APOSPOEY IN FERNS. 
213 
the spores or reproductive bodies are borne normally upon the 
backs of the fronds, a feature constituting one of the most salient 
differences between ferns and flowers, and one which for a long 
time threw observers off the scent regarding their true nature. 
The sori in one family — the Polypods — are destitute of a cover, 
but in the large majority of cases are furnished with an indusium. 
The sorus consists of a variable number of spherical capsules, 
termed sporangia, which, though themselves only just visible to 
the naked eye, contain each a large number of absolutely 
microscopic bodies termed spores, very many millions of which 
are produced upon a fern frond of very moderate size. 
When these spores are ripe, they are scattered by the sudden 
bursting of the sporangium, caused by the contraction of a ring, 
which in most cases wholly or partially surrounds it. 
We here arrive at another great point of difference between 
ferns and flowers, the spores thus scattered differing essentially 
from a seed, inasmuch as they are not the product of a process 
of fertilization. Their function also is not that of reproducing 
the parent form directly, but to give rise to another form capable 
of bearing the equivalents of flowers, and thus bringing about 
that sexual action v/hich Nature seems almost invariably to 
require for the perpetuation of a species. 
The spore then, by a process of cell generation, develops 
into a prothallus, which, after the first few cells have proceeded 
from the spore, develops and supports itself by means of root- 
hairs upon its under surface. After a time the growing point 
stops short, and the sides grow out and on until it assumes 
more or less of a heart shape, at which stage, and when it is on 
an average from to ^ inch across, the antheridia and archegonia 
may be easily detected, the former scattered among the root- 
hairs, and the latter clustered together upon a thickened cushion 
of cells in the indentation of the prothallus. The next process 
is the rupture of the antheridia, from which issue numerous 
