CLASSIFICATION OF THE PTERIDOPHYTES.
There are three well-marked classes of the Pteridophytes: the ferns
(_Filicinæ_); horse-tails (_Equisetinæ_); and the club mosses
(_Lycopodinæ_).
CLASS I.--FERNS (_Filicinæ_).
The ferns constitute by far the greater number of pteridophytes, and
their general structure corresponds with that of the maiden-hair fern
described. There are three orders, of which two, the true ferns
(_Filices_) and the adder-tongues (_Ophioglossaceæ_), are represented
in the United States. A third order, intermediate in some respects
between these two, and called the ringless ferns (_Marattiaceæ_), has
no representatives within our territory.
The classification is at present based largely upon the characters of
the sporophyte, the sexual plants being still very imperfectly known
in many forms.
The adder-tongues (_Ophioglossaceæ_) are mostly plants of rather small
size, ranging from about ten to fifty centimetres in height. There are
two genera in the United States, the true adder-tongues
(_Ophioglossum_) and the grape ferns (_Botrychium_). They send up but
one leaf each year, and this in fruiting specimens (Fig. 70, _A_) is
divided into two portions, the spore bearing (_x_) and the green
vegetative part. In _Botrychium_ the leaves are more or less deeply
divided, and the sporangia distinct (Fig. 71, _B_). In _Ophioglossum_
the sterile division of the leaf is usually smooth and undivided, and
the spore-bearing division forms a sort of spike, and the sporangia
are much less distinct. The sporangia in both differ essentially from
those of the true ferns in not being derived from a single epidermal
cell, but are developed in part from the ground tissue of the leaf.
[Illustration: FIG. 70.--Forms of ferns. _A_, grape fern
(_Botrychium_), × ½. _x_, fertile part of the leaf. _B_, sporangia of
_Botrychium_, × 3. _C_, flowering fern (_Osmunda_). _x_, spore-bearing
leaflets, × ½. _D_, a sporangium of _Osmunda_, × 25. _r_, ring. _E_,
_Polypodium_, × 1. _F_, brake (_Pteris_), × 1. _G_, shield fern
(_Aspidium_), × 2. _H_, spleen-wort (_Asplenium_), × 2. _I_, ostrich
fern (_Onoclea_), × 1. _J_, the same, with the incurved edges of the
leaflet partially raised so as to show the masses of sporangia
beneath, × 2.]
In the true ferns (_Filices_), the sporangia resemble those already
described, arising in all (unless possibly _Osmunda_) from a single
epidermal cell.
One group, the water ferns (_Rhizocarpeæ_), produce two kinds of
spores, large and small. The former produce male, the latter female
prothallia. In both cases the prothallium is small, and often scarcely
protrudes beyond the spore, and may be reduced to a single archegonium
or antheridium (Fig. 71, _B_, _C_) with only one or two cells
representing the vegetative cells of the prothallium (_v_). The water
ferns are all aquatic or semi-aquatic plants, few in number and scarce
or local in their distribution. The commonest are those of the genus
_Marsilia_ (Fig. 71, _A_), looking like a four-leaved clover. Others
(_Salvinia_, _Azolla_) are floating forms (Fig. 71, _D_).
[Illustration: FIG. 71.--_A_, _Marsilia_, one of the _Rhizocarpeæ_
(after Underwood). _sp._ the "fruits" containing the sporangia. _B_, a
small spore of _Pilularia_, with the ripe antheridium protruding,
× 180. _C_, male prothallium removed from the spore, × 180. _D_,
_Azolla_ (after Sprague), × 1.]
Of the true ferns there are a number of families distinguished mainly
by the position of the sporangia, as well as by some differences in
their structure. Of our common ferns, those differing most widely from
the types are the flowering ferns (_Osmunda_), shown in Figure 70,
_C_, _D_. In these the sporangia are large and the ring (_r_)
rudimentary. The leaflets bearing the sporangia are more or less
contracted and covered completely with the sporangia, sometimes all
the leaflets of the spore-bearing leaf being thus changed, sometimes
only a few of them, as in the species figured.
Our other common ferns have the sporangia in groups (_sori_, sing.
_sorus_) on the backs of the leaves. These sori are of different shape
in different genera, and are usually protected by a delicate
membranous covering (indusium). Illustrations of some of the commonest
genera are shown in Figure 70, _E_, _J_.
CLASS II.--HORSE-TAILS (_Equisetinæ_).
