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

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Abstract
Variation in branching pattern and leaf morphology in Sphenophyllum oblongifolium are investigated based on new compression material from the Upper Carboniferous (Stephanian) of France. Morphometric analysis reveals correlation between several parameters (internodal length, maximum diameter of axis, leaf length). Ontogenetic variability of axis diameter and internodal length is also demonstrated. There is no evidence of a rhythmic elongation of stems, which are interpreted as axes with a determinate but continuous growth. The vegetative system of S. oblongifolium is composed of four to five successive orders of axes. Branching is intermittent and diffuse, with an aleatory distribution of proleptic branches in which internode length may equal 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 axes. This contrasts with the situation in the calamitean sphenopsids, where the successive orders of axes show distinct differences in diameter. Heterophylly of S. oblongifolium is documented in detail. As in some other species (e.g., Sphenophyllum emarginatum), heterophylly is interpreted partly as an adaptation to a more or less scrambling habit.