4 Plants vs. Animals-Low

Plants vs. Animals FOCUS Question How do plants defend themselves against herbivores?

Plants Feed the World For plants, the world is a dangerous place. Plants get energy from the Sun, and billions of hungry plant-eating creatures or herbivores, get energy by eating plants. Like other producers, plants are at the bottom of many food chains. Everything from tiny mites to huge elephants eats plants. How do plant species survive? Plants are rooted in one place, so they can't run away and hide. Instead, they have other ways of defending themselves against herbivores. Plants have direct forms of protection. These forms include physical structures and chemicals that deter herbivores. Plants also defend themselves in indirect ways. They get help from the natural enemies of herbivores. Wowser! There are more than 300,000 species of plants on Earth!

Physical Defenses Anyone who has walked in the woods knows about the many thorns and prickles found on plants. These are direct forms of protection. Rose plants and cacti have large thorns to repel large herbivores such as deer. Tomato plants have thin hairs or trichomes (TRY-combs), that make it hard for smaller herbivores to eat them. Sharp trichomes on bean plants can trap insect herbivores by stabbing their feet Other trichomes are poisonous, such as those of stinging nettle plants. Raphides (RAY-fides) are needle-shaped crystals found inside the leaves of many plants. Raphides can irritate an herbivore's skin. These small crystals can also scratch the herbivore's throat, resulting in painful stinging. Kiwi fruit plants and agaves have raphides for defense.

Other plants use waxy leaves as a defense. Slippery wax makes it difficult for caterpillars and other climbing insects to keep a foothold on the leaves. Magnolia and eucalyptus trees have waxy leaves to keep hungry plant eaters away. Some plants, such as passionflowers, have small structures that look like insect eggs. Female butterflies are less likely to lay eggs on leaves that already appear to have eggs. They know that eggs hatch and turn into hungry herbivores. If there are already eggs and caterpillars on a plant, there may not be enough food to feed their own young. They lay their eggs elsewhere.

Chemical Defenses Chemical defenses are another form of direct protection. Many plants make chemicals to help repair injuries caused by munching herbivores. Some plants also make chemicals that taste bad to a plant eater. Herbivores remember the bad taste of the chemicals and may avoid eating the plant in the future. Some plants have special cells called idioblasts in their leaves. These cells produce chemicals for defense and also contain needle-sharp raphides. The prick from the raphides causes pain Then the chemicals cause even more pain. This defense keeps herbivores from eating too much of the plant! Poison ivy and sedum are two of the many plant species that contain idioblasts. Do You Know? Caffeine is a chemical called an alkaloid. It is made in nature by coffee and tea plants to repel herbivore attacks.

Plants such as citrus trees and mint make oils that repel insects. You may enjoy these smells. But the fresh scent of mint as well as lemons oranges, and other citrus fruits keeps many insects away. In Africa, some antelopes eat the leaves of acacia trees. As soon as an antelope starts eating these leaves, the tree produces a bad-tasting chemical called tannin k. This chemical also makes the antelope sick by interfering with its digestion. Just fifteen minutes after an herbivore starts eating acacia leaves, the amount of tannin doubles! You can bet that the antelope will eat something else next time.