Anatomy

Now that you know what a leaf is, let's now take a look at how they are arranged on the twig ... also called leaf phyllotaxy. As already said, leaves are attached to the twig at the same point where the buds are. This point on the twig is referred to as a node. The area of the twig between nodes is referred to as the internode.

  • Leaf alternately 300pxx225

    If there is one leaf (bud) at a node, the leaf arrangement is said to be Alternate. It's called alternate because the leaves tend to alternate going down on either side of the twig.

  • char Leaf basic structures 235x314 1

    Some woody plants have two leaves attached at the same node. This arrangement is called Opposite. These leaf-pairs tend to be arranged at right angles to other pairs as you go down the twig.

  • Leaf subopposite 300pxx225

    An interesting variation to the opposite arrangement is where the leaves are almost opposite, but not exactly. This is referred to as sub-opposite.


Leaf Characteristics

  • char where is the leaf 300pxx225

    Where is the Leaf? Before you can use leaf characteristics to identify a woody plant, you must first know what is considered to be a ... leaf. Woody plants generate a new set of leaves each year. They burst out of their buds and expand into either broad, flattened structures, or into needles such as in the conifers. Since they arise from buds, this is the structure crucial to knowing exactly what a leaf is. A leaf is everything growing out from the bud. Once you locate the bud on the twig, everything distal to it is considered to be a single leaf. Leaves are composed to two basic structures ... the petiole or stalk, and the blade, sometimes called the lamina.

  • basic structures 235x314

    Some woody plants have two leaves attached at the same node. This arrangement is called Opposite. These leaf-pairs tend to be arranged at right angles to other pairs as you go down the twig. In some species, the petiole is absent; the blade is attached directly to the twig. The leaf is referred to as being sessile.


Leaf Composition

  • comp pinnatelycompound 235x314

    A leaf consists of two parts, the petiole and the blade. However, there is quite a bit of variation in the number and arrangement of blades attached to the petiole. First, if there is just one blade attached to the petiole, the leaf is said to be Simple. If there is more than one blade present, it's referred to as a Compound leaf. In this case each blade is called a pinna or leaflet.

  • There are several different ways the pinnae can be attached to the petiole. They all can be attached at the same point at the end of the petiole ... Palmately compound (no image available), or they can be attached at different points along an extension of the petiole called a rachis. This is called a pinnately compound leaf.

  • opposite entire 235x314

    This is an example of 10 simple leaves attached oppositely on the twig. In the picture above, there is just one leaf shown ... a pinnately compound leaf with 21 pinnae or leaflets.


Leaf Margins

The very edge of a leaf or leaflet, is called the margin. Knowing the variation in margin anatomy will be helpful in identifying trees and shrubs.

  • margin smooth 300pxx225

    Margin entire (smooth) or nearly so.

  • margin palmateveined 300pxx225

    Margin entire. Leaf is divided into 5 lobes corresponding to the main veins

  • margin finelyserrated 300pxx225

    Finely serrated or toothed margin.

  • margin doublyserrated 235x314

    Each lateral vein off the main vein ends in a tooth. Additionally, each tooth has teeth. Referred to as Doubly Serrated.

  • margin coarsetoothed 300pxx225

    Lateral veins end in coarse teeth.

  • margin crenate glandular 300pxx225

    Teeth are rounded and tipped with glands.

  • margin wavymargin 300pxx225

    Margin is actually entire, but called wavy.

  • margin arculateveination 300pxx225

    Arcuate veination; lateral veins arch towards the leaf apex.

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