Chapter II: ACROGYNÆ. The apex of the stem or of certain branches is adapted (2)

EXTINCT ISOSPOROUS EQUISETINÆ. In addition to several true
species of fossilized _Equisetums_, the order of the CALAMITES,
which no doubt is closely allied to the Equisetinæ, is also
found in the fossil state. These were gigantic forms, attaining
about twenty times the size of those of the present day, and
stems of nearly 10–12 metres in height are known. They reached
the culminating point of their development in the Carboniferous
period, and died out towards the close of the Palæozoic. The
stems had hollow internodes and alternating grooves, similar
to their relatives of the present day. The leaves must either
have been absent or very perishable, since they have not been
identified with certainty. If the determinations of certain
remains of cones which of late have been discovered are correct,
they were heterosporous and had two kinds of sporangia as in
the following sub-class. A cambium formation and an increase in
thickness has been found in the stems.

Their USES are very limited. A few species, such as _E. hiemale_
are used for polishing on account of the hard siliceous
cell-walls of the epidermis, found in all species of _Equisetum_.

Sub-Class 2. =Heterosporous Equisetinæ.=

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.

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.

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).

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.

=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_).

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.

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.

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.

=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_.

=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.

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.

=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).

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.

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.

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.

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_.

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.

The Phanerogams are separated into two Divisions as follows:--

Division 4. =Gymnospermæ.= The ovules, as well as the seeds, are borne _naked_ on the surface of _open carpels_, or on the apex of a stem (ovary wanting). The pollen-grains are conveyed by the wind to the ovules, and caught by drops of mucilage, secreted by the micropyle. A “stigma” is _wanting_. The entire _female prothallium_ (_the endosperm_), which serves for the nourishment of the embryo, is _formed before fertilisation_. The archegonia are _embedded in the upper part of the prothallium. The pollen-grains are “multicellular,” i.e._ there is always in their interior a distinct prothallium, formed by 1–3 cells, and a larger cell which gives rise to the pollen-tube.

Division 5. =Angiospermæ.= The carpels surround the ovules and form an entirely closed chamber (_ovary_), in which the ovules mature and ripen into seeds. The surface of a portion of the apex of the carpel is transformed into the “stigma,” which, by a sticky fluid and also by hair-structures, is capable of retaining the pollen-grains conveyed to it by the wind, or more frequently by insects. The pollen-tube grows from the stigma, through the “conducting cellular tissue” (_style_), to the ovules. The pollen-grains contain two cells, a vegetative and a free generative cell. The latter passes into the pollen-tube and there divides into two, one of which is the sperm-nucleus. The female prothallium, which is intended to serve as nutritive-tissue, is formed _after fertilisation_. Archegonia are wanting.

DIVISION IV.

GYMNOSPERMÆ.

The following characters should be added to those already given on page 2:--

The Gymnosperms comprise only trees or shrubs. The flowers are always _unisexual_ and destitute of perianth (except _Gnetaceæ_); the female plant of _Cycas_ is the only one which has no flower. The MALE FLOWERS are constructed on the same type as the cones of the Horsetails and Club-Mosses, and are _most frequently long shoots_ (Figs. 243, 258, 260 _A_, 267 _J_) bearing a number of spiral or verticillate stamens. The FEMALE FLOWERS are of a more varied structure (see the orders). The OVULE _is orthotropous_ (except _Podocarpus_ which is anatropous) and projects from the carpel uprightly, inverted, or horizontally; it has usually _only one integument_ (compare however Taxaceæ) which proceeds from the upper part of the nucellus, so that the embryo-sac in part is placed below the integuments (Figs. 251, 264). The drop of mucilage which catches the pollen-grains dries up and draws the pollen-grain through the micropyle to a space just above the nucellus--_the pollen-chamber_--in which the germination of the pollen-grain commences.

In each seed, only one of the many embryos which are formed proceeds to its full development. The seed is always _endospermous_, and the embryo has one, two, or a whorl of several cotyledons. A vigorous primary root is developed on germination. THE VASCULAR BUNDLES in the stem are arranged in a ring, and _increase in thickness_ takes place by a closed cambium-ring which forms bast (_phlœem_) externally, and wood (_xylem_) internally with distinct annual rings, _as in the Dicotyledons_. Only certain of the Cycadeæ deviate from this arrangement. The _secondary wood_ is very uniform, as it is almost exclusively _formed of tracheides_ with bordered pits, but _true vessels are wanting_; this also indicates a relationship with the Pteridophyta (see page 202).

The Gymnosperms are biologically lower than the Angiosperms; they are wind-fertilised, and without extra floral-nectaries.

This Division embraces three classes: CYCADEÆ, CONIFERÆ, And GNETEÆ. It is no doubt monophyletic, and has taken its origin from heterosporous Ferns, now extinct, most nearly related to the Ophioglossaceæ and Marattiaceæ. The Cycadeæ appear to be the oldest class. The Coniferæ are related to these through Ginkgo. The Gnetaceæ are more isolated. The Division is not continued into the higher Flowering-plants; it has evidently attained its highest development, and is now in a retrograde condition. The similarity which has often been pointed out between certain Coniferæ and Lycopodinæ is only in analogous resemblances, and does not entitle one to suppose that there is a nearer relationship, or that the former take their origin from the latter.

Class 1. =Cycadeæ.=

The stem is very _rarely ramified_. The leaves are _large_, _pinnate_, and arranged spirally. The flowers are _diœcious, without perianth_.

There is only one order, the =Cycadaceæ=.--In habit they resemble the Ferns, especially the Tree-Ferns (compare Figs. 207 and 256). The stem is tubercular (Fig. 258), or cylindrical (Fig. 256), but not very tall (as much as about 12 metres), and very rarely ramified. [In Ceylon, unbranched specimens of _Cycas_ are rarely met with in the wild state. The stems of _C. circinalis_ occasionally branch in greenhouses.]

