The two orders which come under this head are united by the
characteristics, that the verticillate leaves are not united
into sheaths (Fig. 226), and that between each whorl of fertile
leaves there is also a whorl of barren ones. The fertile whorls
in ANNULARIÆ are situated about midway between the barren ones
(Fig. 227), but in ASTEROPHYLLITEÆ they occur immediately above
a barren whorl (Fig. 228) and contain only half as many members
as the latter. The lower whorls bear macrosporangia with one
macrospore, the upper, microsporangia with many microspores.
[Illustration: FIG. 226.--A. fragment of _Annularia_.]
[Illustration: FIG. 227.--Fragment of _Annularia longifolia_,
with sporangia; the leaves have partly fallen off: a barren
whorls; _s_ fertile whorls.]
[Illustration: FIG. 228.--Fragment of cone of _Asterophyllites_
(_Volkmannia elongata_): _a_ and _s_ as in Fig. 227.]
The ANNULARIÆ were distichous (Fig. 226), and presumably
floating plants. The ASTEROPHYLLITEÆ had verticellate branches.
These also died out after the Carboniferous period, at the close
of the Palæozoic.
Class 3. =Lycopodinæ= (=Club-Mosses=).
The characteristics of this class have been given on page 205. It
consists of two sub-classes, one embracing isosporous, the other
heterosporous forms.
Sub-Class 1. =Lycopodieæ= (ISOSPOROUS Lycopodinæ).
One kind of spore. Prothallium large, partly green. Leaves without
ligule.
[Illustration: FIG. 229.--_Lycopodium annotinum_: _A_ embryo
(nat. size), with prothallium (_pr_), one embryo is broken off;
_B_ the prothallium (slightly magnified); _C_ section through
the prothallium and embryo in the direction _a-b_ of _A_, and
vertically in the plane of the paper.]
[Illustration: FIG. 230.--_Lycopodium clavatum_: portion of a
stem, bearing cones (_a_); _s_ a spore; _h_ sporangium in the
axil of a leaf, _s_.]
Order 1. =Lycopodiaceæ.= The PROTHALLIUM is only known in a few species
at present, but in these it is more or less tubercular, and bears both
antheridia and archegonia.
In _L. annotinum_ the prothallium is a relatively large mass
of cells, without chlorophyll, and subterranean, in which
the antheridia and archegonia are embedded (Fig. 229). In
the widely distributed tropical species, _L. cernuum_, and
in _L. inundatum_, it is a small tubercular body which has a
subterranean portion, with either little or no chlorophyll;
and an aerial green portion. The prothallia of _L. phlegmaria_
and others live saprophytically in the crevices of the bark of
trees; they are partly filamentous, branched, and possess no
chlorophyll.
The =asexual generation=. PERENNIAL PLANTS. The stem branches
monopodially (often apparently dichotomously), and is thickly covered
by small, simple, triangular or scale-like leaves. The leaves are
spirally arranged in some species (Figs. 229, 230), and in others,
whose stem is compressed with unequal sides, opposite (Fig. 231). The
roots of _Lycopodium_ are dichotomously branched.
The SPORANGIA in _Lycopodium_ are situated singly at the base of the
leaves, almost in their axils; they are reniform, unilocular and open
like a mussel-shell by two valves (Fig. 230 _h_). The sporangia are
developed from a group of surface cells. The archesporium is formed
from one hypodermal cell (or perhaps a cell-row).
[Illustration: FIG. 231.--_Lycopodium complanatum_: _a_ leaves on
the edges of the stem; _d_ leaves on the sides.]
[Illustration: FIG. 232.--_Lycopodium clavatum._ A tetrahedral
spore seen from above, where the three borders join; and a tetrad
of bilateral spores, still lying in the mother-cell.]
The fertile leaves are collected upon definite regions of the stem.
They are either similar to the barren ones, and then the fertile
portions of the stem pass gradually, without any break, into the barren
portion (_L. selago_); or they differ from the barren leaves, and are
then collected into special apical cones (Fig. 230 _a_). The SPORES are
tetrahedral or bilateral (Fig. 232).
About 100 species, chiefly tropical.
Five species of _Lycopodium_ are found in Great Britain. _L.
clavatum_ and _L. selago_ are common in mountainous districts.
_L. annotinum_ is common in the Highlands of Scotland. The other
genus of the order is _Phylloglossum_, with one species, _P.
drummondi_ (Australia, Tasmania, and New Zealand), a small plant
only a few centimetres high, with two tubers, and about eleven
linear leaves at the base of the stem which is terminated by a
cone of sporophylls.--FOSSIL Lycopodiaceæ in the Carboniferous
period.
OFFICINAL: “Lycopodium,” the spores of _L. clavatum_.
Family 2. =Psilotaceæ=. The sporangia are placed on the apex of
short, two-leaved stems, as 2–3, seldom four, small capsules.
Small herbs, with angular stems; leaves small, simple, and one
nerved. Only four species.--_Psilotum_ (Madagascar, Moluccas,
Sandwich Islands, etc.) is destitute of roots, their place being
supplied by special underground stems which bear a few modified
leaves, very much reduced, especially when buried deeply in the
soil. Three species.--_Tmesipteris_ (Australia), one species.
Sub-Class 2. =Selaginelleæ= (HETEROSPOROUS Lycopodinæ).
Micro-and macrospores. The prothallia are very much reduced, especially
the male; the female does not leave the spore. The leaves are ligulate.
[Illustration: FIG. 233.--Germination of the microspores of
_Selaginella_: _A_ the spore rendered transparent, seen from
above. In the interior is seen the prothallium (_f_), and the
first divisions of the antheridium (_a_, _b_, _c_, _d_); in _B_
the spore-wall is removed and all the spermatozoid-mother-cells
formed; in _C_, the microspore has opened and the spermatozoids
and the mother-cells are escaping together.]
