Placed among the busy streets of Washington, D. C. is a large public garden that many Americans have never heard of. The U. S. National Arboretum (植物园) includes     1     collection of flowering plants called azaleas, a field of native plants named ferns, and flowering trees in the dogwood tree area. The garden     2     (establish) by Congress in 1927. It plays a leading part in researching and developing plants like trees, shrubs, flowers, and grasses.

Government scientists are in charge of the 180-hectare area. Their main goal is     3    (strengthen) the U. S. economy by making sure an important kind of agriculture,     4     (call) the nursery industry, continues to perform well. The most recent numbers from the U. S. Department of Agriculture (USDA) show that the ornamental and landscape plant industry creates billions of founds in sales each year.

The arboretum is home to one of the     5     (large) collections of preserved seeds in the world. The National Arboretum Herbarium (植物标本)     6     (house) around 700,000 ex-ample seeds. They include seeds from plants that the USDA considers as     7    (economical) important. Scientists collect many genetically different seeds within a kind of plant from both native and non-native plants. The reason     8     collecting that data is that scientists protect plants from harmful situations like climate change, changed environments, or other new and possible risks.

While the scientists do their work, the public     9     (explore) plants is free to visit the arboretum. Though many people may not know it, the arboretum has had a hand in developing a number of plants     10     are part of American landscaping.