A
leaf is an organ of a vascular plant, as defined in botanical
terms, and in particular in plant morphology. Foliage
is a mass noun that refers to leaves as a feature of plants.
Typically
a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways
almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb
scales), and some are without major photosynthetic function (consider for
example cataphylls, spines, and cotyledons).
The
function of leaves:
1. Place
of photosynthesis
2. Evaporation
3.
Storage of food reserves
4. Vegetative propagation tool
Structure of leaves:
Parts of leaves :
Complete leaf has the following parts:
1.
a petiole (leaf stalk)
2.
a lamina (leaf blade), and
3.
stipules (small structures located to
either side of the base of the petiole)
Complete leaf can be encountered in some
crops, such as: Musa paradisiaca L. ,Areca catechu L. , Bambusa sp. The leaves do not have the
above three sections called incomplete leaf.
Example of complete leave :
Example of incomplete leave :
Leaf characteristics:
- Simple and compound leaf structure:
- Simple leaves have a single blade.
- Compound leaves have more than one blade on a single petiole. The multiple blades of a compound leaf are called leaflets.
- Palmately compound leaves have leaflets arranged like the fingers of a hand.
- Pinnately compound leaves have leaflets arranged on either side of an axis, resembling a feather.
- Trifoliolate leaves have leaflets arranged in threes, like clover.
- Compound leaves are sometimes twice divided. These leaves are called twice-compound.
- Leaf attachment:
- Petiolate - The blade is attached to the stem by a petiole.
- Sessile - The blade is attached directly to the stem without a petiole.
- Leaf arrangement:
- Opposite - Two leaves grow opposite each other at each node.
- Alternate - One leaf grows at each node. The leaves alternate sides along the stem.
- Whorled - Several leaves grow around a single node.
- Leaf shapes:
- Linear - Narrow from base to tip.
- Elliptic - Oval-shaped.
- Ovate - Wide at the base and narrow at the tip.
- Cordate - Heart-shaped.
- Leaf margins:
- Entire - The edge of the leaf is smooth.
- Serrate - The edge of the leaf is finely toothed.
- Lobed - The edge of the leaf is deeply indented.
- Leaf venation: The system of principal veins in the leaf blade.
- Parallel - Major veins arise at the base, remain more or less parallel, and converge at the tip of the leaf.
- Net-veined or Reticulate:
- Pinnate - Major veins diverge from one large mid-vein, with smaller network connections between.
- Palmate - Several large veins arise from the base of the leaf like the fingers of a hand.
- Leaf surfaces: The presence or absence of hairs, the kinds of hairs, and the presence of other surface features, such as glands, combine to give many leaf characteristics. There are over 25 terms used to describe leaf surfaces. This amount of detail is beyond the scope of our class.
Reference :
1. Haupt, Arthur Wing, Plant
morphology. Publisher: McGraw-Hill 1953. Downloable from http://www.archive.org/details/plantmorphology00haup
2. http://en.wikipedia.org/wiki/Leaf
5. http://www.istockphoto.com/stock-photo-7861234-young-mango-leaf.php
6. http://staff.tuhsd.k12.az.us/gfoster/standard/botleaf.htm
6. http://staff.tuhsd.k12.az.us/gfoster/standard/botleaf.htm
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