🌤️ 向光性：植物会“追太阳”🌤️ Phototropism: Plants Chase the Sun
上一片说了：植物用激素“悄悄话”指挥全身。可早在发现激素之前，人们先注意到一个谁都能看见的现象——窗台上的苗，全弯向窗外。这片就学学科学家怎么观察它。Last leaf: plants run on hormone “whispers”. But long before anyone found the hormones, people noticed something anyone could see — seedlings on a windowsill all lean toward the glass. This leaf is about how scientists study that.
第 1 步：先看现象Step 1: Watch the phenomenon first
留心看窗台：盆栽全都歪向玻璃那一边。这是巧合吗？Look at any windowsill: the potted plants all lean toward the glass. A coincidence?
窗台的苗弯向窗外、路边的行道树朝马路中央歪、田埂边一圈麦子朝外探——全都朝着光多的那一侧。把现象记下来，就得到第一条线索：苗生长的方向，跟着光走。这种“追着光长”的本领，叫向光性。Seedlings lean out the window, street trees tip toward the road, the edge of a wheat field reaches outward — always toward the brighter side. Write the pattern down and you get your first clue: the direction a seedling grows follows the light. This “chasing the sun” habit is called phototropism.
“跟着光走”只是猜想。想让猜想站住脚，就得做对照实验。“Growth follows the light” is only a guess. To make it stand, you need a controlled experiment.
第 2 步：控制变量做实验Step 2: The controlled experiment
规矩是：只改一个条件，其他全一样。左边这盆只从右边照光，苗弯向光；右边这盆四面都照光，苗笔直长。两盆唯一的不同是“光的方向”，所以能下结论：让苗弯曲的，正是“单侧光”。转花盆也一样——光虽在一边，但苗每一面轮流受光，照样笔直。The rule: change one thing only. The left pot gets light from the right side only — the seedling bends toward it. The right pot gets even light from all sides — the seedling grows straight. Since the only difference is the direction of light, the verdict is safe: it is one-sided light that makes seedlings bend. Spin the pot and the same happens — each side takes its turn in the light, so it stays straight.
可苗没有眼睛，是哪里“看见”了光？But seedlings have no eyes — which part actually “sees” the light?
第 3 步：达尔文父子的草帽实验Step 3: The Darwins' straw-hat experiment
1880 年，达尔文和儿子给苗“戴帽”做实验：用小帽罩住尖端，苗就不弯了；罩住中段，照样弯。结论：尖端负责“看”光，再通知下面弯曲。几十年后，人们从尖端找到了那个信号——正是上一片的主角生长素：背光侧得到更多生长素、细胞拉得更快，苗就弯向光。In 1880 Charles Darwin and his son Francis tried little “hats”: cover the tip and the seedling stops bending; cover the middle and it bends as usual. Verdict: the tip does the seeing, then tells the rest to curve. Decades later the signal was found in the tip — auxin, last leaf's star: the shaded side gets more of it, stretches faster, and the seedling bends toward the light.
🎮 窗台实验室（30 秒）🎮 Your turn: the windowsill lab (30 seconds)
现在轮到你：拖太阳、转花盆，先预测苗的弯向，再看结果对不对。Over to you: drag the sun, spin the pot, predict the bend first, then check the result.
一句话记住它：向光性 = 苗朝光弯着长。单侧光才让它弯；四面光、转花盆，苗就笔直。Remember it in one line: phototropism = seedlings bend toward the light. Only one-sided light bends them; even light or a spinning pot keeps them straight.
观察现象：窗台苗弯向光 → 提出问题“苗怎么知道光在哪”Observe: windowsill seedlings lean toward the light → ask “how do they know?”控制变量：只改光的方向，其余不动 → 证明单侧光让苗弯Control variables: change only the light's direction → one-sided light causes the bend达尔文父子：尖端“看”光，靠生长素让背光侧拉长The Darwins: the tip “sees” the light; auxin stretches the shaded side
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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