The two main parts of a leaf are the blade and the petiole. The blade is what we commonly think of when we think of a leaf — the broad, flat green bit — while the petiole is the small stem at the end of a leaf that connects it to the stem and through which the veins are connected to the rest of the plant. The blade of the leaf, though thin, contains several layers, like skin, that help the leaf serve its function as energy source for the plant. The outermost layer is the epidermis, which is composed of skin cells and produces a waxy cuticle that forms a protective layer over the epidermis. On the bottom side of the plant, the epidermis is covered with stomata (pores) through which the plant exchanges gases and releases water. The next layer up from the lower epidermis is the spongy mesophyll, which gases pass through on their way in or out of the leaf. This is also the layer in which the veins are positioned, and where they drop off nutrients from the leaves and pick up glucose to carry to the rest of the plant. The next layer up is the palisade mesophyll, and this is where photosynthesis takes place. After that is the upper epidermis and the cuticle again.
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