PTERIDOPHYTES 



145 



margin (Figs. 125, 126), which may be called a ''false in- 

 dusium." 



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 di- 

 vide the work. Certain leaves, 

 or certain leaf-branches, pro- 

 duce spores and do no chloro- 

 phyll work, while others do 

 chlorophyll work and produce 

 no spores. This differentia- 

 tion in the leaves or leaf-re- 

 gions is indicated by appro- 

 priate names. Those leaves 

 which produce only spores are 

 called sporophylls, meaning 

 ''spore leaves," while the leaf 

 branches thus set apart are 

 called sporophyll branches. 

 Those leaves which only do 

 chlorophyll work are called /o- 

 liage leaves ; and such branch- 

 es 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 differentiation 

 may be seen in the ostrich fern and sensitive fern ( Onoclea) 

 (Figs. 127, 128), the climbing fern (Li/godinm), the royal 

 fern {Osmimda), the moon wort {Botriicliium) (Fig. 129), 

 and the adder's tongue {OpJiioghjssu^n) (Fig. 130). 



An ordinary fern sporangium consists of a slender stalk 

 and a bulbous top which is the spore case (Fig. 118, 6). 

 This case has a delicate wall formed of a single layer of 

 cells, and extending around it from the stalk and nearly to 



Fig. 125. Leaflets of two common 

 ferns : A, the common brake 

 (Pteris) ; B, maidenhair (Adian- 

 tum); both showing sori borne 

 at the margin and protected by 

 the infolded margin, which thus 

 forms a false indiisium.— Cald- 



AVELL. 



