PTERLDOPHYTES 



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 sporophylh, meaning 

 " spore leaves," while the leaf 

 branches thus set apart are 

 called sporophyll branches. 

 Those leaves which only do 

 chlorophyll work are called fo- 

 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 (Lygoclium), the royal 

 fern (Osmunda), the moonwort (Botrychium) (Fig. 129), 

 and the adder's tongue (Ophioglossum) (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 

 (Pterti) ; B, maidenhair (Adian- 

 tum); both showing sori borne 

 at the margin and protected by 

 the infolded margin, which thus 

 forms a false indusium.— Calb- 

 weix. 



