XVI 
the gen as, species ha ve often been described as nonindu- 
siate from the involucre having already disappeared. Be- 
tween the peltate centrally fixed involucre of Aspidium 
and the reniform one, attached by its sinus, of Nephro- 
dium, many transfers have been made tending to the ab- 
sorption of Aspidium in the latter genus. 
22. Sori are arranged in definite relation with the 
veins, and their position with regard to these and to the 
margin and midrib must be carefully noted, whether mar- 
ginal or medial, submarginal, &c. ; whether upon, or at end 
of vein or veinlet ; whether on both sides as in Diplazium 
and Anisogonium, In the nonindusiate orders, Polypo- 
dium and Acrostichum, the venation and the relative posi- 
tion of sori must be carefully noticed, often most readily 
by holding up the frond to the light, or to a lamp, and 
looking through. In all cases a small magnifying glass 
is necessary. 
This fructification consists, not of seed, but of minute 
spores grouped in capsules (sporangia), encircled generally 
by an elastic jointed ring, complete or incomplete, and 
covered or not by indusium, through which these spores 
are in various ways discharged, and develop into cellular 
unisexual plants called prothallia, the mal eAntheridia, and 
the female Archonia. From the former a ciliated spiral 
filament emerges, and entering the central canal of the 
archyonia, is secured thereby resulting cell development ; 
and thus fertilised the vesicle within produces the stem 
of the young fern. 
