340 



MOSSES AND FERNS 



small veins. Where there are special sporophylls, the develop- 

 ment of the sporangia begins before the leaves begin to unfold. 

 In Polypodium (Fig. 190) the first evidence of the forma- 

 tion of sporangia is a series of minute depressions upon the 

 lower side of the leaf, much as occurs in Angiopteris. The 

 bottom of this depression is occupied by a low elevation, the 

 placenta, and upon this the sporangia form in an analogous 



Fig. 189. — Polypodium falcatum. A, Cross-section of a sterile leaf, cutting across one 

 of the smaller veins, X260; st, section of a stoma; B, similar section of a sporo- 

 phyll, showing the position of the sorus above the vein, X85. 



way, but are not all developed at the same time, so that a single 

 sorus may contain nearly all stages of development. The spo- 

 rangium here can be readily traced back to a single epidermal 

 cell. 



The sporangial cell protrudes until it is nearly hemispher- 

 ical, when it is cut ofif by a wall level with the surface of the 



