北方的树木在秋季以后,除了少数松柏类常绿树木以外,叶子就会变黄,只要风轻轻一吹就会飘落;叶子的变黄和凋落对于树木来讲,可以说是一种过冬的准备。叶子为什么会变黄呢?因为在落叶以前,叶里所含的营养物质都已经分解,输送到枝干内贮藏起来,于是树叶失去了它固有的绿色。秋季以后,土壤温度逐渐降低,树木的根系要从土壤中吸收水分也日渐困难,为了减少水分的损失,叶子的凋落,实在是一种保护的适应。树木向高里生长是靠枝干的顶端来进行的,生长的部分也是树木最幼嫩的部分。到了冬季,生长停止,这种生长的顶端就被新形成的幼叶包围保护起来,就是我们称做芽的部分。到第二年春天,芽张开以后,顶端继续活动才长成新叶,加长枝干。 After the fall of trees in the north, except for a few coniferous evergreen trees, the leaves will turn yellow. As long as the wind blows, it will fall; the yellowing and falling of the leaves will be a kind of winter for the trees. ready. Why does the leaves turn yellow? Because before leaves fall, the nutrients contained in the leaves have been decomposed and transported to the branches for storage, and the leaves lose their inherent green color. After the fall, the soil temperature gradually decreases, and it is becoming more and more difficult for the roots of trees to absorb water from the soil. To reduce the loss of water, the fall of leaves is actually a protection adaptation. The growth of trees to the height is carried out by the top of the branches, and the growing part is also the youngest part of the tree. In the winter, growth stops, and the top of this growth is protected by newly formed young leaves, which we call the bud. By the spring of the second year, after the buds have opened, the top will continue to grow into new leaves, and the branches will be lengthened.