維管植物莖上任何扁平、綠色的生成物。葉會製造氧和葡萄糖，進而滋養並維持動植物的生命。葉和莖的組織從相同的頂芽生長出來。典型的葉擁有寬大葉片，藉着柄狀的梗附着於莖上。葉可能是簡單的（單一葉片）或複雜的（分立的小葉），也可能退化為棘刺或鱗片。邊緣（葉緣）可能是平滑或鋸齒狀的。葉脈自梗輻射至整個葉片，從葉的組織把物質運入和輸出。葉脈在雙子葉中排列成網狀，而在單子葉（參閲cotyledon）中呈平行狀。葉的外層（表皮）保護着內部（葉肉），其中細胞壁柔軟、未特化的綠色細胞（薄壁組織）藉着光合作用產生碳水化合物。在秋季，落葉樹葉的葉綠素分解，致使其他色素（黃或紅）顯露，樹葉進而掉落。落葉後傷口癒合期間所形成的葉瘢對於辨識冬枝很有用。在球果植物中，常綠的針葉可以維持二或三年。
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.