=The sexual generation.= In the ~MICROSPORES~ are formed: (1) a very
small “vegetative” cell, representing the vegetative part of the
prothallium (_f_ in Fig. 233 _A, B_), and (2) a cell many times larger
and which divides into a number (4–8) of primordial cells, each of
which divides into four spermatozoid-mother-cells, though all of these
may not develope spermatozoids. On germination, when the spore-wall is
ruptured, the spermatozoids and spermatozoid-mother-cells are ejected
into the water.
The ~SPERMATOZOIDS~ in _Selaginella_ are elongated and club-shaped,
with two cilia (Fig. 234); but in _Isoëtes lacustris_ they are
spirally-twisted threads which differ from all other spermatozoids by
having a bunch of cilia _at each end_; the other species of _Isoëtes_
have cilia only at the anterior end.
The MACROSPORES. Shortly after the macrospores have been set free,
or in _Selaginella_, while still enclosed in the sporangium of the
mother-plant, they germinate and soon become filled with the cellular
tissue of the prothallium, and even in _Selaginella_ the archegonium
begins to be formed before the rupture of the spore-cell-wall has
commenced (Fig. 235 _A_).
[Illustration: FIG. 234.--Spermatozoids of _Selaginella_: _b_
with a remnant of cytoplasm.]
[Illustration: FIG. 235.--Macrospore of _Selaginella_: _A_
longitudinal section, before the rupture of the wall, six
weeks after being sown. The endosperm (_e_) has not yet filled
the entire chamber. Cell-formation is still proceeding in the
lower part of the spore. The endosperm and prothallium (_f f_)
are separated by a distinct line (diaphragm). _B_ Germinating
macrospore seen from outside: _s_ wall of the spore; _æ_
archegonia.]
[Illustration: FIG. 236.--Archegonia of _Selaginella_: _A_
unripe, in longitudinal section; _c_ the central cell; _k_
neck-canal-cell, which is wedged in between the two-storied
neck-cells; _B_ ripe; _u_ ventral canal-cell; _C_ seen from
above, open. It will be noticed that the neck is formed of two
tiers of four cells each.]
The ARCHEGONIA are constructed on the same plan as those of the other
Archegoniatæ, but are quite embedded in the prothallium (Figs. 235 _æ_,
236).
=The asexual generation= varies very much in the different orders.
[Illustration: FIG. 237.--_Isoëtes lacustris_ (slightly
diminished): _st_ the stem; _r_ roots; _b_ leaves.]
[Illustration: FIG. 238.--_Isoëtes lacustris_. Longitudinal
section through the base of the leaf with a microsporangium. The
edge of the groove, in which the microspangium is placed, is
continued as a thin covering which envelopes the sporangium. The
inferior edge of the ligular groove (_L_) forms a lip (_J_); _t_
sterile cell-rows (trabeculæ) which divide the sporangium into
compartments; _l_ vascular bundle.]
[Illustration: FIG. 239.--_Selaginella inæqualifolia_. Cone in
longitudinal section; microsporangia are seen on the left side,
macrosporangia on the right (most frequently each with four
macrospores).]
Order 1. =Isoëtaceæ (Quill-worts).= The only known genus, _Isoëtes_
(Quill-wort), has an extremely short, tuberous, _unbranched_ stem
with very short internodes (Fig. 237). The STEM is remarkable as
being the only one among the Vascular Cryptogams which increases in
thickness (see page 202). The meristematic cells are situated round
the axial cylinder, and form, especially, parenchymatous tissue in
two or three directions, giving rise to 2–3 grooves in which the
dichotomously-branched ROOTS are produced. The LEAVES are arranged
spirally in a close rosette. They are awl-shaped and have at the
base a semi-amplexicaul sheath, with a groove (_fovea_), in which a
sporangium is situated (Fig. 238). The ligule is a foliar outgrowth
from the upper edge of the groove.--The MACROSPORANGIA (each with
a number of macrospores), are situated on the outer leaves, the
MICROSPORANGIA (Fig. 238), on the inner ones. Between each cycle of
fertile leaves there are a number of imperfect or barren ones as in
the case of the female plant of _Cycas_. The spores are liberated by
the decay of the sporangium. The two kinds of sporangia develope at
the commencement in the same way. The archesporium is, at first, a
hypodermal layer of cells which grow out in the direction perpendicular
to the surface of the leaf, and divide by a number of walls parallel
to this direction, forming a sporogenous mass of cells. Some of the
cell-rows of this sporogenous mass lose their rich protoplasmic
contents, and are arrested in their growth; thus incomplete divisional
walls of sterile cells, “_trabeculæ_” arise in the sporangium,
dividing it into a number of compartments one above the other (Fig.
238 _t_). (The trabeculæ, according to Goebel, play the same part as
the nutritive cells of the sporangium of _Riella_; the tapetal cells,
as in the Ferns, are in a great measure dissolved at a later period.)
The sporogenous cell-rows, in the microsporangia, give rise to a
large number of spore-mother-cells, but in the macrosporangia only
one spore-mother-cell, with tapetum, is developed from each fertile
archesporial cell.
The two native species, and several others, are aquatic plants, the
remaining species are land plants, or are amphibious. About 50 species.
In temperate and tropical regions.--FOSSIL species in the Tertiary
period.
Order 2. =Selaginellaceæ.= This order contains only one genus,
_Selaginella_. The STEM, in the majority of species, is dorsiventral,
long and slender, and apparently branches dichotomously, but in reality
_monopodially_, with well developed lateral shoots. The LEAVES are
small, round, or ovate, in the majority of species arranged in whorls
of two leaves each; these whorls, however, are not decussate, but are
considerably inclined towards each other, an arrangement by which four
rows of leaves are produced, each whorl having one large and one small
leaf. The two leaves in each whorl are of unequal size, the smaller one
being placed on the upper surface and the larger on the lower surface
of the stem (Fig. 240). Some species have spirally-arranged leaves,
more resembling the arrangement in the _Lycopodiums_.
The ~FERTILE LEAVES~ most frequently differ from the barren ones,
and are collected into spike-like cones (a kind of flower; Fig.
239). Micro-and macrosporangia are found in the same cone (Fig.
