A Survey of Reticulate Venation Among Fossil and Living Land Plants.

Leaves with reticulate venation are recognized as a hallmark of flowering plants. Dicots are usually characterized by reticulate venation and monocots by parallel venation. However, such distinctions are based primarily on the architecture of major veins, and leaves in both groups typically possess several higher orders of intercostal reticulate venation (Gifford and Foster, 1989). Because most major veins in monocots either anastomose progressively toward the apex or form arcuate anastomoses at the laminar margin, Troll (1939a) proposed the term "striate" rather than parallel to reflect the closed venation of most monocots. Reticulate venation is so ubiquitous and comprises so much intertaxonomic variation among the angiosperms that it has become an important component of the systematics of both fossil and extant taxa. Like many other vegetative features though, reticulate venation is not unique to the flowering plants. Rather, as Alvin and Chaloner (1970) point out, it appears to be a grade of morphological complexity shared by several diverse groups of living and fossil vascular plants including ophioglossalean, marattialean, filicalean, and marsilialean ferns, lycopods, cycads, Ginkgo, gnetophytes, pteridosperms, cycadeoids, and some fossil plants of uncertain affinities (Table 2.1). [ABSTRACT FROM AUTHOR]