Montag, 2. Mai 2011

Artcle of the week (May 2nd, 2011) - Phyllotaxis & Leaf Shapes

I. Phyllotaxis

The position of a leaf at the shoot axis is not randomly, but follows a simple, physical principle. Every leaf tries to avoid shading its processor and successor leafs by changing its position a a little bit towards the proceeding and succeeding leaves. In this way the plant tries to find the best possible way of using sunlight. 

In short, they are five types of leaf patterns (s. pictures)


  • Spiral: the spiral pattern is probably the most common pattern. Here we have one leaf per node. Each leaf is a little bit offset from the previous. So, all leaves are forming a repeating spiral, if you look at them from above.

  • Opposite: At the opposite (or decussate) pattern each node has two leaves. This leafs are standing in an angle of nearby ninety degree toward the previous couple. The opposite leaf pattern can found e. g. at the Lamiaceae family.

     the Lamiaceae are a family withe opposite leaf pattern

  • Alternate: The alternate (or distiochus) pattern has only one leaf per node. The leaves are standing in an alternating angle of 180 degree towards the previous at the shoot axis. So the the leafs of one side of the shoot axis are sitting in the gaps of the Other side.


  •  Whorl: At the whorl pattern, you have more than two leaves per node. The leaves are sitting all around the shoot axis. This type of pattern can be found e. g. at the Rubiaceae.
many members of Rubiaceae have
leafs with a whorl pattern
  • Rosette: At a rosettes, the internodes are so short, that all leaves seems to sit a the lower end (in rare cases the top) of the shoot axis. Many plants are forming rosettes; the probably best examples are the dandelion (Taraxacum officinale F. H. Wigg.). During the winter, the leaves of a rosette also protects but from frost.
II. Leaf shapes

As indicated in my last article of the week, leaves are not shaped equal, but existing in many different shapes. I'll not got in to detail at all, because there are to many different shapes. I will only deal with the basic shapes in order to give you an idea about the mater. First, we'll look at the four types of leaf shapes.


  • Simple: the simple leaf shape is the form of the basic foliage leaf, I've introduced last week. We have a leaf blade, a stalk and a leaf base. The leaf blade can be shaped very differently, but this I will show you later.

     a simple leaf
  • Pinnate: at a pinnate leaf, the leaf blade is divided into small leaflets. Sometimes leaflets can be confused with small leaves, but remember, that a leaf always grows from a node; leaflets has no nodes. The axis of the leaflets is call Rachys. Pinnate leafs are typical for the ferns but also e. g. for the roses.

    the pinnate leaf of the rose (Rosa spec.)
  • Digitate: the leaf blade of a digitate (or hand-shaped) leaf is also divided into small leaflets, but here, the leaflets are all coming out from one base. So the leaflets are looking like the fingers of a hand. The leaaves of the conker tree (Aesculus hippocastanum L. are a good example for a digitate leaf.

     a digitate leaf
  • Pedate: The pedate (or foot-shaped) leaf is similar to the digitate leaf but here, the leaflets are not coming together in one point. They have small “branches”, so every leaflet is sold a little bit from the base of the leaf blade. Pedate leaves can be found e. g. at the Christmas rose (Helleborus niger L.).
       
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Leaf blade and leaflets can also be very different in shape. For example, they can bee round, lanceolate, oval, heart shaped (cordat), and so on. This picutre shows you ten different types of leaf blades

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