Leaf

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	Leaf A leaf is the structure in which photosynthesis takes place in most large terrestrial plants. Besides photosynthesis, leaves also takes part in respiration, transpiration and guttation. Leaves can also store food and water. A complete leaf contains a stalk, the blade (lamina) and stipules[?] at either side of the stalk. Many leaves do not have the stipules. Most leaves are green, which comes from its chlorophyll, which takes part in photosynthesis. Leaves on trees in temperate countries can turn yellow, bright orange or red when its carotenoid[?] and anthocyanins[?] (produced only during autumn) are revealed as the tree responds to less amounts of sunlight by ceasing to produce chlorophyll. Public domain (Larry Sanger)

Types of leaves

Leaves can be classified according to

Its type
- Angiosperm leaves (flowering plants)
- Microphyll[?] leaves
- Sheath leaves (e.g. grass)
Its blade
- Simple leaves (which have an undivided blade or divisions that do not reach the vein)
- Compound leaves (which have divided blades with divisions that reach the vein. Each fragment is called a leaflet)
Whether it has a stalk
- Petiolated leaves (which have a stalk—petiole)
- Sessile leaves (which do not have a stalk)
Its arrangement around the stem
- Alternate (one leaf at each level)
- Opposite (two leaves at each level of the stem)
- Whorled (several leaves at each level of the stem)
- Rosulate (leaves form a rosette[?])
Its veins
- Parallel-veined leafs (veins run parallel)
- Pinnate (leaf has a main vein; smaller veins join this main vein)
- Palmate (veins diverge)

The majority of leaves are angiosperm leaves. The angiosperm leaves belong to the flowering plants, which is the prevalent form of plant life. Angiosperm leaves (including maple, oak, chestnut, sunflower, etc.) are generally large and petiolated. Many are also compound (e.g. maple).

Microphyll leaves

Microphyll leaves have a solitary, unbranched vein. They are relatively rare (an example would be horsetails) and would usually be considered revolutionary relics. They generally lack the ability to photosynthesize and are often small in size.

Sheath leaves

Sheath leaves belong to the monocotyledon plants. Sheath leaves are triangular in shape and flat in size. Its veins are usually parallel. A good example of sheath leaves is maize.

Public domain (Nicholas Moreau)

Adaptations

In order to survive in a harsh environment, leaves can adapt in the following ways:

Hairy leaf surface to lessen water loss
Waxy leaf surface to prevent water loss
Small, shiny leaves to deflect the sun's rays
Thicker leaves to store water
Spines instead of leaves (e.g. cactus)
Leaves to trap insects (e.g. pitcher plant)

Parts of a leaf

A leaf commonly has four layers:

Upper epidermis[?]
Palisade (long cells in columns)
Spongy
Lower epidermis

Epidermis

The epidermis is the skin of the leaf blade. It is thin and transparent and it covers both the upper and lower epidermis. The epidermis prevents loss of water. A thin layer of cells, the cuticle, further prevents water loss. The cuticle is thinner on the lower epidermis than on the upper epidermis. Hairs may also grow from the epidermis. The epidermis also has stomata (mostly in the lower epidermis), which enables oxygen and carbon dioxide to travel in and out of the leaf. Water vapor also passes out of the stomata during transpiration. To conserve water, the stomata are closed during the night.

Palisade and spongy layers

The palisade contains of cells packed closely together. The photosynthetic cells are in the palisade and spongy layers. The veins are in the spongy layer and contain xylem, which brings water, and phloem, which takes food. The spongy layer extends into the lower epidermis in most plants.

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