托福考试的阅读文章节选自大学程度的教科书，涉及某种学科或者主题，节选材料尽可能地保持了文章原貌，以衡量考生们阅读学术材料的能力。阅读文章涵盖不同科目的内容，题材主要集中在历史、艺术、社会科学和自然科学方面。在很多情况下托福阅读背景知识对于考生解题时候的帮助都是“隐性”的，但其实，在很多类型的托福阅读文章中，托福阅读背景知识的重要性甚至比技巧还要高。现在我们就来积累一下托福阅读背景知识吧。
Cactus adaptations
The secret to the superior endurance of cacti lies in their adaptations. Over millions of years, through natural selection, only the strongest and best adapted species survived.
As you know, it is very dry in the desert. Plants that adapt to this are known as xerophytes (from xeros, dry and phyton, plant). There are plants that avoid the dry season by sprouting from seed just after the spring rain and growing very fast so that by the time the dry season comes, they have already produced a lot of seeds and died. These seeds lie on the soil for the dry season and sprout again in spring and the cycle repeats. Other xerophytes simply drop their leaves and stay dormant for the winter. But there is another special type of xerophyte which stores water in its fleshy tissues. Such plants are called succulents (from succus, juicy). The cactus is a typical example of a succulent.
If you cut a cactus open, you see a juicy, slimy tissue. This is where the moisture is stored for the dry season. The part between the middle circle (or pith) and just under the very green part of the plant (or palisade parenchyma) just under the skin is allocated for the storage of water and food for the plant. This is a type of spongy parenchyma and can take up up to 85% of the plant's volume. This is a major adaptation in the desert. Because the plant remains completely alive during the dry season and there is no need for it to dry up and lose everything, makes it possible for the plant to grow to large sizes. Another advantage is that the plant retains supplies (in the form of starch) for the winter so that it can flower right away in spring without accumulating more supplies (as most plants need to do in spring). The whole purpose of storing supplies for the winter is mostly to energize flowering in spring but it also lets the cactus start growing much sooner.
Flowering plants breathe and transpire (evaporate water from their surface) through closeable microscopic pores called stomates on the leaves or stems. To do this, their pores have to be open. In most plants these are open all day and on warm nights. But for cacti this is inconvenient as in daytime it is very hot and thus the plant would lose a lot of water through evaporation. So the cactus must close them in the daytime. But then it cannot breathe or photosynthesize (the process where sugars are made from carbon dioxide and water and releasing oxygen using the sun's energy). Succulents have an adaptation to that. Their stomates are closed during the day and are open at night, when it is not that hot and store carbon dioxide in its tissues as crassulean acid and then turn it back to carbon dioxide in the daytime. This process is called crassulean acid metabolism or CAM and it is a very smart way of respiring in the desert.
If we look at the outside of the plant, we notice that there is a tough leathery skin covering the plant, we can also notice the presence of ribs and spines and sometimes fur. These are all very smart adaptations. They serve mainly for surviving heat but are also used as defense.
仙人掌的适应
以仙人掌上的耐力的秘诀在于其适应能力。数百万年来，通过自然选择，只有最强壮的和最佳改编的物种存活了下来。
你知道，在沙漠里很干。植物适应这种被称为旱生植物(从干燥，干燥的植物，植物)。有植物，避免干燥季节发芽的种子在春天的雨和成长的非常快，当旱季来临时，他们已经产生了大量的种子和死亡。这些种子躺在旱季土壤和芽再在春季和周期重复。其他旱生植物是落叶和保持休眠过冬。如果你把一个仙人掌开放，你看到一个多汁的，粘糊糊的组织。这是其中的水分储存干燥的季节。中间圈之间的部分(或芯)和下植物的绿色部分(或栅栏组织)在皮肤下的分配水和食物的储存装置。
中间圈之间的部分(或芯)和下植物的绿色部分(或栅栏组织)在皮肤下的分配水和食物的储存装置。这是一种类型的海绵组织，可以高达85%的植物的体积。这是沙漠中的一个主要的适应。这是沙漠中的一个主要的适应。因为植物仍然活在干燥的季节，没有必要为它干了，失去了一切，使植物生长的大尺寸。另一个优点是，保留植物供应(以淀粉的形式)的冬季可以花就在春天没有积累更多的供应(如大多数植物需要在春季)。另一个优点是，保留植物供应(以淀粉的形式)的冬季可以花就在春天没有积累更多的供应(如大多数植物需要在春季)。贮存过冬物资的整个目的是为了激励在春天开花而且还让仙人掌生长很快的开始。
开花植物的呼吸和蒸腾(蒸发的水从表面)通过闭合微孔呼吁叶或茎气孔。要做到这一点，他们必须开孔。在大多数植物中，这些都是全天开放的，温暖的夜晚。但仙人掌这是不方便在白天很热，从而使植物将失去大量的水通过蒸发。所以仙人掌必须关闭他们在白天。但它不能呼吸和光合作用(在这个过程中，糖是由水和二氧化碳和释放氧气的利用太阳的能量)。肉质植物有一个适应。他们的气孔白天关闭，夜间开放，当它不热，存储在其组织中的二氧化碳，然后重新打开它的智力在白天二氧化碳。这个过程是智力的代谢或凸轮称，这是一个非常聪明的方式呼吸在沙漠。
以上就是本次前程百利小编为大家整理的托福阅读背景知识之仙人掌的适应，希望能够帮助大家备考托福，前程百利祝大家都能取得理想的考试成绩。
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