STRUCTURE. 9 
flowering plants is executed by modifications of leaves. 
The theory, however, applies only to the gyrate ferns. 
In those which are furnished with a transverse ring, he 
suggests, either, that the midrib of the young scale, 
out of which the case is formed, is not so much deve- 
loped : or the case is a nucleus of cellular tissue, separat- 
ing both from that which surrounds it and from its in- 
ternal substance, which latter assumes the form of spo- 
rules, in the same way as the internal tissue of an anther 
separates from the valves under the form of pollen. In 
Ophiogl;ssuir., there is no spore-case beyond the involute 
contracted segments of the spore-bearing leaf. The 
vertical ringed spore-cases, when mature, split suddenly 
with a transverse fissure, thus ejecting the spores. Those 
which are furnished with an horizontal ring, on the other 
hand, burst vertically. The spore-cases of the exannulate 
ferns are regularly two-valved. The accompanying figures 
will convey an idea of the external appearance of these 
organs. 
Spore Casts. 
1. Polypodies: 2. HymenophyUiew, (a) Trichomanog, (4) Hrme- 
nophyllum : 3. Osmundacc* : 4. Ophioglossaces, (o) Botrychinm , 
(6) Ophioglossum. 
