called pinnules. When a frond is tripínnate the last complete divisions are
called ultimate pinnules or segments. A frond is pinnátifid when its lobes
extend halfway or more to the rachis or midvein as in the middle lobes
of the pinnátifid spleenwort (Fig. 3). The pinnæ of a frond are often
pinnátifid when the frond itself is pinnate; and a frond may be pinnate
in its lower part and become pinnátifid higher up as in the pinnátifid
spleenwort just mentioned (Fig. 3).

[Illustration: Fig. 1]

[Illustration: Fig. 2]

[Illustration: Fig. 3]

The divisions of a pinnátifid leaf are called segments; of a bipinnátifid
or tripinnátifid leaf, ultimate segments.


SPORÁNGIA AND FRUIT DOTS

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.