Fern spores are formed in little sacs known as spore-cases or sporángia
(Fig. 4). They are usually clustered in dots or lines on the back or margin
of a frond, either on or at the end of a small vein, or in spike-like
racemes on separate stalks. Sori (singular _sorus_, a heap), or fruit dots
may be naked as in the polypody, but are usually covered with a thin,
delicate membrane, known as the indusium (Greek, a dress, or mantle). The
family or genus of a fern is often determined by the shape of its indusium;
e.g., the indusium of the woodsias is star-shaped; of the Dicksonias,
cup-shaped; of the aspleniums, linear; of the wood ferns, kidney-shaped,
etc.
[Illustration: Fig. 4]
In many ferns the sporangia are surrounded in whole or in part by a
vertical, elastic ring (annulus) reminding one of a small, brown worm
closely coiled (Fig. 4). As the spores mature, the ring contracts and
bursts with considerable force, scattering the spores. The spores of the
different genera mature at different times from May to September. A good
time to collect ferns is just before the fruiting season. (For times of
fruiting see individual descriptions or chronological chart on page 220.)