239). Each sporangium arises from a group of superficial cells of
the stem, directly over the leaf on which it will be situated later
on. Each sporangium has a hypodermal, unicellular archesporium, and
contains a layer of tapetal cells; these are dissolved later, when
the spores are ripe, and not before as in the Ferns. In the very
early stages of their development, the micro-and macrosporangia are
precisely similar, and the differences between them arise later on.
In the microsporangium all the spore-mother-cells divide, and each
forms four tetrahedrically-arranged microspores (Fig. 204); but in
the macrosporangium only four macrospores are formed, by the division
of a _single mother-cell_, while the remaining spore-mother-cells
are aborted. It is rarely that the macrosporangia contain 2 or 8
macrospores.
[Illustration: FIG. 240.--_Selaginella martensii_: _s_ lower
leaves; _r_ upper leaves.]
For the ~GERMINATION OF THE SPORES~, see pages 228, 229. The
prothallium arises in the macrospore (_f-f_, in Fig. 235 _A_),
probably by division of the meniscus-shaped protoplasmic mass,
which is marked off at the apex of the spore; primordial cells
are thus formed which later on are surrounded by a cell-wall. In
six to seven weeks after sowing, the spore-wall is ruptured by
the growing prothallium, which already has developed archegonia
(Fig. 235 _œ-œ_). The prothallium so formed does not occupy
the entire cavity of the spore, but four to five weeks after
sowing, the large-celled parenchyma is developed in the lower
portion of the spore by free cell-formation; this has been
termed by Pfeffer, “endosperm,” since it is similar to the
endosperm of Flowering-plants. Goebel, however, has termed it
“secondary prothallium,” as the homology with the endosperm of
the Angiosperms is very doubtful.
The ~FERTILISED OOSPHERE~ divides into an upper (hypobasal) and a lower
(epibasal) cell; from the latter alone the embryo is developed with
its root, stem, foot, and two _cotyledons_, and the former gives rise
to an organ which appears in this instance for the first time, but
which occurs in all Flowering-plants, viz. the _suspensor_. This forces
the embryo down into the “endosperm,” which is entirely or partially
absorbed by the embryo. In the case of the Flowering-plants the embryo
is developed with its longitudinal axis in the elongation of the
suspensor, but in _Selaginella_ the embryo is situated _transversely_
to it.
_Selaginella_ (300–400 species), is essentially tropical, only one
species living in the North (_S. spinulosa_), but others grow in
Central and South Europe.
Order 3. =Lepidodendraceæ= are extinct, tree-like Lycopods,
which are found especially in the Lower and Middle
Carboniferous. Vegetatively they are most nearly related to
_Lycopodium_, but the stem attained much larger proportions
(about eleven metres in height and one metre in thickness),
and had a cambium by which it increased in thickness. It was
regularly dichotomous, and closely studded with spirally-placed
leaves, which left behind them peculiar rhombic scars. The large
cones resemble Pine-cones, and bore sporangia much larger than
any which are now produced (the male ones as much as 2 cm.’s in
length). The macrosporangia were situated at the base, and the
microsporangia at the apex.
Order 4. =Sigillariaceæ.= These are, presumably, another
group of extinct tree-like Lycopods (especially in the Middle
Carboniferous). The name has been derived from the seal-like
scars, which the fallen leaves have left behind in longitudinal
rows on the grooved stem. The rhizomes of these plants were
formerly termed _Stigmaria_, and placed in a separate genus.
Order 5. =Sphenophyllaceæ= form an entirely extinct group.
They do not definitely belong to any of the three large
classes of Vascular Cryptogams, but it is perhaps best to
place them in juxtaposition to these. They were herbaceous
plants with verticillate, wedge-shaped leaves, with nerves
branching dichotomously into equally strong branches. Micro-and
macrosporangia were formed in the same cone; and were situated
in the axils of the leaves, as in the Lycopods.
The Transition from the Cryptogams to the Phanerogams.
All the plants considered in the preceding chapters are included
in the term CRYPTOGAMS; all in the following chapters under the
head of PHANEROGAMS (see page 3). Hofmeister’s pioneer works
(1851, _Vergleichende Untersuchungen der höheren Kryptogamen_,
etc.) and the numerous researches published later by other
investigators, have closed the gap which was formerly thought
to exist between these plants; so that we now, in the series:
Bryophyta--Pteridophyta--Gymnospermæ--Angiospermæ see the expression of
a single line of development in accordance with a definite plan. The
forms through which this gradual development has taken place have in
course of time, however, to a great extent died out, and only single
links of the chain connecting the lowest to the highest still remain.
THE ALTERNATION OF GENERATIONS, which we found indicated in certain
Thallophytes, can be proved with the greatest clearness in all the
higher Cryptogams, from the Mosses upwards; it is also found in the
Phanerogams, but not in such a pronounced degree, because one of
the generations is so far reduced that it has almost given up its
independence. For the sake of greater clearness, we will begin with the
comparison of the sporophyte, asexual (second) generation.
