The fine structure and development of the trabeculae and the trabecular ring in Selaginella kraussiana.

In the shoot apex of Selaginella kraussiana A. Br., air spaces develop between the endodermal cells, isolating the two steles from the cortex. The endodermal cells elongate to form trabeculae connecting the two tissues. As each trabecular cell develops, cuticularization occurs on the external surface at a point mid-way along the cell and forms the trabecular ring. The plasmalemma of the trabecular cell becomes attached to the wall beneath the main concentration of the ring, resembling the connection between the plasmalemma and the Casparian strip of a primary endodermis. The trabeculae retain a functional cytoplasm throughout growth.