A leaf is an expanded plant organ consisting of a blade and, possibly,
a stalk.
The Shape and Arrangement of Leaves
Deciduous or Broad-leaved Trees
There are two type of leaves: simple, and compound.
- Simple: the leaf consists of a single blade and stalk attached to a twig.
The edge (margin) may be:
- Lobed: the margin has a number of rounded or pointed lobes; the fissure
between the lobes is called the sinus and may be rounded or pointed.
- Pinnately lobed: The lobes are arranged along a central rib (e.g., oak).
- Palmately lobed: The lobes are arranged like fingers on the palm of a hand (e.g., maple).
- Unlobed: the margin is toothless, fine-toothed, large-toothed or
wavy (e.g., willow, elm, linden).
- Compound: the leaf consists of 3 or more leaflets, each leaflet being attached to a
leaf-stalk (note: leaflets are not attached to a twig).
- Pinnately compound: leaflets arranged along a stalk like the barbs of a feather.
(e.g., ash, walnut)
- palmately compound - leaflets arranged like wheel spokes or like fingers on the palm of a hand.
(e.g., horse chestnut, buckeye)
Arrangement
The attachment of leaves to twig may be:
- Opposite: The leaves (two or more) are attached to the twig at the same level; or
- Alternate: The leaves are attached to the twigs at alternate levels.
Conifers
Shape
- scale-shaped; or
- needle-shaped.
Arrangement
The attachment of leaves to twig may be:
- Scales (e.g., red juniper);
- Needles at all angles around the branch (e.g., spruce);
- Needles in 2 rows (e.g., Douglas fir);
- Needles in bunches of 2, 3, 5 (e.g., pine);
- Needles in bunches of more than 5 (e.g., larch).
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