4月15日的托福考试已经过去了，很快就是4月21日的托福考试了，大家对于考试有没有信心呢，下面和出国留学网小编一起来看看2018年4月21日托福考试阅读机经。

　　2018年4月21日托福考试阅读机经预测（部分内容展示）：

　　P1： Vegetation of the sea is more primitive on the evolutionary scale than that of the land. Different varieties of seaweed vary tremendously in form and elaborateness of organization， ranging from single-celled， microscopic flagellates to giant kelp which grow to be five or six hundred feet long. They have no flowers or seeds， their reproduction and distribution being accomplished by asexual spores which are simpler structures than seeds. They do not have roots but are anchored to the substratum by a structure called a “holdfast＂ and absorb necessary mineral nutrients directly from the sea water through their leaf-like fronds. Like land plants， they possess chlorophyll and carry out photosynthesis， through which they utilize the energy of sunlight to synthesize carbohydrates， proteins， and fats from water， carbon dioxide， and， as required， other inorganic chemicals. These products differ chemically from their analogues in land plants， however.

（图片转自百度）

　　•P2： Seaweeds are found throughout the world＇s oceans and seas， inhabiting about 2 percent of the seafloor. Most species of seaweed live directly on the seafloor where they grow on rock， sand， mud， and coral. Other species live on other organisms and as part of artificial surfaces like fouling communities （plants and animals that live on pilings， boat bottoms， and the like）. Some seaweeds are very selective about the surfaces they attach to， whereas other seaweeds do not have this degree of specialization. The presence of benthic （living on the seafloor） seaweeds defines the inner continental shelf， where the marine community largely depends on the food and protection that seaweeds provide. Life on the outer continental shelf and in the deep sea is quite different in the absence of seaweeds. The distinction between the inner and outer shelves is based on the compensation depth of algae. The compensation depth is the depth of water at which there is just enough light for algae to survive. At that depth all the oxygen produced by photosynthesis is consumed by the algae＇s respiration， so that no further growth can occur.

　　•P3： Seaweed boundaries are not necessarily stable. The areas of the world most favorable to seaweed diversity include both sides of the North Pacific Ocean， Australia， southwestern Africa， and the Mediterranean Sea. Several physical and biological factors have been mentioned as potentially restrictive to seaweed distribution. Among these， temperature， sometimes in combination with some specific day length requirements for reproduction， is probably the most important. Some other factors critical in governing the distribution of seaweeds are duration of tidal exposure and desiccation， wave action and surge， salinity， and availability of mineral nutrients.

　　P4： Adaptation as applied to marine algae provides one of the simplest and most attractive examples of the physiological adaptation of plants to the environmental conditions in which they live. It is almost certainly for this reason that the theory was proposed that the vertical distribution of red， brown， and green algae could be explained by their accessory photosynthetic pigments， the presence of which gives the seaweeds their characteristic colors， a concept known as chromatic adaptation. The most frequently cited evidence involving marine algae is a study by Levring （1947）， in which the photosynthesis of green algae was shown to decrease with depth in coastal waters more rapidly than the underwater irradiance. The concept of chromatic adaptation was proposed in 1883 and was accepted for about 100 years， until it was realized that such zonation did not necessarily occur and that the distribution of seaweeds depended more on herbivory （the consumption of plant material）， competition， varying concentration of the specialized pigments， and the ability of seaweeds to alter their forms of growth. Indeed， some recent evidence would seem to support the hypothesis of chromatic adaptation because the depth record （295 meters， or 973 feet） for seaweeds is held by a yet undescribed species of red algae from the Bahamas， However， the green alga Rhipiliopsis profunda is close behind this record at 268 meters （884 feet）.

　　•P5： Temperature determines the performance of seaweeds， and indeed all organisms， at the fundamental levels of enzymatic processes and metabolic function. The greatest diversity of algal species is in tropical waters. Theoretically， increased warmth should fuel the growth of seaweed—as evidenced by seasonal dead zones that form at the mouths of many rivers worldwide when the plants bloom， die and， while decaying， suck up all the available oxygen in the seawater. But temperature is not usually a limiting factor for algae that live in tropical and subtropical seas. Although temperatures in intertidal areas （those areas between high and low tides） may become too warm and contribute to seasonal mass mortality of many seaweeds and the animals they shelter. Some researchers found that increasing temperatures， although initially enhancing the growth of phytoplankton， also allowed increased grazing by zooplankton （microscopic animals） and bacteria. ＂As temperature raises， the zooplankton start to grow faster than the phytoplankton，＂ O＇Connor explains. ＂The zooplankton are more abundant and faster-growing， and are able to eat all the phytoplankton in warmer water. This creates a bottleneck in the food chain that could have large implications for the ocean＇s food web.＂

　　题目：

　　1.word “artificial” in the paragraph 2 is closest in meaning to

　　A. suitable

　　B. human-made

　　C. uneven

　　D. available

　　2. What can be inferred from paragraph 2 about the outer continental shelf？

　　A. The outer continental shelf contains more benthic seaweeds than the deep sea does.

　　B. The outer continental shelf lacks enough sunlight to support the growth of algae.

　　C. The outer continental shelf has a compensation depth that is about the same as that of the inner continental shelf.

　　D. The outer continental shelf has a greater variety of marine life than does the inner continental shelf.

　　3. According to paragraph 2， all of the following are true of seaweeds EXCEPT：

　　A. Seaweeds provide food and protection for other organisms.

　　B. Seaweeds can grow on both living and nonliving things.

　　C. Some seaweeds select specific surfaces to attach to， while others are not selective.

　　D. Two percent of seaweed species live on the ocean floor.

　　4. The word “duration”in the passage is closest in meaning to

　　A. frequency

　　B. force of impact

　　C. length of time

　　D. occurrence