=The asexual (2nd) generation of the Cormophytes.=
The asexual generation which follows from the further development of
the fertilised oosphere, is, in the _Mosses_, only the sporogonium
(according to one theory it is perhaps homologous with a spore-bearing
leaf, situated upon a short stem, see p. 187); in _Filicinæ_,
_Equisetinæ_, and _Lycopodinæ_, on the other hand, it is a highly
developed plant differentiated into stem, leaf, and true root, and
bearing the sporangia on its leaves. The ~MODIFICATION OF THE SHOOT~
is very slight in _Filicinæ_. The first leaves of the embryo are very
simple in form (Fig. 205), but after a certain age all the leaves
which arise are essentially alike. The fertile leaves do not differ
from the barren ones, and are found associated with them, and their
formation does not limit the growth in length of the stem. It is only
in a few of the true Ferns, and in the Hydropterideæ, that the fertile
leaves differ considerably from the barren ones. A division of labour
in which certain leaves are set apart for nutrition, and others for
reproduction, is found more pronouncedly in the _Equisetinæ_ and
_Lycopodinæ_, for in these groups, with a few exceptions, the fertile
and barren leaves are very dissimilar; the former are collected in
special ear-like _cones_, which _terminate the further growth_ of
the short stems on which they are borne. In connection with the cone,
leaves are sometimes developed which form a transition from the barren
to the fertile ones (the “annulus” in Equisetaceæ), and in these cases
the first indication of a flower with perianth or floral-leaves is to
be traced. Among the Cryptogams the highest division of labour is found
in _Selaginella_ and _Isoëtes_, which have the two kinds of sporangia
borne on _different_ leaves. The division of labour (modification) is,
however, still more pronounced in the _Phanerogams_: the leaves which
bear the microsporangia (“pollen-sacs”) have quite different forms
from those which bear macrosporangia (the “nucellus” in the ovule),
the former are termed _stamens_, the latter _carpels_; in certain
instances, too, there is even a contrast between the “male plants” and
the “female plants.” Moreover, a division of labour, in a much greater
degree, takes place in the leaves which do not directly take part in
reproduction, and it is thus possible in many plants to draw a sharp
line not only between stamens and carpels, but also between four or
five distinct kinds of leaves, which differ in _form_, _structure_, and
corresponding _functions_, and which appear in regular sequence on the
shoot: namely, between “scale-leaves” and “foliage-leaves,”[21] both
of which occur in the Cryptogams, and the “floral-leaves,” including
the bracts and leaves of the “perianth,” which latter often differ from
each other in form and colour, and are then separated into _sepals_ and
_petals_. The _leaves_--stamens and carpels--_which bear the sporangia_
are termed sporophylls, and the shoot, or extremity of a shoot, whose
leaves are modified into sporophylls, is _terminated in its further
growth by their production, and is known as a flower_. The flower
which is most completely furnished has calyx, corolla, stamens, and
carpels arranged in this order. If the only sporophylls present are
stamens, then it is said to be a _male_ (_staminate_) flower, and if
only carpels, then a _female_ (_pistillate_) flower, and in both these
cases the flowers are _unisexual_, or diclinous. If stamens and carpels
are both present in the same flower, it is termed _hermaphrodite_.
Diclinous plants in which the female flowers are situated on one plant,
and the male flowers on another, are termed _diœcious_; and those
in which the same plant bears the two kinds of flowers are termed
_monœcious_. When the male, female, and hermaphrodite flowers are found
in the same species, the plant is said to be _polygamous_.
=The sporangia-bearing leaves--Sporophylls.= In the Mosses the
asexual generation is only represented by the sporogonium, and if the
theory is correct which considers the sporogonium to be an embryo
consisting of a rudimentary stem and terminal leaf, then the spores are
produced on the leaves in these plants. The sporangia in the Filicinæ
are situated in groups (sori) on the back or on the edge of the leaves.
The number of sporangia in the sorus diminishes very greatly in the
Marattiaceæ and Gleicheniaceæ (three to four in the latter, Fig. 213).
In the Equisetinæ the sporangia are situated in a small number on the
underside of shield-like leaves, and in Lycopodinæ, singly, in the
axils of the fertile leaves, which are alike and bear either micro- or
macrosporangia. In the Phanerogams there is a great difference between
the stamens and carpels.
[Illustration: FIG. 241.--_Cycas_: _a_ stamen (nat. size) seen
from the under side; _b_ four pollen-sacs, not yet open, forming
a “sorus”; _c_ three open pollen-sacs; _d_ a pollen-grain.]
[Illustration: FIG. 242.--Stamens of _Araucaria_ (pollen-sacs
long and pendulous).]
[Illustration: FIG. 243.--Male flower of _Taxus_.]
[Illustration: FIG. 244.--_A_ Cross section through a
quadrilocular anther in different stages of development: _s_ the
seam where it bursts open; _vf_ vascular bundle; _k_ connective.
_B_ A stamen. _C_ Another stamen seen from the front (_f_) and
from the back (_b_).]
=Stamens.= In the lowest Phanerogams (_Cycadeæ_) there are many
indications of relationship to the Ferns. The stamens are flat and
broad, and have _on the back many pollen-sacs_ (_microsporangia_)
arranged in small groups (true _sori_), which even have a small
“placenta,” similar to the one possessed by the Ferns, and open towards
the inside by a longitudinal cleft (Fig. 241, compare Fig. 213). A
section of the _Coniferæ_ agree more closely with the Equisetaceæ, in
having a few (three to eight) pollen-sacs arranged on the underside of
more or less shield-like leaves (Figs. 242, 243, compare with Fig. 224
_a_, _c_, _d_). In the Abietaceæ the number of sporangia is diminished
to two, which are placed also on the lower side (Fig. 267) of a stamen.
The number of _pollen-sacs_ (microsporangia) in the _Angiosperms_ is
nearly always four to each stamen; they are longitudinal projections
which are placed in pairs on each side of the central line of the
stamen, two on the edge, and the other two generally on the side which
is turned inwards; the pollen-sacs generally dehisce longitudinally
(quadrilocular anthers, Fig. 244). A few, for instance Orchidaceæ and
Asclepiadaceæ, have only two pollen-sacs (bilocular anthers); and in
others, such as _Solanum_ and the Ericaceæ, they open by pores; in
Lauraceæ and Berberidaceæ, by valves. The part of the stamen which
bears the pollen-sacs is termed the _anther_. Most frequently this is
supported by a stalk known as the _filament_.
[Illustration: FIG. 245.--A carpel of _Cycas revoluta_ with 5
ovules (_s_), at half to one-third nat. size.]
[Illustration: FIG. 246.--Carpel with 2 ovules of _Ceratozamia
robusta_ (1/1).]
