Venation

Generally, the patterns of the leaf veins, or vascular bundles (which can be seen readily by holding the specimen up to a strong light), are pinnate, and the veins are free; that is, they all diverge and never coalesce, either along their sides or at the ends. Nevertheless, there are numerous fern groups in which netted, or reticulate, venation is found. These have vein patterns like those of other ferns for the most part, except that various systems of networks and areolae (areas enclosed within loops of veins) have developed between the major, pinnately arranged veins. There are many reticulate patterns known. One of the more striking is that in which each loop or areola contains one or more free included veinlets, as seen in various members of the family Polypodiaceae. Another is the herringbone pattern, believed to result from an evolutionary concrescence (growing together) of pinnae, as shown by certain tree ferns (Cyathea), lady ferns (Athyrium), and marsh ferns (Thelypteris).