THE OLDER SPOROPHYTE 



207 



The dehiscence of the individual locuH is affected by the contraction of the 

 thinner-walled cells surrounded by firmer tissue. The sporangium oi Angiopteris 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 Osmundaceje. By the 

 contraction of this thickened annulus the longitudinal sHt 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 Dan^a, 7,500 for 

 Kaulfussia, 2,500 for Marattia, and 1,450 for Angiopteris. 



Which type of sporangium in the Marattiaceae is the more primitive is very 

 difficult to say, as both the free sporangium like that of Angiopteris and the compact 



synangium like that oi Marattia 

 and Danaa are of about equal 

 antiquity, so far as the geological 

 record goes. It must be remem- 

 bered that the living Marattiaceae 

 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 Uke 

 Botrychium or Heltninthostachys, 

 while the genera with the solid 

 synangium like Dancea may have come from forms with completely united spo- 

 rangia like Ophioglossum. 



It appears from a study of the most ancient ferns that these had dimorphic 

 leaves, the sporophyll probably resembling the fertile spikes of Botrychium or the 

 fertile leaf segments of Osmunda. How the modern Marattiaceae originated from 

 forms of this type is not by any means clear. The development in Helminthostachys 

 of sterile leaf-like lobes associated with the sporangia may perhaps afford a clew to 

 the method of sterilization by which the sporangiophores of some type allied to the 

 Ophioglossaceae may gradually have developed sterile green leaf segments, bearing 

 upon their lower surface sporangia or synangia like those of the modern Marattiaceae. 

 These sterile segments found in Helminthostachys are occasionally strikingly leaf- 

 like, and as the groups of sporangia in Helminthostachys are sometimes united into 

 small synangia, the development of similar synangia upon the lower surface of such 

 green sterile segments of the sporangiophore is quite conceivable. 



Another explanation might be the coalescence of the fertile and the sterile parts 

 of a sporophyll like that found in the Ophioglossaceae, but the fact that the spo- 



FiG. 187. — Angiopteris. 



.\, B, sections of young sporangia (after Goebel). C, an older sporangium. 

 r. the annulus; t, the persistent tapetum. X75. 



