Secondary Growth and Deformation of Stem Tissues in the Lower Carboniferous Seed Fern Calamopitys

Abstract

Some Carboniferous seed ferns undergo extensive secondary growth, with secondary vascular tissues occupying increased space within the axis. Mechanisms of accommodating secondary growth in the stem cortex vary significantly between different seed ferns and depend on the degree of secondary growth and position of secondary tissues relative to the cortex. Cortical tissue adjacent to the secondary vascular cylinder is often highly deformed, whereas primary cortex tissue farther away from the secondary growth may be less strained. Developmental patterns of primary and secondary tissues are observed in two species of Calamopitys from the Lower Carboniferous and are compared with patterns of the Upper Carboniferous “pteridosperm” Lyginopteris oldhamia. Calamopitys embergeri and Calamopitys schweitzeri show different organizations, development, and deformations of the cortex compared with Lyginopteris. The main differences include (1) retention of a massive cortex in relation to the degree of development of the vascular cambial tissues, (2) cortical deformation confined to areas around the periphery of the vascular cylinder rather than toward the periphery of the stem, and (3) absence of a discrete periderm layer. Our findings also indicate that cambial growth in Calamopitys was capable of producing relatively large cylinders of vascular tissues within the primary body without causing a sloughing of the outer cortex.