The ontogeny of the sporangia of Sphaeropteris cooperi was studied using cleared whole mounts of sporangia in different stages as well as sori embedded in paraffin and sectioned. The sporangia develop from a single superficial primordial cell that becomes divided into five initials or "segments." Segment 0, located at the level of the surface receptacular cells, does not become subdivided and does not contribute further to the structure of the mature sporangium. Segments I, II, III and IV each become subdivided through a series of divisions to produce the mature sporangia. The four-rowed sporangial stalks are formed from Segment I and part of Seg- ment II, and the capsules develop from a part of Segment II and Segments III and IV. The annulus develops in Segments II and IV. The developmental pattern of the sporangia of Sphaeropteris cooperi is compared to that of the sporangia of higher leptsoporangiate ferns. The most familiar and most frequently illustrated leptosporangia are those of the higher leptosporangiate ferns. The development of the sporangia of the higher leptosporangiate ferns was described in a series of papers (Wil- son,1958a, b, 1960) and is now well understood. In the sporangia of the ad- vanced leptosporangiate ferns, as illustrated by species in the Polypodiaceae, Grammitidaceae, and Vittariaceae, it was shown that the stalk and the capsule of the leptosporangium develops from a single epidermal primordial cell that becomes divided into five initials or "segments," rather than from the activity of an apical cell. Each one of these "segments" in turn divides, through a series of divisions to produce the mature sporangium. Segment 0 contributes only to the formation of the stalk; Segment I to a portion of the stalk and part of the proximal face of the capsule; Segment II to the stomial region, the stalk, and to the proximal and distal faces of the capsule; and Segments III and IV to the rest of the annulus and to both the proximal and distal faces of the capsule. Although the stalk may be one-, two- or three-rowed at its base, the capsule is always subtended by a three-rowed stalk. The one-rowed stalk results di- rectly from the horizontal orientation of the first division of the sporangial initial, whereas the two- and three-rowed stalks depend on the orientation of both the first division and also the division that produces Segment I. A review of the history of our knowledge of the nature of the leptosporan- gium and its development was published in the introduction to the study of the ontogeny of the sporangia of Phlebodium aureum (L.) J. Sm. (Wilson, 1958a). Recent descriptions of sporangial development continue to reproduce the erroneous pattern apparently originated by Campbell (1905) that the spo- rangial initial produces a three-sided apical cell that cuts off several basal cells to form the stalk until a transverse division stops its activity by cutting off the cap cell. Other accounts are unclear, incomplete and often incorrect. (see Gif- ford and Foster, 1989; Bold et.al., 1987; Holttum et. al., 1970). No detailed ontogenetic studies have been published since the appearance of the paper on