312 TlMEHRI. 
one in which the capsules have discharged their contents. 
In the latter case the surface is dusty and rough, and 
the exploded capsules are clearly and alone seen. In 
the former the surface is quite smooth, different in 
colour to the former, and felt-like. Inspection by a lens 
reveals the fact that this smooth appearance is caused by a 
thin coating of minute corpuscles, which form a continuous 
unbroken covering quite concealing the layer of sori 
beneath. These corpuscles, whatever their shape, 
for they varv much in the different varieties or 
types, are attached by slender tubes which penetrate 
between the sporangia to the surface of the leaf. 
As is known, in this sub-order {Polypodiacese) the 
sporangia are rather compressed, stipitate, orbicular 
bodies, girt by a jointed elastic vertical band, which, 
at the maturity of the vessel, bursts transversely 
on one side for the emission of the dust-like contents 
(spores). This a6l is produced by the contraction of the 
vertical band, and as the tension, through the band not 
reaching to the base on one side of the capsule, is not 
equal, the spontaneous and immediate result is an irreg- 
ular elongation of that body, which then appears with a 
wide-gaping mouth where it opened. Without this 
elongation of the sporangia the layer of corpuscles cover- 
ing them would remain intact, and effectually prevent 
the dispersion of the spores ; but in the act of bursting 
and elongating simultaneously the sporangia force 
through this covering, entirely displacing it, and leave 
on the external surface no trace whatever of its 
existence. In gathering botanical specimens, if this 
change has not already occurred, it takes place in drying 
them, so that herbaria very rarely exhibit the feature. 
