BIOMECHANICAL ANALYSIS OF PITUS-DAYI - EARLY SEED PLANT VEGETATIVE MORPHOLOGY AND ITS IMPLICATIONS ON GROWTH HABIT

A biomechanical method to distinguish self-supporting and non self-supporting growth habits is applied to exceptionally preserved ''twigs'' of Pitus dayi Gordon, The analysis investigates whether these isolated stem segments are consistent with a self-supporting tree-like habit as suggested by the stumps, trunks and branches of the genus Pitus Witham preserved more commonly in the fossil record. Because of difficulties in accurately identifying certain fossil tissues, three centrisymmetrical models were constructed to test a range of possible tissue combinations over five ontogenetic stages. The results suggest a self-supporting habit with trends in mechanical parameters during ontogeny similar to those of extant, self-supporting plants. Less explicitly constrained to the analysis of habit, the investigation also examines the structural significance of specific tissues during ontogeny as observed from contributions of individual tissues to cross-sectional area, axial second moment of area and flexural stiffness. Pitus dayi produced a physiologically ''cheap'' primary cauline cortex which was rapidly replaced by the development of a rhytidome. A mechanically significant, cauline hypoderm comprising thick-walled sclerenchymatous tissue is absent. This arrangement differs from other tested Palaeozoic pteridosperms interpreted as semi-self-supporting such as Lyginopteris oldharnia and Calamopitys sp. in which the primary cortex is mechanically significant and secondary growth of the wood does not reach mechanically significant thresholds within the primary body.