Leaf
In botany, a leaf is an above-ground plant organ specialized for the process of photosynthesis.
For this purpose, a leaf is typically flat (laminar) and thin.
As an evolutionary trait, the flatness of leaves works to expose the chloroplasts to more light and to increase the absorption of carbon dioxide at the expense of water loss.
In the Devonian period, when carbon dioxide concentration was at several times its present value, plants did not have leaves or flat stems.
Many bryophytes have flat, photosynthetic organs, but these are not true leaves.
Neither are the microphylls of lycophytes.
The leaves of ferns, gymnosperms, and angiosperms are variously referred to as macrophyll, megaphylls, or euphylls.
Leaves are also the sites in most plants where transpiration and guttation take place.
Leaves can store food and water, and are modified in some plants for other purposes.
The comparable structures of ferns are correctly referred to as fronds.
Furthermore, leaves are prominent in the human diet as leaf vegetables.
Interactions with other Organisms
Although not as nutritious as other organs such as fruit, leaves provide a food source for many organisms.
Animals which eat leaves are known as folivores.
The leaf is one of the most vital parts of the plant, and plants have evolved protection against folivores such as tannins, chemicals which hinder the digestion of proteins and have an unpleasant taste.
Some animals have cryptic adaptations to avoid their own predators.
For example, some caterpillars will create a small home in the leaf by folding it over themselves, while other herbivores and their prey mimic the appearance of the leaf.
Some insects, such as the katydid, take this even further, moving from side to side much like a leaf does in the wind.