The second class of the pteridophytes includes the horse-tails
(_Equisetinæ_) of which all living forms belong to a single genus
(_Equisetum_). Formerly they were much more numerous than at present,
remains of many different forms being especially abundant in the coal
formations.
[Illustration: FIG. 72.--_A_, spore-bearing stem of the field
horse-tail (_Equisetum_), × 1. _x_, the spore-bearing cone. _B_,
sterile stem of the same, × ½. _C_, underground stem, with tubers
(_o_), × ½. _D_, cross-section of an aerial stem, × 5. _f.b._
fibro-vascular bundle. _E_, a single fibro-vascular bundle, × 150.
_tr._ vessels. _F_, a single leaf from the cone, × 5. _G_, the same
cut lengthwise, through a spore sac (_sp._), × 5. _H_, a spore, × 50.
_I_, the same, moistened so that the elaters are coiled up, × 150.
_J_, a male prothallium, × 50. _an._ an antheridium. _K_,
spermatozoids, × 300.]
One of the commonest forms is the field horse-tail (_Equisetum
arvense_), a very abundant and widely distributed species. It grows in
low, moist ground, and is often found in great abundance growing in
the sand or gravel used as "ballast" for railway tracks.
The plant sends up branches of two kinds from a creeping underground
stem that may reach a length of a metre or more. This stem (Fig. 72,
_C_) is distinctly jointed, bearing at each joint a toothed sheath,
best seen in the younger portions, as they are apt to be destroyed in
the older parts. Sometimes attached to this are small tubers (_o_)
which are much-shortened branches and under favorable circumstances
give rise to new stems. They have a hard, brown rind, and are composed
within mainly of a firm, white tissue, filled with starch.
The surface of the stem is marked with furrows, and a section across
it shows that corresponding to these are as many large air spaces that
traverse the stem from joint to joint. From the joints numerous roots,
quite like those of the ferns, arise.
If the stem is dug up in the late fall or winter, numerous short
branches of a lighter color will be found growing from the joints.
These later grow up above ground into branches of two sorts. Those
produced first (Fig. 72, _A_), in April or May, are stouter than the
others, and nearly destitute of chlorophyll. They are usually twenty
to thirty centimetres in height, of a light reddish brown color, and,
like all the stems, distinctly jointed. The sheaths about the joints
(_L_) are much larger than in the others, and have from ten to twelve
large black teeth at the top. These sheaths are the leaves. At the top
of the branch the joints are very close together, and the leaves of
different form, and closely set so as to form a compact cone (_x_).
A cross-section of the stem (_D_) shows much the same structure as the
underground stem, but the number of air spaces is larger, and in
addition there is a large central cavity. The fibro-vascular bundles
(_f.b._) are arranged in a circle, alternating with the air channels,
and each one has running through it a small air passage.
The cone at the top of the branch is made up of closely set,
shield-shaped leaves, which are mostly six-sided, on account of the
pressure. These leaves (_F_, _G_) have short stalks, and are arranged
in circles about the stem. Each one has a number of spore cases
hanging down from the edge, and opening by a cleft on the inner side
(_G_, _sp._). They are filled with a mass of greenish spores that
shake out at the slightest jar when ripe.
The sterile branches (_B_) are more slender than the spore-bearing
ones, and the sheaths shorter. Surrounding the joints, apparently just
below the sheaths, but really breaking through their bases, are
circles of slender branches resembling the main branch, but more
slender. The sterile branches grow to a height of forty to fifty
centimetres, and from their bushy form the popular name of the plant,
"horse-tail," is taken. The surface of the plant is hard and rough,
due to the presence of great quantities of flint in the epidermis,--a
peculiarity common to all the species.
The stem is mainly composed of large, thin-walled cells, becoming
smaller as they approach the epidermis. The outer cells of the
ground tissue in the green branches contain chlorophyll, and the
walls of some of them are thickened. The fibro-vascular bundles
differ entirely from those of the ferns. Each bundle is nearly
triangular in section (_E_), with the point inward, and the inner
end occupied by a large air space. The tracheary tissue is only
slightly developed, being represented by a few vessels[9] (_tr._) at
the outer angles of the bundle, and one or two smaller ones close to
the air channel. The rest of the bundle is made up of nearly
uniform, rather thin-walled, colorless cells, some of which,
however, are larger, and have perforated cross-walls, representing
the sieve tubes of the fern bundle. There is no individual bundle
sheath, but the whole circle of bundles has a common outer sheath.
