tailieunhanh - Đề ôn thi đại học môn Tiếng Anh năm 2013-Đề 3

Tham khảo đề thi - kiểm tra đề ôn thi đại học môn tiếng anh năm 2013-đề 3 , tài liệu phổ thông, ôn thi đh-cđ phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | However, as soon as climatic conditions change in favour of plant life, a fascinating sequence of changes, called a primary succession, occurs first to colonize the barren land and the lichen surviving on bare rock. Slowly, the acids produced by these organisms crack the rock’s surface, plants debris accumulate and mosses establish shallow root-holes. Ferns may allow and, with short grasses and shrubs, gradually form a covering of plant life. Roots broke even deeper into the developing soil and eventually large shrubs give way to the first trees. These grow rapidly, cutting off sunlight from the smaller plants, and soon establish complete domination – closing their ranks and forming a climax community which may endure for thousands of years. Yet even this community is not everlasting. Fire may destroy it outright and settlers may cut it down to gain land for pasture or cultivations. If the land is then abandoned, a secondary succession will take over, developing much faster on the more hospitable soil. Shrubs and trees are among the early invaders, their seeds carried by the wind, by birds and lodged in the coat of mammals.