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雅思口语重要植物相关的话题。下面为大家整理了 雅思口语考试中重要植物相关的话题,同学们可适当 进行下载参考,这样才能更好地复习雅思口语话题。 雅思口语重要植物 A cactus is a member of the plant family Cactaceae, within the order Caryophyllales. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north—except forRhipsalis baccifera, which also grows in Africa and Sri Lanka. Most cacti live in habitats subject to at least some drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. 全国免费咨询电话:400-0123-267 As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled. In the absence of leaves, enlarged stems carry out photosynthesis. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Cactus stems also store water, and are often ribbed or fluted, which allows them to expand and contract easily. Cacti occur in a wide range of shapes and sizes. The tallest free-standing cactus is Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft),[2] and the smallest is Blossfeldia 全国免费咨询电话:400-0123-267 liliputiana, only about 1 cm (0.4 in) in diameter at maturity.[3] The smaller cacti usually have globe-shaped stems, combining the highest possible volume with the lowest possible surface area. Many cacti have short growing seasons and long dormancies, and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 200 US gallons (760 l; 170 imp gal) of water during a rainstorm.[4] Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis. Transpiration, during which carbon dioxide enters the plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night. The 全国免费咨询电话:400-0123-267 plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in photosynthesis. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced. A few species differ significantly in appearance from most of the family. At least superficially, plants of the genus Pereskia resemble other trees and shrubs growing around them. They have persistent leaves, and when older, bark-covered stems. Their areoles identify them as cacti, and in spite of their appearance, they, too, have many adaptations for water conservation. Pereskia is considered close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers and epiphytes (plants that grow on trees). Their 全国免费咨询电话:400-0123-267 stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines, such as the well-known Christmas cactus or Thanksgiving cactus (in the genus Schlumbergera). Cacti have a variety of uses: many species are used as ornamental plants, others are grown for fodder or forage, and others for food (particularly their fruit). Cochineal is the product of an insect that lives on some cacti. . 全国免费咨询电话:400-0123-267 全国免费咨询电话:400-0123-267 全国免费咨询电话:400-0123-267 . 来源于:小马过河 小马过河资料下载频道,欢迎您来下载! 全国免费咨询电话:400-0123-267