托福听力背景知识之Sunflower

Sunflower common name for annual and perennialherbs of a genus of the family of composite flowers. The genus, whichcontains about 67 species, is thought to be native to South Americaoriginally, although sunflowers are now distributed almost worldwide.Some of the tall-growing forms may attain a height of 3 m (12 ft). Thelarge, solitary blossom, sometimes as large as a meter in diameter, iscomposed of yellow ray flowers and a central disk of either yellow,brown, or purple flowers, depending on the species. The dailyorientation of the flower to the sun is a direct result of differentialgrowth of the stem. A plant-growth regulator, or auxin, accumulates onthe shaded side of a plant when conditions of unequal light prevail.Because of this accumulation, the darker side grows faster than thesunlit side. Thus, the stem bends toward the sun. lost waxing方法制作 CirePerdue (French, ¡§lost wax¡§), process of waxcasting used in making metal sculpture. A model is coated with wax. Thesolidified wax is encased in a two-layer mold of plaster or clay. It isthen melted or otherwise removed from the mold, and metal is pouredinto the space where the wax had been. After cooling, the mold isbroken to free the metal object. This ancient method is used to producesculpture, jewelry, and utilitarian products such as dentures.