=Carpels.= The simplest forms of carpels are found in _Cycas_. In
this genus both the foliage and fertile leaves are pinnate, and hence
present great similarity; the ovules (macrosporangia) are situated on
the margin of the central portion, just as the sporangia are placed
on the edge of the fertile leaf of _Ophioglossum_ (Fig. 245, compare
with Fig. 209). The carpels of the other Cycadeæ present greater
divergence from the foliage-leaves, being peltate, for instance, in
_Zamia_ and _Ceratozamia_ (Fig. 246). The ovules in the Coniferæ are
situated on the upper side and near the base of the ovuliferous scales,
almost in the same position as the sporangia in the Lycopodinæ (Figs.
269, 272, 273 _H_, compare Figs. 230, 239). In _Taxus_ the uninclosed
ovule is placed on the apex of a shoot (Fig. 264). In all these plants
the ovules are _not enclosed_ by the carpels, that is, they are not
enclosed in chambers formed by the turning in of the walls of the
carpel, and hence the name _Gymnospermæ_ is given to them. In the
higher Flowering-plants, the _Angiospermæ_, the ovules are distinctly
situated on the edge, the upper surface, or base of the carpel; but
the carpel closes round the ovules which are therefore enclosed in a
chamber--the _ovary_. In a few cases, for example in the Polygonaceæ,
an ovule is situated apparently on the apex of the stem itself, as
in the Yew; in other cases, as in the Primulaceæ, many ovules are
apparently developed on the apex of the stem, which seems to have been
specially adapted as a placenta, but it is also possible and correct
in these cases to suppose that the ovules are in reality developed
on the carpels.[22] A single fully-developed carpel or a collection
of carpels joined together is termed the _pistil_. The extremity of
the carpel, which is specially developed to catch the pollen-grains
and form a suitable nidus on which they may germinate, is called the
_stigma_. The united edges of a carpel which bear the ovules are termed
the _ventral suture_. The back of the carpel forms the _dorsal suture_.
The Marsiliaceæ take a position among the Hydropterideæ analogous to
that occupied by the Angiosperms; the sporangia are in a corresponding
manner enveloped in a closed leaf.
The collection of stamens in a flower is termed the _andrœcium_,
and all the carpels, whether individually free or united into one
pistil, the _gynœceum_.
The =Sporangia=. The asexual generation of the _Mosses_ is the
sporogonium, in which the spores arise in tetrads from the
mother-cells. The sporangia in the _Filicinæ_ take their origin either
from a single cell (Leptosporangiatæ) or, what probably may be regarded
as an older stand-point, from a group of cells (Eusporangiatæ). In both
cases there may be distinguished in a mature sporangium three tissues,
which have different significance (Fig. 204): (1) an external layer,
the _sporangium-wall_, most frequently composed of one layer of cells
made up of cells of dissimilar structure, so that on desiccation the
wall is ruptured and the sporangium opens in a definite manner; (2)
an internal group of cells, consisting of the _spore-mother-cells_,
developed from an archesporium, and which by division into four gives
rise to the _spores_; (3) a layer of cells lying between the two
already mentioned, which is dissolved before maturity. The intermediate
cellular layer, which directly surrounds the spore-forming cells, is in
form and contents more worthy of note than the others, and is termed
the _tapetum_. The construction of the sporangium in the _Equisetinæ_
and _Lycopodinæ_ is in the main the same.
[Illustration: FIG. 247.--Development of an anther. _A_
Transverse section of a young anther of _Doronicum macrophyllum_.
The formation of the 4 pollen-sacs commences by divisions of
the hypodermal cells (at _m_, for instance). These cells divide
by periclinal walls into external cells which only take part in
forming the anther-wall; and internal cells, which correspond to
the Archesporium, and from which the spores are derived. These
spore-forming cells are drawn with thicker walls in _B-E_. The
commencement of the vascular bundle is seen in the centre. _B_ An
older stage; the pollen-sacs already project considerably. It is
the cells in the hypodermal layer which are active and in which
tangential divisions particularly occur; _fv_ vascular bundle.
_C_ A corresponding longitudinal section. _D_ Transverse section
through an older anther, the thickness of the wall outside the
mother-cells of the pollen-grains is already increased, and it
becomes still thicker by the division of the hypodermal cells:
its most external layer of cells but one, becomes transformed
into the “fibrous cells.” _E_ Transverse section of a still
older pollen-sac of _Menyanthes_; _sm_ are the mother-cells of
the pollen-grains surrounded by the tapetum (_t_), external to
the tapetum is the anther-wall, which is still far from being
fully developed. The sub-epidermal layer becomes “fibrous,” and
the cells lying inside it become dissolved, together with the
tapetum.]
In the PHANEROGAMS the =Microsporangia= are termed =Pollen-sacs=.
They take their origin from a large group of cells, which, in the
Angiosperms, lie immediately beneath the epidermal cells of the anther.
In the developed, but not yet mature, sporangium (pollen-sac) there are
to be found: (as in the Vascular Cryptogams) (1) an internal group of
mother-cells which give rise to the _pollen-grains_ (_microspores_),
in this case also formed in tetrads; (2) a group of cells surrounding
these, of which the internal ones form a _tapetal layer_, similar to
that in the Vascular Cryptogams; the tapetum and some of the cells
surrounding it in this group, become dissolved before maturity; the
more external ones, on the other hand, are provided with peculiar
thickenings, and form the “fibrous” layer by the aid of which the
dehiscence of the anther takes place; (3) an external layer, the
epidermis, enclosing all the other layers (Fig. 247).
In some Coniferæ (_Cupressus_, _Thuja_, and several species
of _Juniperus_) the microsporangia (pollen-sacs), which are
situated on the under side of the stamen, are covered by a
thin structure which seems to be a continuation of the lamina
and which is supposed to be homologous with the indusium of the
Ferns.
[Illustration: FIG. 248.--Development of the ovule in the Red
Currant, _Ribes rubrum_, arranged alphabetically in the order
of development. _A_ Is the youngest stage, _E_ the oldest. _ii_
Inner integument; _ie_ outer integument; _nc_ nucellus; _m_
archespore (mother-cell of the embryo-sac).]
