Historical development of the present classification of morphological types of stomates

There is a long-standing confusion between morphologic and ontogenetic classifications of stomates. The earliest scheme, by Vesque (1889) was proposed as basically ontogenetic, but it was soon widely applied to mature leaves. The ontogenetic distinction between haplocheilic and syndetocheilic stomates in gymnosperms, proposed by Florin (1931, 1933) soon suffered a similar fate. Continuing studies over the past half-century have shown that stomatal ontogeny is only poorly correlated with the mature morphology. Efforts to combine ontogenetic and morphologic features in a single scheme have led to classifications so complex as to be impractical. The explicitly morphological classification by Metcalfe and Chalk (1950) is the foundation for the most widely used present scheme, in which some 14 morphological types are recognized. The distinctions among these types are conceptually useful, though often arbitrary in practice.