[9] A vessel differs from a tracheid in being composed of several
cells placed end to end, the partitions being wholly or partially
absorbed, so as to throw the cells into close communication.
The epidermis is composed of elongated cells whose walls present a
peculiar beaded appearance, due to the deposition of flint within
them. The breathing pores are arranged in vertical lines, and
resemble in general appearance those of the ferns, though differing
in some minor details. Like the other epidermal cells the guard
cells have heavy deposits of flint, which here are in the form of
thick transverse bars.
The spore cases have thin walls whose cells, shortly before
maturity, develop thickenings upon their walls, which have to do
with the opening of the spore case. The spores (_H_, _I_) are round
cells containing much chlorophyll and provided with four peculiar
appendages called elaters. The elaters are extremely sensitive to
changes in moisture, coiling up tightly when moistened (_I_), but
quickly springing out again when dry (_H_). By dusting a few dry
spores upon a slide, and putting it under the microscope without any
water, the movement may be easily examined. Lightly breathing upon
them will cause the elaters to contract, but in a moment, as soon as
the moisture of the breath has evaporated, they will uncoil with a
quick jerk, causing the spores to move about considerably.
The fresh spores begin to germinate within about twenty-four hours,
and the early stages, which closely resemble those of the ferns, may
be easily followed by sowing the spores in water. With care it is
possible to get the mature prothallia, which should be treated as
described for the fern prothallia. Under favorable conditions, the
first antheridia are ripe in about five weeks; the archegonia, which
are borne on separate plants, a few weeks later. The antheridia
(Fig. 72, _J_, _an._) are larger than those of the ferns, and the
spermatozoids (_K_) are thicker and with fewer coils, but otherwise
much like fern spermatozoids.
The archegonia have a shorter neck than those of the ferns, and the
neck is straight.
Both male and female prothallia are much branched and very irregular
in shape.
There are a number of common species of _Equisetum_. Some of them,
like the common scouring rush (_E. hiemale_), are unbranched, and the
spores borne at the top of ordinary green branches; others have all
the stems branching like the sterile stems of the field horse-tail,
but produce a spore-bearing cone at the top of some of them.
CLASS III.--THE CLUB MOSSES (_Lycopodinæ_).
The last class of the pteridophytes includes the ground pines, club
mosses, etc., and among cultivated plants numerous species of the
smaller club mosses (_Selaginella_).
Two orders are generally recognized, although there is some doubt as
to the relationship of the members of the second order. The first
order, the larger club mosses (_Lycopodiaceæ_) is represented in the
northern states by a single genus (_Lycopodium_), of which the common
ground pine (_L. dendroideum_) (Fig. 73) is a familiar species. The
plant grows in the evergreen forests of the northern United States as
well as in the mountains further south, and in the larger northern
cities is often sold in large quantities at the holidays for
decorating. It sends up from a creeping, woody, subterranean stem,
numerous smaller stems which branch extensively, and are thickly set
with small moss-like leaves, the whole looking much like a little
tree. At the ends of some of the branches are small cones (_A_, _x_,
_B_) composed of closely overlapping, scale-like leaves, much as in a
fir cone. Near the base, on the inner surface of each of these scales,
is a kidney-shaped capsule (_C_, _sp._) opening by a cleft along the
upper edge and filled with a mass of fine yellow powder. These
capsules are the spore cases.
The bases of the upright stems are almost bare, but become covered
with leaves higher up. The leaves are in shape like those of a moss,
but are thicker. The spore-bearing leaves are broader and when
slightly magnified show a toothed margin.
The stem is traversed by a central fibro-vascular cylinder that
separates easily from the surrounding tissue, owing to the rupture of
the cells of the bundle sheath, this being particularly frequent in
dried specimens. When slightly magnified the arrangement of the
tissues may be seen (Fig. 73, _E_). Within the epidermis is a mass of
ground tissue of firm, woody texture surrounding the central oval or
circular fibro-vascular cylinder. This shows a number of white bars
(xylem) surrounded by a more delicate tissue (phloem).
On magnifying the section more strongly, the cells of the ground
tissue (_G_) are seen to be oval in outline, with thick striated
walls and small intercellular spaces. Examined in longitudinal
sections they are long and pointed, belonging to the class of cells
known as "fibres."
