The alternation of generations is as distinct in this Division as in
the Mosses, but the sexual generation consists of only a small thallus,
the prothallium, which bears directly the sexual organs, _antheridia_
and _archegonia_; and the asexual generation, which arises from the
fertilisation of the oosphere, is no longer a single short-lived
sporangium, but a highly developed, generally perennial, plant provided
with stem, leaves and _true roots_ (Ferns, Horsetails, etc.), the
sporangia being borne on the leaves. In this latter generation the
tissues are differentiated into epidermis, ground tissue and vascular
tissue; in the last named the bundles are closed, and in the majority
of cases concentric.
The =sexual generation=, =gametophyte=, or =prothallium=, is _always
a thallus_, although not always green and leaf-like (Figs. 205, 215,
222, 229, 235, etc.) It is very small, even in cases where it attains
the greatest development, and consists only of parenchymatous cells.
The prothallium is nourished by hair-like roots (rhizoids) and has
only a transitory existence, dying soon after the fertilisation of its
oosphere.
The ANTHERIDIA exhibit great variations in structure which, however,
must be considered as modifications of the fundamental type which is
found in the Mosses. These modifications will be mentioned under the
various families. The _spermatozoids_ are always spirally-coiled,
self-motile, protoplasmic bodies, with most frequently a large number
of fine cilia on the anterior end (Figs. 206, 223, 234). They are
formed principally from the nucleus of the mother-cell, and portions of
the cytoplasm often remain for a time attached to their posterior end.
The ARCHEGONIA are more uniform throughout the entire Division,
and more closely resemble those of the Mosses. They are, as in the
previous Division, principally flask-shaped; but the central portion,
which encloses the oosphere, is always embedded in the tissue of the
prothallium, so that the neck, which is formed of 4 rows of cells,
projects above the surface (Figs. 201 ^3, 222 _h_). The development
of the archegonium in a Fern is seen in the accompanying figure (Fig.
201). The archegonium is developed from a surface cell, which divides
into three cells by two walls in a direction parallel to the surface
of the prothallium (Fig. 201). The most internal cell becomes the
ventral portion of the archegonium. The external one (_b_) divides
perpendicularly to the surface of the prothallium into four cells,
which again divide parallel to the surface and form the neck (_b_,
in 2 and 3). The intermediate cell projects upwards into the neck
and divides into two, the lower one, after the separation of the
ventral canal-cell, becoming the _oosphere_, and the upper one the
_neck-canal-cell_ (_c_, in 2 and 3).
[Illustration: FIG. 201.--_Pteris serrulata._ Development of
archegonia.]
As in the Mosses, the divisional walls of the neck-canal-cells become
mucilaginous, causing the rupture of the neck of the archegonium.
Fertilisation takes place as in the Mosses, and the passage of the
spermatozoids, along the neck, to the oosphere, has been observed.
Water (rain or dew) is similarly necessary for the movements of the
spermatozoids, and hence for fertilisation. The other classes of the
Division chiefly deviate from the Ferns in having the archegonium sunk
deeper into the prothallium, and the neck reduced in length (compare
Fig. 201 with Figs. 216, 222, 235, 236).
According to the nature of the spores, the three classes of the
Vascular Cryptogams are each divided into isosporous and heterosporous
groups.
I. The =isosporous= Vascular Cryptogams have _only one kind of spore_.
The prothallium developed from this is in some cases monœcious, bearing
both antheridia and archegonia; but in others there is a distinct
tendency for each prothallium to bear only antheridia or archegonia
(diœcious)--true Ferns and _Lycopodium_.
In _Equisetum_ there is only one kind of spore, but two kinds of
prothallia are developed, one of which bears only antheridia (male),
the other only archegonia (female); but the one that bears antheridia
may be transformed into the one that bears archegonia and vice versa.
II. In the higher group, =heterosporous= Vascular Cryptogams
(_Selaginella_ and _Isoëtes_, etc.), there are two distinct kinds
of spores, the _small_, microspores, and the _large_, macrospores.
The _microspores_ are male, and produce prothallia which bear only
antheridia. The _macrospores_ are female, and produce prothallia which
bear only archegonia.
Corresponding to this difference in the spores, there is also found
a difference in the development of the prothallium. In the Isosporeæ
the prothallium is large, and either green, leaf-like, and provided
with rhizoids (most of the Ferns, Horsetails, etc.), or subterranean,
pale-coloured, and globular (_Ophioglossum_, _Lycopodium_). It lives
vegetatively for a fairly long time, and generally produces a large
and varying number of archegonia and antheridia. The prothallium in
the Heterosporeæ is gradually more and more reduced, its independent
and vegetative life becomes of less and less importance, it becomes
more dependent on the mother-plant, and projects from the spore very
slightly, or not at all. The antheridia and archegonia become reduced
in number to one, and also degenerate in point of development.