=The Ovule= in the Phanerogams arises most frequently on a projecting
portion of the carpel, termed the _placenta_. The ovules (compare the
sporangium of the Eusporangiatæ and especially the pollen-sac) take
their origin from a _group of cells which lies beneath the epidermis_
(Fig. 248 _A_, _B_). First of all a small papilla is formed, which is
later on provided with a _vascular bundle_ and becomes the _funicle_;
this probably has the same value as the projections (“placenta”) on
which the sori in the Ferns are attached. Only _one_ =macrosporangium=
(_nucellus_; Fig. 248 _nc_) is developed at the apex of the funicle.
This arises by a process of cell-division exactly corresponding to
that by which the pollen-sacs are formed (Fig. 248 _C-E_), with this
difference only, that while a great _many_ cells may be distinguished
in each pollen-sac, which forms pollen-grains by tetrad-division, only
a few are found in the ovule, and all these moreover are _suppressed,
with one single exception_ which developes into the =macrospore=
(=embryo-sac=) without undergoing a division into tetrads. The wall of
the embryo-sac, in the Gymnosperms, may be thick and divided into two
layers and partly cuticularized, as in the spores of the Cryptogams
which are to be set free. In the Angiosperms, on the other hand, the
wall is extremely thin.
The pollen-sac thus stands in the same relation to the nucellus as the
microsporangium does to the macrosporangium: in the pollen-sacs and
microsporangia a _number_ of spores arise by the tetrad-division of
several mother-cells; in the nucellus and macrosporangium, a reduction
of the cells already formed takes place to such an extent that the
number of macrospores becomes one (_Salvinia_, _Marsilia_, Phanerogams)
or four (_Selaginella_), or rarely a large number as in _Isoëtes_.
In the Ferns, as stated on page 210, etc., _indusia_ covering the
sori very often occur. Horsetails and Club-Mosses have no indusium;
but in all Phanerogams cupular or sac-like structures (_integuments_)
are found which envelop the nucellus. These develope from the upper
end of the funicle (_ii_ and _ie_, in Fig. 248; _y_ and _i_, in Fig.
249) and enclose the nucellus on all sides as a sac, leaving only a
small channel at the apex of the nucellus--the _micropyle_--(Fig. 249)
through which the pollen-tube proceeds to the embryo-sac. The ovules
of the Gymnosperms have only one integument (Figs. 251, 264, 269, 274)
and the same is the case with the majority of the Sympetalæ and a few
Choripetalæ; but the Monocotyledons and most of the Choripetalæ have
two integuments (Fig. 249).
[Illustration: FIG. 249.--Various forms of ovules: _A_ an erect
ovule (_orthotropous_); _B_ reversed (_anatropous_); _C_ curved
(_campylotropous_): _k_ the nucellus (shaded in all the figures);
_s_ the embryo-sac; _ch_ the base of the ovule (chalaza); _y_
and _i_ the external and internal integuments, the dotted line
denotes the place where the scar (_hilum_) will form when the
seed is detached from the funicle.]
In shape the integuments resemble very closely the cupular indusium of
the Hymenophyllaceæ, certain Cyatheaceæ (Fig. 212 _E_), and _Salvinia_
(Fig. 218); that they are really homologous with these is probable, but
is not proven. Some authorities regard them as structures found only in
the Phanerogams.
The ovule is thus a “_monangic_” (_i.e._ reduced to 1 sporangium, the
_nucellus_) _sorus_, situated on a funicle, and enclosed by one or two
cupular _indusia_, the integuments. Some of the ovules are _erect_
(_orthotropous_), others _curved_ (_campylotropous_), the majority
_reversed_ (_anatropous_) (Fig. 249).
[Goebel (1884 and earlier) with Strasburger considered the
entire ovule of the Phanerogams as homologous with the
macrosporangium, the integuments however as new structures in
contradistinction to the Ferns: the funicle then corresponds
to the stalk of the sporangium. The integuments of the ovule
(according to Goebel, 1882) differ from the indusium of the
Fern-like plants in being developed from the basal portion of
the nucellus and are not, as in the Ferns and _Isoëtes_,
a portion (outgrowth) of the leaf which bears the sporangia
(_K_).]
The nucellus is the only macrosporangium which never opens; _the
macrospore remains enclosed in it_, and _the macrosporangium remains
attached to the mother-plant_. It is therefore essential that the
_method of fertilisation_ which is employed should be very different
from that of the Cryptogams. _The pollen-grains must be transferred
to the ovule_, and retained either by a drop of mucilage at the
micropyle (Gymnosperms) or by the stigma on the carpels (Angiosperms).
Fertilisation by spermatozoids, which are freely motile in water, is
abandoned in the Phanerogams.
Many other modifications, unknown in plants of more simple structure,
take place, for instance, in the shoots which bear the fertile
leaves; especially in the form of the stem or _thalamus_ (hypogynous,
perigynous, epigynous); in the development of the perianth which stands
in intimate connection with the special means employed to effect
fertilisation; with respect to the different grades of union found
in the leaves; in the union of the flowers into aggregations of a
higher order (inflorescences), and at the same time the production of
“floral-leaves” (page 235).
=The sexual generation. The Fertilisation.=
The sexual generation in the _Mosses_ is relatively well developed,
because not only the protonema, but all the other vegetative parts
of the Moss-plant, in addition to the archegonia and antheridia,
belong to it. In the groups which follow, a gradual but increasing
reduction of the sexual generation takes place, and at the same
time an indication of sex is found in the prothallia, which finds
expression in the forms of the spores themselves. In the majority
of cases among the _isosporous_ Vascular Cryptogams, the sexual
generation--prothallium--is a green, leafy expansion which can sustain
itself by the assimilation of carbonic acid, and by the absorption
of nutriment from the soil by means of root-hairs. In some plants
(_Ophioglossaceæ_, _Lycopodium annotinum_) the prothallium is a
subterranean, pale, tubercular body, but in these instances it is
relatively large. In the _heterosporous_ Vascular Cryptogams and
in the _Phanerogams_, the prothallium is much more reduced, both as
regards its size, and also with respect to the number and structure of
the antheridia and archegonia.