[Illustration: FIG. 73.--_A_, a club moss (_Lycopodium_), × ⅓. _x_,
cone. _r_, root. _B_, a cone, × 1. _C_, single scale with sporangium
(_sp._). _D_, spores: i, from above; ii, from below, × 325. _E_, cross
section of stem, × 8. _f.b._ fibro-vascular bundle. _F_, portion of
the fibro-vascular bundle, × 150. _G_, cells of the ground tissue,
× 150.]
The xylem (_F_, _xy._) of the fibro-vascular bundle is composed of
tracheids, much like those of the ferns; the phloem is composed of
narrow cells, pretty much all alike.
The spores (_D_) are destitute of chlorophyll and have upon the
outside a network of ridges, except on one side where three straight
lines converge, the spore being slightly flattened between them.
Almost nothing is known of the prothallia of our native species.
The second order (_Ligulatæ_) is represented by two very distinct
families: the smaller club mosses (_Selaginelleæ_) and the quill-worts
(_Isoeteæ_). Of the former the majority are tropical, but are common
in greenhouses where they are prized for their delicate moss-like
foliage (Fig. 74, _A_).
[Illustration: FIG. 74.--_A_, one of the smaller club mosses
(_Selaginella_). _sp._ spore-bearing branch, × 2. _B_, part of a stem,
sending down naked rooting branches (_r_), × 1. _C_, longitudinal
section of a spike, with a single macrosporangium at the base; the
others, microsporangia, × 3. _D_, a scale and microsporangium, × 5.
_E_, young microsporangium, × 150. The shaded cells are the spore
mother cells. _F_, a young macrospore, × 150. _G_, section of the
stem, × 50. _H_, a single fibro-vascular bundle, × 150. _I_, vertical
section of the female prothallium of _Selaginella_, × 50. _ar._
archegonium. _J_, section of an open archegonium, × 300. _o_, the egg
cell. _K_, microspore, with the contained male prothallium, × 300.
_x_, vegetative cell. _sp._ sperm cells. _L_, young plant, with the
attached macrospore, × 6. _r_, the first root. _l_, the first leaves.]
The leaves in most species are like those of the larger club mosses,
but more delicate. They are arranged in four rows on the upper side of
the stem, two being larger than the others. The smaller branches grow
out sideways so that the whole branch appears flattened, reminding one
of the habit of the higher liverworts. Special leafless branches (_B_,
_r_) often grow downward from the lower side of the main branches, and
on touching the ground develop roots which fork regularly.
The sporangia are much like those of the ground pines, and produced
singly at the bases of scale leaves arranged in a spike or cone (_A_,
_sp._), but two kinds of spores, large and small, are formed. In the
species figured the lower sporangium produces four large spores
(macrospores); the others, numerous small spores (microspores).
Even before the spores are ripe the development of the prothallium
begins, and this is significant, as it shows an undoubted
relationship between these plants and the lowest of the seed plants,
as we shall see when we study that group.
If ripe spores can be obtained by sowing them upon moist earth, the
young plants will appear in about a month. The microspore (Fig. 74,
_K_) produces a prothallium not unlike that of some of the water
ferns, there being a single vegetative cell (_x_), and the rest of
the prothallium forming a single antheridium. The spermatozoids are
excessively small, and resemble those of the bryophytes.
The macrospore divides into two cells, a large lower one, and a
smaller upper one. The latter gives rise to a flat disc of cells
producing a number of small archegonia of simple structure (Fig. 74,
_I_, _J_). The lower cell produces later a tissue that serves to
nourish the young embryo.
The development of the embryo recalls in some particulars that of
the seed plants, and this in connection with the peculiarities of
the sporangia warrants us in regarding the _Ligulatæ_ as the highest
of existing pteridophytes, and to a certain extent connecting them
with the lowest of the spermaphytes.
Resembling the smaller club mosses in their development, but differing
in some important points, are the quill-worts (_Isoeteæ_). They are
mostly aquatic forms, growing partially or completely submerged, and
look like grasses or rushes. They vary from a few centimetres to half
a metre in height. The stem is very short, and the long cylindrical
leaves closely crowded together. The leaves which are narrow above are
widely expanded and overlapping at the base. The spores are of two
kinds, as in _Selaginella_, but the macrosporangia contain numerous
macrospores. The very large sporangia (_M_, _sp._) are in cavities at
the bases of the leaves, and above each sporangium is a little pointed
outgrowth (ligula), which is also found in the leaves of
_Selaginella_. The quill-worts are not common plants, and owing to
their habits of growth and resemblance to other plants, are likely to
be overlooked unless careful search is made.