Fig. 2

The formation and differentiation of sporangium wall. a A sporangial primordium includes primary wall cells (PWC) and sporogenous cell (SC) stained with deep density. b An epidermal initial cell is performing a periclinal division indicated by an arrow. MT mitosis. c An epidermal cell is divided into two daughter cells (DC), one inner daughter cell to forming sporogenous cell and one outer daughter cell forming primary mantle cells in the late development. d An image shows a periclinal division (indicated by an arrow, MT mitosis) of a cell derived from the primary mantle cells, the result of which increases the number of layer. Those deep stained cells next to this dividing cells develop into spores. e An epidermal cell derived from the mantle cells is performing an anticlinal division (indicated by an arrow, MT mitosis) to expand the number of the outmost cell layer. f This image shows a 2–3-cell layer (indicated by a two-end arrow) forming the wall structure (SW sporangium wall) of a sporangium during the primary sporogenous cell (PSC) stage. g This image shows a 4-cell layer (indicated by a two-end arrow, SW sporangium wall) mature wall of a sporangium during the stage of secondary sporogenous cells (SSC); EP epidermis, M middle layer, Ta tapetum. h An image of a longitudinal section of one sporangium shows structures of a mature wall (SW sporangium wall) and spore mother cells (SMC) during the stage of secondary sporogenous development. i, j Images show disruption (indicated by DIS) of the sporangial wall in the stage of spore mother cells (SMC)