Reading — Advanced Level
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Cacti occur in a wide range of shapes and sizes. 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. Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of leaves, enlarged stems carry out photosynthesis.

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. 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 that quickly absorbs any water reaching the ground surface. Cactus stems are often ribbed or fluted, which allows them to expand and contract easily for quick water absorption after rain, followed by long drought periods.

Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" 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 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.
Many smaller cacti have globe-shaped stems, combining the highest possible volume for water storage, with the lowest possible surface area for water loss from transpiration.
Source: Wikipedia
  1. A drought is usually characterized by   .

  2. Cacti survive in dry places because they are   .

  3. Most cacti are devoid of   .

  4.   is the process by which the plant exhale water vapor through the stomata.

  5. Cacti are dormant for a long period of time. This means that they   .


Practice your writing skills by discussing the questions below

  1. Where do cacti usually grow? Why?

  2. What are succulent plants?

  3. Do you have cacti at home? If yes, how many kinds do you have?


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      • (absorbed) take in (something, such as a liquid) in a natural or gradual way
      • (adapted) adjust, accomodate
      • large in size or amount : very full or complete
      • the place or type of place where a plant or animal naturally or normally lives or grows
      • a piece of machinery : a mechanical part or group of parts having a particular function
      • the chemical processes by which a plant or an animal uses food, water, etc., to grow and heal and to make energy
      • the area of variation between upper and lower limits on a particular scale
      • a series of vertebrae extending from the skull to the small of the back, enclosing the spinal cord and providing support for the thorax and abdomen; the backbone
      • of plants : having thick, heavy leaves or stems that store water
      • densely filled or covered with


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