Structure and development of Sphenophyllum oblongifolium from the Upper Carboniferous of France

Variation in branching pattern and leaf morphology in Sphenophyllum oblongi folium are investigated based on new compression material from the Upper Ca rboniferous (Stephanian) of France. Morphometric analysis reveals correlati on between several parameters (internodal length, maximum diameter of axis, leaf length). Ontogenetic variability of axis diameter and internodal leng th is also demonstrated. There is no evidence of a rhythmic elongation of s tems, which are interpreted as axes with a determinate but continuous growt h. The vegetative system of S. oblongifolium is composed of four to five su ccessive orders of axes. Branching is intermittent and diffuse, with an ale atory distribution of proleptic branches in which internode length may equa l that in the parent stem. In the slender axes of the vegetative system of S. oblongifolium there is no clear hierarchy among successive orders of axe s. This contrasts with the situation in the calamitean sphenopsids, where t he successive orders of axes show distinct differences in diameter. Heterop hylly of S. oblongifolium is documented in detail. As in some other species (e.g., Sphenophyllum emarginatum), heterophylly is interpreted partly as a n adaptation to a more or less scrambling habit.