ry fine yellow powder, the spores.

493. The Selaginellas have been separated from Lycopodium, which they
much resemble, because they produce two kinds of spores, in separate
spore-cases. One kind (MICROSPORES) is just that of Lycopodium; the
other consists of only four large spores (MACROSPORES), in a spore-case
which usually breaks in pieces at maturity (Fig. 513-515).

494. =The Quillworts, Isoetes= (Fig. 516-519), are very unlike Club
Mosses in aspect, but have been associated with them. They look more
like Rushes, and live in water, or partly out of it. A very short stem,
like a corm, bears a cluster of roots underneath; above it is covered by
the broad bases of a cluster of awl-shaped or thread-shaped leaves. The
spore-cases are immersed in the bases of the leaves. The outer
leaf-bases contain numerous macrospores; the inner are filled with
innumerable microspores.

[Illustration: Fig. 520. Plant of Marsilia quadrifoliata, reduced in
size; at the right a pair of sporocarps of about natural size.]

495. =The Pillworts= (_Marsilia_ and _Pilularia_) are low aquatics,
which bear globular or pill-shaped fruit (SPOROCARPS) on the lower part
of their leaf-stalks or on their slender creeping stems. The leaves of
the commoner species of Marsilia might be taken for four-leaved Clover.
(See Fig. 520.) The sporocarps are usually raised on a short stalk.
Within they are divided lengthwise by a partition, and then crosswise by
several partitions. These partitions bear numerous delicate sacs or
spore-cases of two kinds, intermixed. The larger ones contain each a
large spore, or macrospore; the smaller contain numerous microspores,
immersed in mucilage. At maturity the fruit bursts or splits open at
top, and the two kinds of spores are discharged. The large ones in
germination produce a small prothallus; upon which the contents of the
microspores act in the same way as in Ferns, and with a similar result.

496. =Azolla= is a little floating plant, looking like a small Liverwort
or Moss. Its branches are covered with minute and scale-shaped leaves.
On the under side of the branches are found egg-shaped thin-walled
sporocarps of two kinds. The small ones open across and discharge
microspores; the larger burst irregularly, and bring to view globose
spore-cases, attached to the bottom of the sporocarp by a slender stalk.
These delicate spore-cases burst and set free about four macrospores,
which are fertilized at germination, in