Anabaena azollae - 1, an oblique longitudinal section of a dorsal leaf lobe of the water fern Azolla caroliniana; the large cavity (C) containing filaments of A. azollae (A) is conspicuous in the proximal half of the lamina; 2, habit of a trichome with 2 solitary heterocysts. A. flos-aquae - 3, habit of trichome with akinetes (A), heterocysts (H) and gas vacuoles (G). A. siamensis - 4, 5, habits of trichomes with terminal heterocysts and akinetes (arrows).