Animals can move from place to place, but plants cannot. When an animal is attacked, it can run away or fight back. Plants, however, certainly cannot run away, and they lack teeth and claws (爪子). But plants can defend themselves by using both physical and chemical means (方法).
Some plants have their own ways to keep animals away. For example, the leaves of the holly (冬青树) plant have sharp spines (刺) that discourage grass-eating animals. Holly leaves on lower branches have more spines than leaves on upper branches, which is because the lower leaves are easier for most animals to reach.
Some plants, such as the oak tree (橡树), 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 (尼古丁). 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. In order to defend themselves, oak trees use ______.