The lower pinnæ (frondlets) sterile, roundish, five to seven lobed, distant in pairs with simple veins; the upper fertile, contracted, several times forked, forming a terminal panicle; the ultimate segments crowded, and bearing the sporangia, which are similar to those of curly grass, and fixed to a veinlet by the inner side next the base, one or rarely two covered by each indusium.

A, small branch of pulmonary artery; B, twigs of the pulmonary artery anastomosing to form peripheral network of the primitive air cells; C, capillary network around the walls of the air sacs; D, branches of network converging for form the veinlets of the pulmonary veins.