a mass of protoplasm that usually forms by fusion of cell protoplasts of the amoeboid tapetum (as in the sporangia of numerous ferns and the microsporangia of certain angiosperms and gymnosperms). In selaginellas and horsetails certain sporogenous cells also participate in periplas-modium formation, whereas in the Psilotophyta and the Iso-ëtales the periplasmodium forms by fusion of the protoplasts of some of the sporogenous cells.
The periplasmodium penetrates the cavity of the sporangium and is used in feeding the developing sporogenous cells and spores. In seed plants it also nourishes the male gametophytes (pollen grains) in the early stages of development. In a number of Pteridophyta, periplasmodium substances are deposited on the surface of the spore’s exine, forming an outer covering—the perisporium. In the sporangia of Salviniaceae, the periplasmodium hardens to form massulae.