126 
COMMON HART’S TONGUE. 
The dwarf fronds are thick and of a leathery tex¬ 
ture, the larger thinner and less rigid ; their outline 
is what is called strap-shaped, that is narrow oblong 
lanceolate much elongated : they taper toward a point 
at the apex, and become narrow downwards, cor¬ 
date (rounded like the thick end of the heart in cards) 
at the base; the margin is entire, or very slightly 
wavy ; and the stipes is shaggy and about half the 
length of the leaf. The venation consists of a strong 
midvein or midrib, extending the whole length of the 
frond, from which forked veins, their branches paral¬ 
lel and proceeding direct toward the margin, termin¬ 
ating just within it in a club-shaped apex. The veins 
are usually forked twice, but are not constant to any 
exact number of divisions. The sori, which are ob¬ 
long patches of unequal length, lying in the direction 
of the veins at short intervals along the upper two- 
thirds of the frond, are composed each of two lines 
of fructification united at their sides, each of these 
lines, however, consisting of a complete sorus, so that 
the two united are properly called a twin sorus. This 
twin sorus is always produced between two fascicles 
of veins; that Is, the lowermost venule produced by 
one vein and the uppermost of another below become 
each a receptacle upon which one of the two conti¬ 
guous lines of spore-cases is produced. Their indusia 
are attached on the upper and lower sides of the 
venules, the other edges overlapping, so that the free 
margin is outside the fascicle of venules to which it 
belongs. Yfhen very young, the separation where 
they overlap is not apparent, but becomes so as they 
