ch grow out of the
veins of the leaf. Sometimes these are distributed over the whole lower
surface of the leaf or frond, or over the whole surface when there are
no proper leaf-blades to the frond, but all is reduced to stalks.
Commonly the spore-cases occupy only detached spots or lines, each of
which is called a SORUS, or in English merely a Fruit-dot. In many Ferns
these fruit-dots are naked; in others they are produced under a
scale-like bit of membrane, called an INDUSIUM. In Maidenhair-Ferns a
little lobe of the leaf is folded back over each fruit-dot, to serve as
its shield or indusium. In the true Brake or Bracken (Pteris) the whole
edge of the fruit-bearing part of the leaf is folded back over it like a
hem.

488. The form and structure of the spore-cases can be made out with a
common hand magnifying glass. The commonest kind (shown in Fig. 503) has
a stalk formed of a row of jointed cells, and is itself composed of a
layer of thin-walled cells, but is incompletely surrounded by a border
of thicker-walled cells, forming the RING. This extends from the stalk
up one side of the spore-case, round its summit, descends on the other
side, but there gradually vanishes. In ripening and drying the shrinking
of the cells of the ring on the outer side causes it to straighten; in
doing so it tears the spore-case open on the weaker side and discharges
the minute spores that fill it, commonly with a jerk which scatters them
to the wind. Another kind of spore-case (Fig. 507) is stalkless, and has
its ring-cells forming a kind of cap at the top: at maturity it splits
from top to bottom by a regular dehiscence. A third kind is of firm
texture and opens across into two valves, like a clam-shell (Fig. 508a):
this kind makes an approach to the next family.

[Illustration: Fig. 509. A young prothallus of a Maiden-hair, moderately
enlarged, and an older one with the first fern-leaf developed from near
the notch. 510. Middle portion of the young one, much magnified, showing
below, partly among the rootlets, the _antheridia_ or fertilizing
organs, and above, near the notch, three _pistillidia_ to be
fertilized.]

489. The spores germinate on moistened ground. In a conservatory they
may be found germinating on a damp wall or on the edges of a
well-watered flower-pot. Instead of directly forming a fern-plantlet,
the spore grows first into a body which closely resembles a small
Liverwort. This is named a PROTHALLUS (Fig. 509): from so