So far in this chapter, we have looked at the structure of a typical plant cell in the prime of its life. Like organisms, cells have a life cycle that includes youth, maturity, and old age. At the end of their life cycle, cells can either divide or die. For plant cells, death is not necessarily the end of their usefulness because the cell walls of dead cells become permanent parts of the plant body. In large trees, for example, more than 99% of the cells in the trunk are dead. These cells serve a variety of useful functions, including conducting water and providing the structural support necessary to lift the leaf canopy into the sunlight. 到本章为止 我们已经看过了植物成熟时期的构造 就像一个生物的生命周期 细胞也有幼 年 成熟 老年期 当一个周期完成时 细胞可以选择分裂或者死亡 就植物而言 死亡并 不代表失去功能 像死细胞的细胞壁就成了植物体永久的一部分 例如一棵大树 99%的树干细胞是死的 这些细胞仍然有着重要的功能 包括输送水分 支撑树冠层使其 可以吸收阳光 无聊翻译一下 --

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