Leaf

Leaf, green outgrowth from the stems of higher plants; the main site of photosynthesis. The form of leaves varies from species to species, but the basic features are similar. Each leaf consists of a flat blade or lamina attached to the main stem by a leaf stalk or petiole. Leaflike stipules may be found at the base of the petiole. The green coloration is produced by chlorophyll, located in the chloroplasts. Most leaves are covered by a waterproof covering or cuticle. Gaseous exchange takes place through small openings called stomata, through which water vapor also passes. The blade of the leaf is strengthened by veins that contain the vascular tissue responsible for conducting water and the substances essential for metabolism through the plant. Leaves may be adapted to catch insects or to reduce water loss. Bracts, leaves produced immediately below the flowers, may be highly colored and thus mistaken for flowers (as in the poinsettia).