‘Paedomorphosis’ and ‘juvenility’ in secondary xylem: (not such) useful constructs?

Abstract

In 1962 Carlquist published “A Theory of Paedomorphosis in Dicotyledonous Woods” based on his observations that some species of non-tree growth forms such as stem succulents, lianas, woody herbs, and tree rosettes exhibit, in their secondary xylem, cellular characteristics more commonly associated with primary xylem. He considered the appearance of these characters an expression of protracted juvenility, and that they reflected paedomorphosis. Since then, many others have observed similar suites of traits across multiple plant lineages, as these traits also characterize secondary and insular woodiness, and have used ‘paedomorphic’ and ‘juvenile’ to describe these traits. In this essay, it is argued that the use of the term paedomorphosis should be restricted to cases where ancestor/descendant relationships are known or used in a comparative sense among closely related taxa, to retain its significance as a descriptor of an evolutionary pattern. Second, it is argued that primary growth, and therefore primary xylem, should not be considered ‘juvenile’ because it is not characteristic only of the young, pre-reproductive phases of plant growth but instead persists for the life of the plant. Because both the concepts of juvenility and paedomorphosis possess more or less well-accepted modern meanings that differ in important ways from Carlquist’s use of the terms, a different approach is suggested that focuses on the functional significance of this suit of traits, in addition to their cellular characteristics.