. Carnegie Institution of Washington publication. THE OLDER SPOROPHYTE 207 The dehiscence of the individual loculi is affected by the contraction of the thinner-walled cells surrounded by firmer tissue. The sporangium of An giopteris is undoubtedly a more specialized structure than the synangium of the other Maratti- aceae. In Angiopteris each individual sporangium has the wall on the outer side much thicker than that on the inner one, and the superficial cells have their walls much thickened. The inner wall is sometimes composed of but one layer of cells outside of the tapetum, but more commo
. Carnegie Institution of Washington publication. THE OLDER SPOROPHYTE 207 The dehiscence of the individual loculi is affected by the contraction of the thinner-walled cells surrounded by firmer tissue. The sporangium of An giopteris is undoubtedly a more specialized structure than the synangium of the other Maratti- aceae. In Angiopteris each individual sporangium has the wall on the outer side much thicker than that on the inner one, and the superficial cells have their walls much thickened. The inner wall is sometimes composed of but one layer of cells outside of the tapetum, but more commonly there are one or two layers of cells between the tapetum and the epidermis. Near the top of the sporangium on its outer side there is a transverse band of cells with thicker walls, and these constitute a rudimentary annulus very much like that found in the Osmundaceae. By the contraction of this thickened annulus the longitudinal slit on the inner face of the sporangium is made to open widely at maturity. The number of spores produced in each loculus, according to Bower, is approximately 1,750 for DancEa, 7,500 for , 2,500 for Marattia, and 1,450 for Angiopteris. Which type of sporangium in the Marattiacea? is the more primitive is very difficult to say, as both the free sporangium like that of Angiopteris and the compact synangium like that of Marattia and Dancea are of about equal antiquity, so far as the geological record goes. It must be remem- bered that the living Marattiaceaj are almost certainly merely a few isolated fragments of a once large group, and it is by no means necessary to assume that the spo- rangia of the living forms must necessarily all conform to a com- mon primitive type. It seems quite as likely that the free spo- rangia, like those of Angiopteris, may have originated directly from some ancient prototype which resembled, perhaps, forms like Botrychtum or Helminthostach ys, while the genera with the solid synangium like Dancea may have c
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