328 



PLANT STUDIES 



gated, and is usually covered by a delicate flap (indusium) 

 which arises from the epidermis (Fig. 296). Occasionally 

 the sori are extended along the under surface of the mar- 

 gin of the leaf, as in maidenhair fern [Adiantum)^ and the 

 common brake (Pteris), in which case they are protected 

 by the inrolled margin (Fig. 298), which may be called a 

 "false indusium." 



It is evident that such leaves are doing two distinct 

 kinds of work — chlorophyll work and spore formation. 

 This is true of most of the ordinary Ferns, but some of 



them show a tendency to divide 

 the work. Certain leaves, or 

 certain leaf-branches, produce 

 spores and do no chlorophyll 

 work, while others do chloro- 

 phyll work and produce no 

 spores. This differentiation in 

 the leaves or leaf-regions is in- 

 dicated by appropriate names. 

 Those leaves which produce 

 only spores are called sporo- 

 phylls, meaning " spore leaves," 

 while the leaf branches thus 

 set apart are called sporophyll 

 branches. Those leaves which 

 only do chlorophyll work are 

 called foliage leaves ; and such 

 branches are foliage branches. 

 As sporophylls are not called 

 upon for chlorophyll work they 

 often become much modified, being much more compact, 

 and not at all resembling the foliage leaves. Such a differ- 

 entiation may be seen in the ostrich fern and sensitive 

 fern (Onoclea) (Fig. 299), the climbing fern (Lygodium), 

 the royal fern {Osmu?ida), the moonwort {BotrycMum) 

 (Fig. 300), and the adder's tongue (Ophioglossum). 



Fig. 298. Leaflets of two common 

 ferns : A, the common brake 

 (Pteris) ; B, maidenhair {Adian- 

 turri) ; both showing sori borne 

 at the margin and protected by 

 the infolded margin, which thus 

 forms a false indusium.— Cald- 

 well. 



