Tham khảo tài liệu 'toefl ibt internet based test 2006 - 2007 part 41', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | MODEL TEST 3 READING SECTION 275 can get more sunlight and rain. Some mosses ferns and many vines also make use of the surfaces of trees in this way. In the ocean many sharks have a smaller fish known as a remora attached to them. Remoras have a sucker on the top of their heads that they can use to attach to the shark. In this way they can hitchhike a ride as the shark swims along. When the shark feeds the remora frees itself and obtains small bits of food that the shark misses. Then the remora reattaches. The shark does not appear to be positively or negatively affected by remoras. - Mutualism is another kind of symbiotic relationship and is actually beneficial to both species involved. In many mutualistic relationships the relationship is obligatory the species cannot live without each other. In others the species can exist separately but are more successful when they are involved in a mutualistic relationship. Some species of Acacia a thorny tree provide food in the form of sugar solutions in little structures on their stems. Certain species of ants feed on the solutions and live in the tree which they will protect from other animals by attacking any animal that begins to feed on the tree. Both organisms benefit the ants receive food and a place to live and the tree is protected from animals that would use it as food. - One soil nutrient that is usually a limiting factor for plant growth is nitrogen. Many kinds of plants such as beans clover and alder trees have bacteria that live in their roots in little nodules. The roots form these nodules when they are infected with certain kinds of bacteria. The bacteria do not cause disease but provide the plants with nitrogen-containing molecules that the plants can use for growth. The nitrogen-fixing bacteria from the living site and nutrients that the plants provide and the plants benefit from the nitrogen they receive. Glossary sucker an adaptation for sucking nourishment or sticking to a surface nodules growths in the
