Animals can move from place to place, but plants cannot. When an animal is unde

      Animals can move from place to place, but plants cannot. When an animal is under attack, it can run away or fight back. Plants certainly cannot run away, and they lack teeth and claws. But plants can defend themselves by using both physical and chemical means.

     Some plants have their own ways to keep animals away. For example, the leaves of the holly plant have sharp spines (刺) that discourage grass-eating animals. Holly leaves on lower branches have more spines than leaves on upper branches. This is because the lower leaves are easier for most animals to reach.

      Some plants, such as the oak tree, have thick and hard leaves that are difficult for animals to eat. Some grasses may contain a sandy material; eating such grasses wears down the animal's teeth.

      Many plants also have chemical defenses. Some plants produce chemicals that taste bitter or cause an unpleasant reaction. Some plants may fight against an attack by increasing the production of these chemicals. When a caterpillar (毛虫) bites a tobacco leaf, the leaf produces a chemical messenger. This messenger sends to the roots the information to produce more nicotine. The higher levels of nicotine discourage the caterpillar.

      Many plants depend on both physical and chemical defenses. A certain plant in China, for instance, has prickly (多刺的) leaves, and each prickle contains poisonous venom (毒液) A single experience with this kind of plant will teach an animal to stay away from it in the future.

68. The holly plant has more spines on the lower leaves because most animals________.

A. are not tall enough             B. like the lower leaves only

C. are not clever enough        D. can get the lower leaves easily

69. To defend themselves, oak trees use________.

A. chemical means                     B. physical means

C. bitter chemicals                     D. sandy materials

70. How does tobacco protect itself against an attack from a caterpillar?

A. Its leaves fight against the attack by physical means.

B. Its roots send a messenger to discourage the caterpillar.

C. Its roots increase the production of nicotine when it is attacked.

D. Its leaves produce poisonous sand to drive the caterpillar away.

71. What would be the best title for this passage?

A. Plants and Animals            B. How Plants Defend Themselves

C. Attacks and Defenses       D. How Animals Eat Plant Leaves

68.D 69.B 70.C 71.B

68.解析：这是一道细节题。第二段“Holly leaves on lower branches have more spines than leaves on upper branches. This is because the lower leaves are easier for most animals to reach.”告诉我们，冬青树底部叶子上的刺比上部多的原因是动物更容易得到底部的叶子。

69.解析：这是一道细节推断题。根据第三段“Some plants，such as the oak tree，have thick and hard leaves that are difficult for animals to eat.”可判断出橡树采用了物理学的办法来保护自己。

70.解析：这是一道细节推断题。根据倒数第二段“The higher levels of nicotine discourage the caterpillar.”可推断出烟草在受到毛虫攻击时，根部产生的尼古丁增多。

71.解析：这是一道主旨题。这篇短文讲述了植物采用各种手段来防止外来攻击，保护自己。