primary. photosynthetic organ of a plant that is connected to a stem by a petiole.

Merriam-Webster Online Dictionary
leaf (noun)
a) (1) a lateral outgrowth from a plant stem that is typically a flattened expanded variably shaped greenish organ, constitutes a unit of the foliage, and functions primarily in food manufacture by photosynthesis
(2) a modified leaf (as a bract or sepal) primarily engaged in functions other than food manufacture
b) (1) - foliage trees in full leaf
(2) the leaves of a plant as an article of commerce
something suggestive of a leaf as
a) a part of a book or folded sheet containing a page on each side
b) (1) a part (as of window shutters, folding doors, or gates) that slides or is hinged
(2) the movable parts of a table top
c) (1) a thin sheet or plate of any substance - lamina
(2) metal (as gold or silver) in sheets usually thinner than foil
(3) one of the plates of a leaf spring
leaf (verb)
intransitive verb
to shoot out or produce leaves - will leaf out in spring
transitive verb
to turn over pages especially to browse or skim - leaf through a book to turn over the pages of
leaf (Wikipedia)
For other uses, see Leaf (disambiguation).
Leaf of Tilia tomentosa (Silver lime tree)

A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage."

Diagram of a simple leaf.
  1. Apex
  2. Midvein (Primary vein)
  3. Secondary vein.
  4. Lamina.
  5. Leaf margin
  6. Petiole
  7. Bud
  8. Stem

Although leaves can be seen in many different textures and sizes, typically a leaf is a thin, dorsiventrally flattened organ, borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the (palisade mesophyll), is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinctive upper (adaxial) and lower (abaxial) surfaces that differ in colour, hairiness, the number of stomata (pores that intake and output gases), epicuticular wax amount and structure and other features.

Broad, flat leaves with complex venation are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In others, such as the clubmosses, with different evolutionary origins, the leaves are simple, with only a single vein and are known as microphylls.

Some leaves, such as bulb scales are not above ground, and in many aquatic species the leaves are submerged in water. Succulent plants often have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls, and spines). Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin. Many structures of non-vascular plants, such as the phyllids of mosses and liverworts and even of some foliose lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), look and function much like leaves.

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