A leaf may be viewed as a solar collector filled with photosynthetic
cells. Plants are the only photosynthetic organisms to have
leaves, although not all plants have leaves.
raw materials of photosynthesis, water and carbon dioxide,
enter the cells of the leaf. Oxygen, a by-product of photosynthesis,
and water vapor exit the leaf.
In most land plants, water enters the roots and is transported
up to the leaves through specialized cells known as xylem
(pronounced zigh-lem). Plants have a waxy cuticle on
their leaves to prevent desiccation, or drying out. Carbon
dioxide and oxygen cannot pass through the cuticle, but move
in and out of leaves through openings called stomata (stoma
= "hole"). Guard cells control the opening and
closing of stomata. When stomata are open to allow gases to
cross the leaf surface, the plant loses water vapor to the