Leaves are metabolic factories equipped with food-producing, photosynthetic cells. They develop as outgrowths of apical meristems. Monocot leaves form a sheath around the stem and lack a petiole. Dicot leaves have a blade and a petiole by which the leaf is attached to the stem. Dicots may have "compound" leaves of many leaflets.
Leaves are usually thin and offer a high surface-to-volume ratio that promotes diffusion of carbon dioxide in and oxygen out. Leaves are arranged to capture sunlight. They are held perpendicular to rays of the sun and are arranged so they don't shade one another.
The epidermis covers all leaf surfaces that are exposed to the surroundings. Its surface may be smooth or covered with a variety of hairs and scales. Cuticle minimizes water loss. Stomata are located mostly on the lower epidermis.
Mesophyll is a photosynthetic ground tissue containing chloroplasts. Photosynthetic parenchyma cells (in the mesophyll layer) are located between the extensive surface areas of the upper and lower epidermis. Air spaces in the leaf participate in gaseous exchange. In dicots there are two layers of mesophyll called palisade mesophyll and spongy mesophyll. Columnar parenchyma cells attached to the upper epidermis (palisade cells) have more chloroplasts than the spongy cells below.
Veins are the leaf's vascular bundles composed of xylem and phloem. Veins form a network for water, solutes, and photosynthetic products. They are often strengthened with fibers. In dicots, the veins are netlike and repeatedly branch into smaller ones embedded in the mesophyll. In monocots, veins are quite similar in length and run parallel with the leaf's long axis.
This animation (Audio - Important) describes the structure of leaves.
REVIEW: The main photosynthetic area of a leaf is composed of
REVIEW: The openings in leaves that function to exchange gases are called
REVIEW: Buds give rise to ____ .
a. leaves
b. flowers
c. stems
d. all of the above
REVIEW: Mesophyll consists of _____ .
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