It may here be remarked that the gradual development of the asexual
generation, the development of the two kinds of spores, and the
progressive reduction of the prothallium and sexual organs which
is found in this Division, is continued to the Gymnosperms and
Angiosperms. The microspores are in these called pollen-grains, and
the male prothallium is very rudimentary. The macrospores are termed
embryo-sacs, and the female prothallium, the endosperm.
The =asexual generation=, =sporophyte=. When the oosphere, which in
this case as in all others is a primordial cell, is fertilised, it
surrounds itself with a cell-wall and commences to divide into a number
of cells, to form the embryo.
The first dividing wall (basal wall) is nearly horizontal, and
in the direction of the longitudinal axis of the archegonium.
The next wall is vertical, and the next perpendicular to the
other two. The oosphere, therefore, is now divided into eight
octants by these three walls. The basal wall divides the
embryo into a hypobasal and an epibasal half. From the first
one, by continued divisions, the first root is developed; from
the latter, the stem and leaves. After the formation of the
octants the development proceeds in somewhat different ways in
the various classes. In addition to the stem, leaf, and root,
a “foot” is developed from the hypobasal half which remains
enclosed in the prothallium, and conveys nourishment from the
prothallium to the young plant until it is able to sustain
itself (Fig. 202). The formation of these members in the embryo
depends on the position of the oosphere in the archegonium and
prothallium, and is independent of gravity.
[Illustration: FIG. 202.--_Adiantum capillus veneris._ Vertical
section through a prothallium (_f f_), with a young plant
attached on its under side (mag. about 10 times); _r_ the first
root, and _b_ the first leaf of the young Fern-plant; _m_ the
foot. In the angle between _m_ and _b_ lies the apex of the
stem: _h_ the rhizoids of the prothallium; _æ æ_ unfertilised
archegonia.]
In the Mosses the asexual generation is the sporogonium, which is
limited in its development and in a great measure dependent upon the
sexual generation, upon which it is situated; but in the Pteridophyta
this generation is an independent and highly developed plant,
provided with stem, leaf, and true roots, and has in many instances
an unlimited development. The Pteridophyta are the lowest Division
with _true roots_. The root which is first formed is very similar in
nature to the primary root of the Monocotyledons; it very soon dies
and is replaced by others which are more permanent, and developed
upon the stem (adventitious roots); roots are wanting in _Salvinia_,
_Psilotum_, and some Hymenophyllaceæ. The differentiation is, however,
not so complete as in the Flowering-plants, and so many leafy forms are
not found. The various members of these plants are anatomically much
higher than in the Mosses, having an epidermis, a ground tissue with
variously differentiated cells, and a highly developed vascular system.
The vascular bundles, like those in the Monocotyledons, are without
cambium, and closed; they are therefore incapable of any increase in
thickness. In general the bundles are concentric, with the bast round
the wood (Fig. 203). The wood is almost entirely made up of scalariform
tracheides.
In _Isoëtes_ a secondary thickening takes place by a cambium,
which is formed inside the cortex, constructing secondary
cortex to the exterior, and secondary wood towards the
interior.--_Botrychium_ has also a thickening growth. Collateral
vascular bundles occur in _Osmundaceæ_, _Equisetaceæ_, and the
leaves of many _Polypodiaceæ_, etc.
[Illustration: FIG. 203.--Portion of the stem of a Fern. Above is
seen the transverse section, with vascular bundles of different
form and size. The rhombic figures on the side of the stem are
leaf-scars.]
It is a point of special interest, that the gigantic forms of Ferns,
Equisetums, and Club-Mosses (which flourished in earlier geological
periods, when these classes attained their highest development)
possessed some means of increasing in thickness.
The _sporangia_ are in all cases _capsule-like_, and burst open when
ripe to eject the spores. They are nearly always situated on the leaves
(in _Lycopodiaceæ_, in the axils of the leaves, or above these, on the
stems themselves). In some forms (LEPTOSPORANGIATÆ), the sporangia are
developed from a single epidermal cell; in others (EUSPORANGIATÆ),
from a group of epidermal cells, or from cells which lie beneath the
epidermis. In the first group a primitive mother-cell (archesporium)
is formed, which divides commonly into sixteen special mother-cells.
In the latter group, on the other hand, a number of primitive
spore-mother-cells are developed. In each sporangium three different
tissues are generally developed; an innermost _sporogenous_ one (_s_ in
Fig. 204 _A_), which arises from the archesporangium; an outermost one,
which forms the _wall_ (_a_), and may be one or, more rarely, several
layers in thickness; and an intermediate one, the _tapetum_ (Fig. 204
_A_, _B_, _b t_), which is rich in protoplasm, and whose cells are
dissolved so that the spores float freely in the fluid thus provided.
