🌿 茎秆：植物体内的"管道与支架"🌿 Stem: the Plant's Pipeline and Scaffold
根把水喝进来了，可叶子长在几十米高的地方——水怎么"爬"上去？这一片，🦠 小细胞跟着一滴水钻进茎秆，看清两条方向相反的管道。The roots have drunk the water in — but the leaves sit metres up. How does water climb? In this leaf, 🦠 Little Cell rides one drop into the stem and finds two pipelines running opposite ways.
第 1 步：一根茎，两根管道Step 1: One stem, two pipelines
先切开茎，看看里面藏着什么。Cut the stem open and see what's hidden inside.
茎里藏着两条方向相反的运输线。导管（xylem）又细又硬，把根吸来的水和无机盐一路向上送到叶子；筛管（phloem）把叶子造好的糖向下运给根和果实。一根向上、一根向下，把全身连成一家。Inside a stem run two transport lines going opposite ways. The xylem is thin and stiff, carrying water and minerals from the roots upward to the leaves. The phloem carries sugars made by the leaves downward to roots and fruits. One up, one down — wiring the whole plant into one family.
水往上走要力气，这力气从哪来？Lifting water takes force — where does it come from?
第 2 步：蒸腾：水往上爬的"拉力"Step 2: Transpiration — the upward pull
把芹菜插进红墨水，几小时后切开茎，导管被染成红色——水真的在往上走。动力来自叶子：叶子通过气孔不停蒸发水分（蒸腾作用），像吸管一样把下面的水整条拉上来。光照越强、风越大，蒸腾越快，水也运得越快。Stand celery in red ink; cut the stem hours later — the xylem is stained red: water really climbs. The pull comes from the leaves: they constantly evaporate water through their stomata (transpiration), sipping the water column up like a straw. Stronger light and wind mean faster transpiration — and faster delivery.
茎还有自己的"日记本"和"小仓库"。A stem keeps its own diary — and its own pantry.
树干里的形成层每年长一圈，圈出年轮——数一圈圈，就知道树几岁。有些茎索性改行当仓库：土豆（块茎）、洋葱（鳞茎）和仙人掌的胖茎，都存着淀粉或水，全是茎的"变形记"。Each year the cambium adds one ring of wood — count the rings to know the tree's age. Some stems switch careers and become pantries: the potato (a tuber), the onion bulb, and the cactus's fat green stem all store starch or water — true stem shape-shifters.
🎮 你来当运输实验员（约 1 分钟）🎮 Your turn: the transport lab (1 minute)
同一张实验台，这次是芹菜和彩色水。调光照和风，看彩色水爬得多快，完成 4 张实验单。Same bench, now with celery and colored water. Adjust light and wind, watch the colour climb, and finish 4 lab sheets.
一句话记住它：茎 = 植物的"支架 + 双向快递"：导管向上送水，筛管向下送糖，年轮记年龄。Remember it in one line: a stem is a scaffold plus a two-way delivery — xylem up, phloem down, rings keep the calendar.
茎的两件大事：支起枝叶、运输物质Two big jobs: holding leaves up and moving materials导管（xylem）向上运水和无机盐；筛管（phloem）向下运糖Xylem carries water & minerals up; phloem carries sugar down蒸腾（光照、风越大越快）拉动水上升；数年轮知树龄；土豆、洋葱、仙人掌都是变态茎Transpiration (faster with light and wind) pulls water up; rings count age; potato, onion and cactus are modified stems
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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