Pharmaceutical botany - Lighthouse Survival Blog

FILICALES 67
stems, sheath-like whorls of united leaves and terminal cone-like fruc-
tifications. Their bodies contain large amounts of silicon, hence the
name scouring rushes.
In some varieties the fruiting cone is borne on the ordinary stem, in
others on a special stem of slightly different form. In the latter the
spores are provided with elaters, which, being hygroscopic, coil and un-
coil with increase or decrease in the amount of moisture present, thus
aiding in the ejection of spores from the sporangia. The number of
species is small and included under one genus Equisetum.
SUBDIVISION III.—FILICALES
The group Filicales is the largest among the vascular cryptogams
and includes all the plants commonly known as Ferns. The main axis
of a typical fern is a creeping underground stem or rhizome which at
its various nodes bears rootlets below and fronds above. These fronds
are highly developed, each being provided with a petiole-like portion
called a stipe which is extended into a lamina usually showing a forked
venation. Some ferns possess lamina; which are lobed, each lobe being
called a pinna. If a pinna be further divided, its divisions are called
pinnules. The unfolding of a frond is circinate and it increases in length
by apical growth. On the under surface of the laminte, pinnae, or
pinnules may be seen small brown patches each of which is called a
sorus, and usually covered by a membrane called the indusium. Each
sorus consists of a number of sporangia (spore cases) developed from
epidermal cells. In some ferns the entire leaf becomes a spore-bearing
organ (sporophyll). Most sporangia have a row of cells around the
margin, the whole being called the annulus. Each cell of the annulus
has a U-shaped thickened cell wall. Water is present within these
cells and when it evaporates it pulls the cell walls together, straightening
the ring and tearing open the weak side. The annulus then recoils
and hurls the spores out of the sporangium. Upon coming in contact
with damp earth each spore germinates, producing a green septate
filament called a protonema. This later becomes a green heart-shaped
body called a prothallus. It develops on its under surface antheridia
or male organs and archegonia or female organs as well as numerous
rhizoids. Within the antheridia are developed motile sperm, while
ova are produced within the archegonia. The many ciliate sperms
escape from the antheridia of one prothallus during a wet season and