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)

The diversity of leaves
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
Top and Right: Staghorn Sumac, Rhus typhina (Compound Leaf)
Bottom: Skunk Cabbage, Symplocarpus foetidus (Simple Leaf)
1. Apex
2. Primary Vein
3. Secondary Vein
4. Lamina
5. Leaf Margin
6. Petiole

A leaf is a dorsiventrally flattened organ, usually 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 distinct upper surface (adaxial) and lower surface (abaxial) that differ in color, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax and other features. Leaves are mostly green in color. This is due to the presence of a compound called chlorophyll. This compound is essential for photosynthesis as it absorbs light energy from the sun. A leaf with white patches or edges is called a variegated leaf.

Leaves can have many different shapes, sizes, and textures. The broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In 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. 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. Some structures of non-vascular plants look and function much like leaves. Examples include the phyllids of mosses and liverworts.

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