die. The bark of a tree is in two layers, a thick outer layer and a thin inner layer. The outer layer is made up of dead plant cells, which the trees shed. The inner layer is living cells, but they too will eventually die. You know… there is a whole science dealing with the counting of bark tree rings. Specialists can determine if there was a great deal of rainfall in a given year by measuring the growth of the bark and the size of the ring. Of course, they have to cut down the tree to do this. Anyway, so the bark allows water and nutrients to be absorbed by the tree. It also protects the tree from losing too much water and from some diseases. However, in areas with lots of acid rain, the bark can be worn off, and the trees can easily die. Leaves are the farmers of the tree and givers of life to humans. If there were no plants on Earth, we would all die from a lack of oxygen. Leaves perform two jobs for the tree: they produce sugars for the tree to absorb and remove water through evaporation. This is done by a process called photosynthesis. Carbon dioxide and water combine with sunlight and the green chlorophyll in the leaves to make a chemical reaction that produces a sugar-like substance that the trees feed on. As this chemical reaction takes place, oxygen is byproduct and is released into the air that we breathe. So trees do us two favors: they soak up harmful carbon dioxide, and they produce life-giving oxygen. I highly recommend everyone have a few plants in their home or dorm room. Not only will they make it look nicer, but they will also improve the quality of your air. Unfortunately, man has not heeded the necessity of trees, and they are being cut down in record numbers. While some logging companies replant them immediately, others are not so generous. Of particular worry now is the Amazon rainforest, one of our main sources of oxygen. 请听一段植物学课堂讲座。教授：大家好。今天我们讨论一个吸引人的话题——树。好，谁能告诉我两个主要树种是什么?学生：啊，我不确定，但我想应该是宽叶树和像圣诞树那样的有针叶及球果的树。教授：非常接近。宽叶树被称为落叶树，而有针叶及球果的树被称为针叶树。落叶树是被子植物的一种，也就是说，它是一种有种子的开花植物。针叶树是裸子植物，也就是说，它们没有花但有球果。好，我们先讨论一下这两种树的主要区别，然后谈一下树木的大体结构。首先，让我们来看看落叶树家族。在你家前院的草坪及街道上长着的枫树、榆树、橡树等都是落叶树。它们的叶子比针叶树的叶子更宽大，在温带，秋天叶子变色、落下，为了过冬。这是因为它们很脆弱，难以撑过冬天。然而，针叶树终年长有针叶，啊，最古老的落叶针叶树除外。松树、云杉、冷杉都是针叶树。我提到过，它们有球果，没有花和种子，这些球果掉落后会长出新的针叶树。好了，这些是主要区别。落叶树有花、种子和宽叶，秋天落叶，而针叶树有针叶和球果，几乎终年长有针叶，有时掉落球果。而且它们闻起来更香，不是吗?哦，今年我就有一棵上好的圣诞树。它的香气仍让我记忆犹新。好了，现在我们再来谈谈树木的结构。除了花、种子和球果，树木还有它们赖以生存的三个重要部分：根、树皮、叶子或针叶。根深深地长在地下，通过渗透性极好的树木系统来吸收水分。然而，不是所有的根部系统都一样。主要有两种：浅根和深根。浅根多而小，长得不是很深。有时，它们甚至长出地表。有些树，像松树，有很多根，但有一个主要的根，叫主根，长在地下很深的地方。有人想移动这棵树时，主根就会引发一些问题，因为一旦你把主根砍了，树就会死掉。一棵树有两层树皮，厚的外层和薄的内层。外层是树木脱落的死植物细胞。内层是活细胞，但最后也会坏死。要知道，有一个学科专门计算树木的年轮。专家可以通过测量树皮的生长及年轮的大小来确定特定年代的降雨量。当然，他们得把树砍掉才能完成这项工作。不管怎样，树木通过树皮吸收水分和养料，并且通过树皮防止水分流失和抵御一些疾病。然而，在酸雨多发的地区，树皮受到腐蚀会脱落，树木很容易就死掉了。树叶是树木的培育者，人类的养育者。如果地球上没有植物，我们就会因缺氧而死。树叶对树木起到两个作用：它们产生糖分供树木吸收，并通过蒸发去除水分。这通过光合作用来完成。二氧化碳、水与太阳光以及树叶中的叶绿素相结合，发生化学反应，从而产生树木赖以生存的糖类物质。发生这种化学反应时，氧气就是一种副产品，被释放到我们所呼吸的空气中。因此，树木带给我们两个好处：他们吸收有害的二氧化碳，释放出维持生命的氧气。我强烈推荐每个人在家中或寝室养一些植物。它们不仅可以美化环境，还可改善空气质量。遗憾的是，人们没有注意到树木的必要性，它们正在被大量砍伐。有些伐木公司会立即重新种植，而另一些就没那么大方了。现在最令人担忧的是亚马逊雨林，它是我们氧气的主要来源之一。 12．教授主要讨论了树木的哪个方面? A.落叶树和针叶树的区别 B.树根、树皮及树叶的作用 C.人们过度伐木的危险性 D.树木造福人类的途径解析教授虽然提及了树的两大种类，但讲课的中心内容还是树的报、茎、叶的构造和功能