A leaf is a plant's principal organ of photosynthesis, the
process by which sunlight is used to form foods from carbon
dioxide and water. Leaves also help in the process of
transpiration, or the loss of water vapor from a plant. A
typical leaf is an outgrowth of a stem and has two main parts:
the blade (flattened portion) and the petiole (pronounced PET-
ee-ole; the stalk connecting the blade to the stem). Some leaves
also have stipules, small paired outgrowths at the base of the
petiole. Scientists are not quite sure of the function of
stipules. Leaf size and shape differ widely among different
species of plants. Duckweeds are tiny aquatic plants with leaves
that are less than 0.04 inch (1 millimeter) in diameter, the
smallest of any plant species. Certain species of palm trees
have the largest known leaves, more than 230 feet (70 meters) in
length. Words to Know Abscission layer: Barrier of special cells
created at the base of petioles in autumn. Blade: Flattened part
of a leaf. Chloroplasts: Small structure