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. Why does the holly plant have more spines on the lower leaves
A. Because most animals are not tall enough.
B. Because most animals can get the lower leaves easily.
C. Because most animals are not clever enough.
D. Because most animals like the lower leaves only.