1. =The Microspores.= The PROTHALLIUM in all Vascular Cryptogams which
have unequal spores, consists of a single, vegetative (barren) cell,
which plays a very unimportant part in the life of the prothallium
(Fig. 233 _A_). In _Salvinia_ it is somewhat elongated and tubular,
because it must break through the sporangium (Fig. 214); but in other
cases it is very small and lenticular. In all these plants only one
antheridium is formed. In _Salvinia_ it consists of 2 cells whose walls
are ruptured in order that the spermatozoids may be liberated (Fig. 214
_B_, _C_). In _Marsilia_, _Isoëtes_, and _Selaginella_ the prothallium
does not leave the spore, and consists for the most part of primordial
spermatozoid-mother-cells _without cell-wall_, which on germination are
ejected so that the spermatozoids are set free.
In the Phanerogams, the microspores have from olden times been termed
_pollen-grains_.
In the GYMNOSPERMS the prothallium is reduced to 1, 2 or 3 small cells,
placed on one side of the mature pollen-grain (at the top in Fig. 250
_I_, _II_, and in Fig. 267 _N_) and which do not play any part in
the germination of the pollen-grain. The antheridium is represented
by the remaining portions of the interior of the pollen-grain, that
is, it consists of a large cell with a nucleus which does not even go
so far as the antheridium of _Selaginella_ and become divided into
spermatozoid-mother-cells without cell-wall, for even these cells are
not formed. The unicellular antheridium grows, on the germination of
the pollen-grain, into a tubular body known as the _pollen-tube_,
formed from the inner wall of the pollen-grain (Fig. 250), which works
its way down the micropyle to the oosphere. The fertilisation takes
place by diosmosis through the cell-wall, and consists here also of the
coalescence of the nucleus of the pollen-tube (the sperm-nucleus, male
pronucleus) with that of the oosphere.
In the ANGIOSPERMS the reductions proceed still further. The barren
cell, which represents the prothallium, was in the last group separated
from the antheridium by a true cell-wall, but in the Angiosperms a
membrane at most, but no firm cell-wall, is formed. The pollen-grain
contains two cells, a vegetative and a free generative cell. Both these
pass into the pollen-tube, but the vegetative cell disappears about
the time the pollen-tube reaches the ovule; while the generative cell
divides into two: one, the sperm-nucleus coalescing with the nucleus of
the oosphere, the other being absorbed (_Lilium_, after Guinard).
The Gymnosperms prove in yet another point that they are more
closely related to the Cryptogams than are the Angiosperms. When the
pollen-grain begins to germinate the external wall ruptures as in the
Cryptogams (Fig. 250), but in the Angiosperms special germ-pores are
formed in the cell-wall for the emergence of the pollen-tube.
[Illustration: FIG. 250.--_I_ Pollen-grains of _Cupressus_; at
the top is seen one prothallium-cell. _II_ Germinating; _c_
pollen-tube; _a_ the extine; _b_ the intine.]
2. =The Macrospores.= The prothallium in _Salvinia_ and _Marsilia_ is
still rather large, green, and capable of the independent assimilation
of carbon. It projects more or less from the macrospore and bears
(in _Marsilia_ only one, in _Salvinia_ several) archegonia, which
however are embedded to a greater degree in the prothallium, and are
more reduced than the archegonia of the true Ferns and Horsetails
(Figs. 215, 216). The prothallium is still more reduced in _Isoëtes_
and _Selaginella_; _partly_ because it is smaller and is in a higher
degree enclosed in the spore, it also contains less chlorophyll, or
is entirely without chlorophyll, and in consequence incapable of
independent existence, whilst the number of archegonia is less; and
_partly_ because the archegonia are themselves reduced, the cells of
the neck are fewer and embedded to the level of the surface of the
prothallium without any, or with only a very slight projection (Figs.
235, 236).--Finally, the prothallium with its archegonia begins to
develope in _Selaginella_ while the macrospore is still within its
sporangium, and before it is set free from the mother-plant. After
the spores are set free and germination has commenced, the spore-wall
ruptures and the prothallium is exposed.
[Illustration: FIG. 251.--Longitudinal section of ovule of _Abies
canadensis_. Inside the integument (_i_) is seen the nucellus,
_n_; _m_ the micropyle. In the interior of the nucellus is
seen an oval mass of cells, the endosperm, and at its top two
archegonia, _c_. The ovule is turned in such a way that the
micropyle points upwards, but usually it turns downward in the
_Abietineæ_.]
[Illustration: FIG. 252.--The apex of the nucellus (_n_) of
an ovule of _Abies_: _l_ long-shaped cells which guide the
pollen-tube; _s_ the wall of the macrospore (embryo-sac); _h_ the
neck-cells of the archegonium; _k_ the ventral canal-cell; and
_c_ the central cell (oosphere). The archegonia of the Cryptogams
should be compared with this (see pages 181, 208, 216).]
The GYMNOSPERMS go still further. The macrospore (embryo-sac)
germinates and forms internally a cellular tissue, designated in former
times by the name of _albumen_ (endosperm), which is _homologous with
the prothallium_. It always _remains enclosed in the embryo-sac_, and
is a parenchymatous mass containing a large supply of nourishment. In
the upper part of the endosperm a number of archegonia are developed
which are in the main constructed in the same manner as those in the
Cryptogams, but are still more reduced, the neck consisting only of 4,
2, or 1 cell (Figs. 251, 252). The ventral canal-cell is also formed,
in the majority, as a small portion cut off from the large central
cell just beneath the neck; the larger remaining portion becomes the
oosphere. When the pollen-tube has passed down to the oosphere (Fig.
253) and fertilisation has been effected, the oospore commences a
cell-formation, the final result of which is the formation of _an
embryo_ (_the asexual generation_) which is provided with a thinner,
lower end, termed the suspensor. The embryo is forced more or less
into the endosperm in which it may rest for a longer or shorter
time, and generally is developed to such an extent that it has a
distinct primary-root (radicle) and stem (plumule) with one or more
embryo-leaves (cotyledons).
