Metaphyta Pteris. 



123 



face of the leaf more towards its centre, there are developed 

 scale-like outgrowths from the epidermis of the leaf, which 

 have for their function the protection of the sporangia before 

 they are mature. The epidermal covering is known as an 

 indusmm, and a group of sporangia enclosed by an indusium 

 is termed a sorus. In Pteris^ however, the edge of the leaf 

 forms a false indusium, whilst the term sorus must be applied 

 to the entire infolded margin and its contents. All the 

 leaves of the fern do not carry sporangia a point of some 



FIG. 51. Pteris aqiiUina.- SORUS AND INDUSIUM. (Maout and Decaisne ) 



Pinnule with incurved edge. 



Sporangia exposed by rup- 

 ture of indusium. 



importance, as we shall see subsequently. Those leaves 

 which do carry sori, i.e. the fertile leaves, are termed sporo- 

 phylla. 



It will be necessary now to examine a sorus in greater 

 detail. 



If a section of the margin of a sporophyllum be made 

 the sporangia will be found to be stalked capsules springing 

 from the under surface of the leaf (fig. 51). Each sporan- 

 gium is multicellular, and continuous with the epidermis of 

 the leaf, the point of origin of a sporangium being termed 

 the placenta. 



The stalk of the sporangium, or funicle, is itself multi- 

 cellular, but does not contain any fibro-vascular or funda- 

 mental tissue. In its mature state it usually consists of two or 

 more rows of cells, although very young sporangia have only 