The spores arise as in the Mosses (in tetrads), by the cross-division
of the special mother-cells, and according to the manner in which they
are arranged in the mother-cell have either a tetrahedral form, with
a large base resembling a segment of a ball, or are oblong (bilateral
spores). Their construction is the same as in the Mosses (p. 187).
[Illustration: FIG. 204.--_Selaginella inæqualifolia. A_ A
young sporangium, which may develope either into a macro-, or a
microsporangium. _B_ A microsporangium.]
The spore-formation in its earliest commencement takes place in the
same way in the Isosporous and the Heterosporous Vascular Cryptogams;
but from a certain point, after the tetrahedral division, a difference
occurs with regard to the macrosporangia. All the spores formed in the
microsporangium may complete their development; but those which are
formed in the macrosporangium are generally aborted, with the exception
of one or four, and these consequently attain a much larger size (see
Fig. 239.--The series to the left are microsporangia; those to the
right, macrosporangia).
APOGAMY. In some Ferns (_Pteris cretica_; _Aspidium filix mas_,
var. _cristatum_; _A. falcatum_; _Todea africana_) the young
plant is not developed as a consequence of fertilisation, but as
a bud from the prothallium. This is known as apogamy, or loss of
the power of sexual reproduction. The antheridia are generally
more or less developed; archegonia are entirely wanting in _Asp.
filix mas_, var. _cristatum_. This variety has probably only
become apogamous through cultivation. Many specimens of _Isoëtes
lacustris_, in a lake in the Vosges mountains, produce in the
place where the sporangia are usually found, a vegetative shoot
which grows into a new plant, so that the sexual generation is
wanting in this case. Some specimens have sporangia on some
leaves, and shoots on others.
Apospory, or the formation of prothallia instead of sporangia
and spores on the leaves, is found in _Athyrium filix
femina_, var. _clarissimum_. In this case the development
of the sporangia proceeds only to a certain point, and from
these arrested sporangia the prothallia are produced. Normal
sporangia are entirely wanting in this variety, and in _Aspidium
angulare_, var. _pulcherrimum_, sporangia are completely
wanting. Compare the Mosses (page 188).
The Vascular Cryptogams are divided into _three large classes_,
in each of which a progressive development can be traced from the
isosporous to the heterosporous forms, but some of these are now only
known as fossils.
Class 1. =Filicinæ= (=Ferns=).--The stem is small in comparison with
the leaves, and branches only seldom, and then by lateral shoots. The
leaves are scattered, large, often deeply divided, and of various
highly developed forms. The undeveloped leaves are rolled up in the
bud, having what is termed circinate venation. The sporangia are
situated on the edge or on the lower side of the leaves, those on
which the sporangia are borne (_sporophylls_) being often the ordinary
foliage-leaves; but in a few cases the fertile differ from the barren
ones (a higher stage in development). The fertile leaves are not
confined to definite parts of the shoot, and do not limit its growth.
The archesporium is most frequently unicellular.
_A_. =Isosporous=: Sub-Class 1. Filices (True Ferns).
_B_. =Heterosporous=: Sub-Class 2. Hydropterideæ (Water Ferns).
Class 2. =Equisetinæ= (=Horsetails=), in its widest meaning.--The
leaves in this class are small in comparison with the stem. They are
arranged in whorls, and unite to form a sheath. The sporangia are
situated on specially modified, shield-like leaves, which are closely
packed together and form a “cone.” The cone is borne terminally, and
limits the growth of the shoot. The sporangia are developed from a
large group of epidermal cells, the archesporium being unicellular. The
branches are arranged in whorls, and develope acropetally.
_A_. =Isosporous=: Sub-Class 1. Equisetaceæ. Existing forms.
_B_. =Heterosporous=: Sub-Class 2. Extinct forms.
Class 3. =Lycopodinæ= (=Club-Mosses=).--Roots generally branching
dichotomously. The leaves are scattered or opposite, and in proportion
to the stem very small, undivided, and simple. They are scale-like and
triangular, tapering from a broad base to a point. The sporangia are
situated singly (except in _Psilotaceæ_), and almost in every case on
the upper side of the leaf or in the axil of a leaf; but in some cases
they are borne on the stem, just above the leaf-axil. The sporangia
arise from groups of epidermal cells. The sporophylls are often
modified, and differ from the foliage-leaves; they are then arranged in
cones placed terminally on branches, thus limiting their growth.
_A_. =Isosporous=: Sub-Class 1. Lycopodieæ.
_B._ =Heterosporous=: Sub-Class 2. Selaginelleæ.
Class 1. =Filicinæ= (=Ferns=).
The characteristics of this class have already been given on page 204.