The farmers kept the cotton seeds to plant again next year.
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usage
籽 specifically refers to small seeds within fruits and plants. Common compounds: 瓜籽 (guā zǐ, melon seeds), 棉籽 (mián zǐ, cottonseeds), 亚麻籽 (yàmá zǐ, flaxseeds). Compare 种子 (zhǒngzi, seed in a broader/agricultural sense).