Leaf Description Terminology
The classification of living plants was rationalized in more or less its present form by Linnaeus (1753). Like others before him Linnaeus relied heavily on the morphology of the reproductive organs for characters by which plants could be divided into groups. Compared to vegetative organs, angiosperm flowers exhibit little phenotypic (environmental) variation at the species level and thus provide a more or less stable and consistently reliable set of characters. This is largely because reproductive morphology is often associated with specific pollination mechanisms which, in order for fertilization to be successful between spatially separated individuals, must be consistent and compatible irrespective of the local environment.
Vegetative organs, on the other hand, have to function as efficiently as possible under the variety of conditions to which they are exposed and the competitive advantage goes to those plants which can appropriately modify their morphology, anatomy and/or chemistry. The variability of vegetative organs has deterred their use in systematics and as a result the taxonomic potential of such organs, particularly leaves, has been overlooked.
Paleobotanists, however, rarely have the luxury of studying fossil flowers. Flowers are often produced only in limited numbers, are delicate structures which are unlikely to become fossilized, and normally develop into fruits or seeds which are sought as food by animals, or germinate and are thereby destroyed. These kinds of reproductive organs are therefore comparatively rare in the fossil record and even more rarely are they attached to a branch bearing leaves.
By necessity paleobotanists have long been interested in the taxonomic potential of leaf remains and have sought to describe them in concise and unambiguous language. Ettingshausen (1861) was the first to develope a comprehensive terminology for leaf architecture which, although not rigorously adhered to, was utilized in part by a few workers including Lesquereux (1878), Berry (1916) and Hollick (1930, 1936). Others, often (but not always) with paleobotanical interests have subsequently proposed using leaf characters for taxonomic or environmental research: Lam (1925), Lee (1948), Mouton (1966, 1970), Ferguson (1971), Hickey (1973, 1979), Dilcher (1974), Dolph (1975, 1976a, 1976b), Madler (1975), Melville (1976), Hill (1980). Most of the proposed schemes of leaf character analysis were intended as aids to written descriptions (for example Hickey, 1973, 1979), but a few authors either have modified the descriptive terminologies for numerical analyses (Dolph, 1976a, 1976b) or have devised original schemes for use with computer techniques (Hill, 1980).
Manual of Leaf Architecture
Perhaps one of the more widely accepted descriptive terminologies began with the work of Hickey (1973) which was subsequently republished with the addition of cuticular characters by Dilcher (1974). Hickey (1979) subsequently expanded his original version and over the years it has evolved into what is now known as 'The Manual of Leaf Architecture' (Ellis et al., 2009).
The scheme described in the Manual of Leaf Architecture, although partly based on experience with fossil leaves, cannot adequately encompass the intergradation of architecture seen in mid- and Late Cretaceous forms. This marked integradation of form, including venation patterns, appears to be the result of reticulate evolution and undermines attempts to recognise vein orders even at the most basic level of determining which are primary and which are secondary veins. This is unfortunate because much of the subsequent descriptive terminology in the Manual of Leaf Architecture relies on getting this early categorization of vein orders correct.