When the oosphere has been fertilised its nucleus sinks down
to its lower end, and by repeated division into two, forms
four cells lying in one plane (Fig. 253, see base of the left
archegonium). Three tiers of cells are now formed by transverse
division of these four. It is the intermediate one of these
which elongates and forms the suspensor, or four suspensors, if
they separate from each other, which push the lowermost four
cells deep down into the endosperm. It is from these four lower
cells that the embryo (or four embryos when the suspensors
separate) is developed, but never more than one embryo attains
full development. As several archegonia are contained in one and
the same ovule, all of which are capable of forming embryos,
there is the possibility that several embryos may be developed
in a seed (polyembryony), but usually only one embryo attains
perfect development.
[Illustration: FIG. 253.--The apex of the nucellus (_n_) of
_Abies_ in longitudinal section: _c_, _c_ the oospheres of
the two archegonia; the embryo-formation has commenced at the
bottom of the left archegonium; _s_ wall of the macrospore; _p_
pollen-grains; _r_ pollen-tubes.]
[Illustration: FIG. 254.--Embryo-sac of _Carex præcox_: _syn_
synergidæ; _kb_ the oosphere; _c_ the central nucleus; _ant_ the
antipodal cells.]
At the same time that the embryo is being developed, other changes
are taking place in the ovule, especially in the integument which
becomes the shell of the seed (_testa_). The endosperm grows, and the
embryo-sac supplants the cells of the nucellus. The _seed_ is now
formed, and it consists in its most complete development, as in this
instance, of three parts:
(1) The _testa of the seed_, formed by the enveloping integuments,
with the remainder of the tissue of the nucellus lying outside the
embryo-sac (the macrosporangium).
(2) The _endosperm_ or prothallium.
(3) The _embryo_.
[Illustration: FIG. 255.--Diagrammatic longitudinal section
through an anatropous ovule shortly after fertilisation; _a_
and _i_ are the two integuments; _f_ the funicle; _k_ the
nucellus; _S_ the embryo-sac, with the incipient formation of
nutritive-tissue; _E_ the embryo; _P_ the pollen-tube passing
through the micropyle (_n_) to the oosphere.]
The reduction in the ANGIOSPERMS is carried to the extreme limit. In
the embryo-sac (the macrospore) the nucleus by continued division
produces a prothallium consisting of primordial cells (Fig. 254).
In the upper end of the embryo-sac (which is nearest the micropyle)
are three cells, two of which are termed the “co-operating cells”
(_synergidæ_) and the third is the _oosphere_. Three others are
placed at the opposite end of the embryo-sac and are therefore termed
the “antipodal cells.” Finally, a large cell is also formed, which
occupies the space between the two groups and whose cell-nucleus, the
central definitive nucleus, lies in the centre of the embryo-sac.
These primordial cells are the slight remnant of the prothallium. The
entire structure of the archegonium, with its neck and canal-cells,
has disappeared, and nothing is left but the indispensable _oosphere_.
When the oosphere has been fertilised, and has commenced the cellular
divisions which lead to the formation of the embryo (Fig. 255), the
synergidæ and antipodal cells are absorbed, and a cell-formation
begins by a new process which emanates from the definitive nucleus and
by which a parenchymatous cell-tissue, the nutritive-tissue, arises
which may perhaps be considered as homologous with the endosperm of
the Gymnosperms. The difference is that the nutritive-tissue of the
Angiosperms is formed in two parts with an intervening interruption;
the primary nutritive-tissue is first formed, and after fertilisation
is absorbed, with the exception of one cell, which continues the
development and gives rise to the nutritive-tissue proper, which is
formed in the first instance of primordial cells, and later on of a
cellular tissue; this nutritive-tissue formed in the embryo-sac is
termed “endosperm”; in a few instances[23] a tissue which is derived
from the nucellus functions as nutritive-tissue, and is termed
“perisperm.” In many plants the seeds, when ripe, contain a very
rich nutritive-tissue, in addition to the embryo, for the purpose
of its nourishment during germination. These are termed albuminous
(endospermous) seeds, in distinction to the ex-albuminous, or those in
which the nutritive-tissue is stored in the embryo itself, before it is
completely developed, and used for its sustenance.
In addition to the changes which fertilisation produces in the ovule
itself, it also gives the impetus to a series of changes in the
entire shoot which bears the ovule. The perianth, stamens, and style,
generally wither, because the part they play is at an end; the wall
of the ovary grows and becomes the wall of the fruit (pericarp).
The entire gynœcium of a flower, transformed as a consequence of
fertilisation, is termed a _fruit_. It consists of two parts, the
_pericarp_ and the _seeds_, and according to the nature of the
pericarp, the fruit is termed a capsule, nut, berry, or drupe.
The chief characteristic of the Phanerogams does not lie in the
formation of the flower (although they may quite properly be termed
“Flowering-plants”), because Equisetums and Lycopods have reproductive
shoots as highly differentiated as those of certain Gymnosperms and
other Phanerogams. As regards the SEXUAL GENERATION the characteristics
are found:--(1) in its great reduction; (2) in the transmission of the
microspore (pollen-grain) to the macrosporangium, and its germination,
with the formation of a _pollen-tube_ (antheridium), the protoplasm of
which is not differentiated into spermatozoids; (3) in the fact that
the macrospore (embryo-sac) never leaves its sporangium (nucellus);
and further in the Angiosperms, (4) in the peculiar development of the
nutritive-tissue in two parts; and (5) in the great reduction of the
archegonium.
As regards the ASEXUAL GENERATION the characteristic feature is that
this generation is formed whilst the sporangium is still attached to
the mother-plant, and for a long time is nourished by it; and that
after the sporangium has become detached from the mother-plant, it
spends a longer or shorter resting period as the embryo in the seed
(enveloped by the testa), and does not make its appearance until
the “germination” of the seed. In addition the shoot which bears
sporangia undergoes greater modification than in the case of the
Flowerless-plants.