叶(leaf)
维管植物茎上任何扁平、绿色的生成物。叶会制造氧和葡萄糖，进而滋养并维持动植物的生命。叶和茎的组织从相同的顶芽生长出来。典型的叶拥有宽大叶片，藉着柄状的梗附着於茎上。叶可能是简单的（单一叶片）或复杂的（分立的小叶），也可能退化为棘刺或鳞片。边缘（叶缘）可能是平滑或锯齿状的。叶脉自梗辐射至整个叶片，从叶的组织把物质运入和输出。叶脉在双子叶中排列成网状，而在单子叶（参阅cotyledon）中呈平行状。叶的外层（表皮）保护着内部（叶肉），其中细胞壁柔软、未特化的绿色细胞（薄壁组织）藉着光合作用产生碳水化合物。在秋季，落叶树叶的叶绿素分解，致使其他色素（黄或红）显露，树叶进而掉落。落叶後伤口癒合期间所形成的叶瘢对於辨识冬枝很有用。在球果植物中，常绿的针叶可以维持二或三年。
English version：
leaf
Any flattened, green outgrowth from the stem of a vascular plant. Leaves manufacture oxygen and glucose, which nourishes and sustains both plants and animals. Leaves and stem tissue grow from the same apical bud. A typical leaf has a broad, expanded blade (lamina), attached to the stem by a stalklike petiole. The leaf may be simple (a single blade), compound (separate leaflets), or reduced to a spine or scale. The edge (margin) may be smooth or jagged. Veins transport materials to and from the leaf tissues, radiating from the petiole through the blade. They are arranged in a netlike pattern in dicot leaves and are parallel in monocot leaves (see cotyledon). The leaf's outer layer (epidermis) protects the interior (mesophyll), whose soft-walled, unspecialized green cells (parenchyma) produce carbohydrate food by photosynthesis. In autumn the green chlorophyll pigments of deciduous leaves break down, revealing other pigment colors (yellow to red), and the leaves drop off the tree. Leaf scars that form during wound healing after the leaves drop are useful for identifying winter twigs. In conifers, evergreen needles, which are a type of leaf, persist for two or three years.