The LEAVES are arranged spirally, and so closely together that no free stem-surface is left between them, and have only a slight sheath (which is not amplexicaul, as in the Palms). They are compound (most frequently pinnate; in _Bowenia_, bipinnate); in some genera the leaves are rolled up in various ways, resembling the vernation in Ferns (Fig. 257); they are leathery and perennial. In some, stipules are present, as in the Marattiaceæ. Groups of scale-leaves alternate in the majority with groups of foliage-leaves.

The FLOWERS are without perianth. The MALE FLOWER is sometimes an enormous collection of stamens (Fig. 258), which are flat in some (_Cycas_, Fig. 241), shield-like in others (_Zamia_, _Ceratozamia_) like the sporophylls in Horsetail (Fig. 259); but in all, the pollen-sacs are situated in large and varying numbers on the back of the stamens, and arranged in groups of 2–5, like the sporangia in the sori of the Ferns (Fig. 241 _b_, _c_). FEMALE FLOWERS _are wanting_ in _Cycas_, because the carpels do not terminate the apical growth of the stem. After a group of foliage-and of scale-leaves, a group of carpels is developed, which are pinnate and resemble the foliage-leaves, bearing on their edges a number of ovules (most frequently 5–6) (Figs. 245, 256); the same stem produces successively scale-leaves, foliage-leaves, and carpels. The differentiation is not much more advanced than in certain Ferns (_Struthiopteris_, _Blechnum_), where barren and fertile leaves of different form regularly alternate. _The other genera have female flowers_; the carpels are shield-like in _Zamia_ and _Ceratozamia_ (Fig. 246), and collected into cone-like flowers, which terminate the growth of the stem (Fig. 259). The number of ovules in these instances is two to each carpel.

The SEEDS are large (most frequently 2–6 centimetres long) and plum-like; the external layer of the testa is fleshy, while the internal one is hard and horny. There are two systems of vascular bundles in the testa, one outside, the other inside the stone. The embryo is straight, attached to the end of the suspensor, which is often long, filamentous, and rolled up; it has one or two cotyledons.

The embryo in _Ceratozamia_ and others is very slightly
developed, at the time when the ripe seed is detached from
the carpel; and it is not until after sowing that its further
development and germination proceed. This calls to mind the
Cryptogams, especially _Selaginella_, whose macrospores are
thrown off filled with endosperm; but the oosphere is not
fertilised till after the separation of the macrospore from the
parent-plant, while in the Cycadeæ fertilisation is effected
before the separation. In _Cycas_ the testa may rupture, and
the endosperm grow and become green in the light, even though
no embryo has been formed. This also is an indication of its
prothalloid nature.

Gum-passages are present in all organs. Collateral vascular
bundles, with spiral and scalariform tracheides, are found;
and normal thickening takes place by means of a cambium.
An exceptional mode of growth is found in _Cycas_ and
_Encephalartos_, the cambium ceases to divide after a time
and is replaced by a new cambium which arises in the cortical
parenchyma just outside the bast, and which forms a new ring
of xylem and phlœem. This may be repeated so that a number of
concentric rings are produced. In _Ceratozamia_, structures
resembling corals extend from the roots in a vertical direction
and appear on the surface of the soil; these are peculiar roots,
in which a symbiotic Alga (_Anabæna_) is found.

The Cycadeæ were formerly (from the Coal period to the Later
Cretaceous) far more numerous than at the present day. They
appear to have been most numerous in the Trias and Jurassic.
The remnant (75 species) which have persisted to the present
time are found in all tropical countries. _Cycas_ (Trop. and
Sub-trop., Eastern Hemisphere); _Dioon_ (Mexico); _Macrozamia_
(Australia); _Encephalartos_ (Trop. and S. Africa); _Stangeria_
(Fig. 258, Sub-trop. South and East Africa); _Bowenia_ (Trop.
Australia); _Ceratozamia_ (Mexico, New Granada, Western Brazil);
_Microcycas_ (Cuba); _Zamia_ (Trop. and Sub-trop. N. America.)

USES. Sago is made from the starch-containing pith of _Cycas
revoluta_ and _circinalis_. The leaves are often used at
funerals and church festivals, under the name of “palm-branches.”

Class 2. =Coniferæ= (=Pine-trees=).

The stem _branches freely_. The leaves are _entire_, relatively small, linear or reduced to scales. The flowers are without perianth. The ovules naked. It is seldom that the female flower is reduced to only one carpel.

Whilst the Cycadeæ principally resemble the Ferns, the Conifers partly resemble the Lycopods, and partly the Equisetums--the former especially in the _needle- or scale-like_, leathery, simple, and often perennial leaves (“evergreen plants”), which _never possess stipules_ (Figs. 263, 270, 272). _Ginkgo_ deviates from this, being no doubt the oldest, and the Conifer which stands nearest to the Cycadeæ (Fig. 260). The resemblance to the Equisetums is especially owing to the fact that the stem ramifies abundantly, and often very regularly, forming a pyramid with verticillate branches. In addition to the foliage-leaves, scale-leaves (bud-scales) are present in the majority of species.

The FLOWERS are monœcious or more rarely diœcious. _Perianth is wanting._ The stamens of the _catkin-like male flowers_ (Fig. 267, _J_) are of different forms, but as a rule more or less shield-like. As in the Cycadeæ, the pollen-sacs are in all cases situated _on the underside_. There are, as a rule, two pollen-sacs (the Abietaceæ, Fig. 267), or 3–5, (the Cupressaceæ and Taxaceæ, Fig. 243); a few have more, _e.g. Araucaria_ (Fig. 242); they dehisce by clefts.