Leaf Veins
If you have ever been walking along and picked up a leaf, one of the first things you will notice is that you can see a pattern of leaf veins on the surface. To a plant, this pattern of veins is similar to the circulatory system for a human being. There are large veins, medium-sized veins and very tiny veins, but all of them work together for two purposes. The first is that the veins are the supportive structure to the leaf, giving it its shape. The second function of veins is much the same as to what veins do within our own bodies--they transport substances from one place to another.
Leaf veins are more correctly referred to as vascular bundles. That’s because each vein is really a bundle of two types of tissue, xylem and phloem. Xylem carries water and minerals from the roots to the leaves of the plant, where the process of photosynthesis takes place. Photosynthesis creates carbohydrates--food which the phloem carries from the leaves to the rest of the plant where it is used or stored.
The network of veins in different types of leaves can be complex. There are two general types of veins: these are the craspedodromous, where the large veins in the leaf expand all the way to the edge or margin of the leaf, or camptodromous, where the major veins spread out close to the edge of the leaf, but turn away before they actually reach the edge. An example of the first type is a hydrangea leaf, while the latter is exemplified by a maple leaf.
Beyond these two large distinctions, leaf veins then take on a wide array of smaller variations. For instance, the word pinnately is used to describe veins which have one major axis up the middle of the leaf with many smaller veins or veinlets arranged on both sides. These are also described as pinnate-netted, penniveined, penninerved, or penni-ribbed. Feather-veined means that the veins are pinnate but once they have divided off the main vein, they continue to subdivide many times.
Palmate is the word used to describe leaf veins when there are many main veins, not just one. These main veins all move in different directions to the leaf edge. Maple trees have these type of leaves, which are also described as fan-veined, palmate-veined or palmate-netted. In addition to these, leaf veins can have other patterns, such as three main veins, veins that run parallel from the base to the apex of the leaf, and dichotomous veins, where there is no main vein, but the veins fork into twos throughout the surface of the leaf.
No matter what the pattern of leaf veins, they all work in the same way with xylem and phloem carrying either water and minerals or food respectively, through a network to feed the plant and produce oxygen for the atmosphere.