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- 
pliylls, 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 (Osmunda), the moon wort (Botrychium) 
(Fig. 300), and the adder's tongue ( Ophioglossum). 
Fro. 298. 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 indusium. CALD- 
WELL. 
