Chapter II: ACROGYNÆ. The apex of the stem or of certain branches is adapted (1)

for the formation of female shoots. The archegonia are most frequently aggregated on the apex of the shoots, and are encircled by their leaves (perichætium). Between these and the archegonia, enclosing the latter, a peculiar cup-shaped organ (the involucre) is formed. This group only includes leaf-bearing genera: _Frullania_, _Radula_, _Madotheca_, _Ptilidium_, _Calypogeia_, _Lepidozia_, _Mastigobryum_, _Lophocolea_, _Jungermannia_, _Scapania_, _Plagiochila_.

Class 2. =Musci frondosi or veri (True Mosses).=

In this class the protonema is well developed, and resembles a branched filamentous Alga, from which it can be easily distinguished by its oblique septa (in _Sphagnum_ it is a cellular expansion). The Moss-plant, which is developed directly from the protonema, generally has an erect, thick, cylindrical stem similarly constructed on all sides. The leaves are arranged spirally, the most frequent divergence being 2/5 or 3/8 (Fig. 200 _A_). A midrib is often present and also marginal veins formed by longitudinally elongated cells; at these veins the leaf is more than one layer in thickness. In _Leucobryum_ the leaves are generally constructed of more than one layer.

The stem grows by means of a three-sided, pyramidal, apical cell which gives rise to three rows of segments, each segment forming a leaf. The lateral branches arise from the lower portions of the segments, the upper portion of which does not take any part in the construction of the leaf. From their mode of origin the branches are not axillary, and differ in this respect from the Flowering-plants.

The ventral portion of the archegonium is very early ruptured _at its base_ by the growing sporogonium, upon which it remains, and it is thus raised into the air, forming a “hood,” the calyptra (Figs. 192; 200 _B_). In the Sphagnaceæ the hood is not present; in this order, as in the Liverworts, the archegonium remains at the base of the sporogonium. The sporangium opens by circumsessile dehiscence, the upper portion (_operculum_) being separated along a specially constructed ring of cells, and falls off like a “lid” (Fig. 200). Only in a few forms (families 2 and 3) does any variation of this take place. Elaters are never found, but (with the exception of _Archidium_) there is always present in the sporangium a central mass of cells, the _columella_, which take no part in the formation of the spores. The columella, in some, does not reach quite to the operculum and in these cases the spore-sac is bell-shaped and covers the columella (_Andreæa_, Fig. 190; _Sphagnum_, Fig. 199 _D_); but in the majority of Mosses the columella extends to the lid, so that the space containing the spores becomes a hollow cylinder.

The _sporangium_ is generally raised on a long stalk; in the great majority this stalk is formed from the lower half of the oospore and belongs to the asexual generation--it is then known as the _seta_. In _Andreæa_ and _Sphagnum_ the seta is very short, and the sporangia are raised upon a long stalk (_pseudopodium_) developed from the summit of the sexual generation (Figs. 190, 192). In the latter figure an archegonium (_a_) is seen attached to the pseudopodium, having been carried up with this during the course of its development. The summit of the pseudopodium is enlarged to embrace the foot of the sporogonium (Figs. 192, 199 _D_).

A. The sporangium is supported on a pseudopodium; the columella
does not extend to the operculum.

Family 1. =Sphagneæ (Bog-Mosses).=

The protonema has been already described. The stem is regularly branched owing to the fact that a branch, or collection of branches, arises at every fourth leaf. These branches are closely covered with leaves, some are erect, while others hang down and surround the stem. No rhizoids are developed. These Mosses are of a whitish-green colour, and when water is present are always saturated with it like a sponge, the reason for this being found in the construction of the stem and leaves. The stems are covered by an external layer of large clear cells, without chlorophyll, but with annular or spiral thickenings on the walls, which are also perforated by large holes. By means of capillary attraction, water is thus raised to the summit of the stem. Similarly constructed cells are also found in the leaves, but they are surrounded by a net of very narrow, chlorophyll-containing cells (Fig. 199 _F_), whose colour is thus to a great extent lost amongst those which are colourless. This anatomical structure is an essential condition for the formation of peat. The Bog-Mosses grow by preference on moors, which they cover with a thick carpet saturated with water. The lower extremities of the plants perish very rapidly, and gradually become converted into peat, and the branches thus separated from each other become independent plants. The sporangia (Fig. 199 _D_, _E_) are spherical, but with a very short stalk. They open by a _lid_, but have no _annulus_. The _archegonium_ (Fig. 199 _C_) persists at the _base of the sporogonium_ as in the Liverworts. Only one genus, _Sphagnum_.

Family 2. =Schizocarpeæ.=

The Mosses which constitute this family are of a brownish-black
colour and are found living on rocks. The sporangium resembles
that of the Liverworts inasmuch as it opens by four valves, but
these continue attached to each other at the apex as well as at
the base (Fig. 193).--There is only one genus: _Andreæa_.

=B.= The stalk is formed from the lower portion of the
sporogonium. The columella is continued to the summit of the
sporangium and united with it (_Archidium_ has no columella.)

Family 3. =Cleistocarpeæ.=

The fruit does not dehisce in the regular way, but the spores
are liberated by decay. They are small Mosses which remain
in connection with their protonema until the sporangium is
mature. The archegonium remains sessile at the base of the
short capsule-stalk, and is not raised into the air (compare
Hepaticæ).--_Phascum, Ephemerum, Archidium, Pleuridium._

Family 4. =Stegocarpeæ.=

To this belong the majority of the Mosses, about 3,000 species.

The capsule opens as in _Sphagnum_ by means of a _lid_ (_operculum_), which is often prolonged into a beak. Round the mouth of the opened capsule, a number of peculiar yellow or red teeth are to be found. These constitute the _peristome_; their number is four, or a multiple of four (8, 16, 32 or 64). The form and thickenings of these teeth are widely different, and on this account are used by Systematists for the purposes of classification. In some Mosses (Fig. 200 _C_, _D_) there is a double row of teeth. Except in _Tetraphis_ they are not formed from entire cells, but from the strongly thickened portions of the wall of certain layers of cells belonging to the lid, and persist when this falls off. They are strongly hygroscopic, and assist greatly in the ejection of the lid, in which operation they are considerably aided by a ring of elastic cells with thickened walls, situated in the wall of the lid near the base of the teeth. This ring is known as the _annulus_. The archegonium is raised into the air like a hood, the calyptra, which either covers the sporangium on all sides (having the shape of a bell), or is split on one side (Fig. 200 _B_, _h_).

Among peculiar forms may be mentioned: _Splachnum_, which is
especially remarkable for the collar-like expansion at the base
of the capsule. _Fissidens_ deviates in having a flat stem and
leaves arranged in two rows. The leaves are boat-shaped and
half embrace the stem.--_Schistostega_ has two kinds of stems.
The barren ones resemble Fern-leaves; they have two rows of
leaves, which are attached together vertically, are decurrent
and coalesce at their bases. The fertile ones have an ordinary
appearance.--_Tetraphis_: the peristome is composed of four
teeth, which are formed from entire cells. _T. pellucida_ has
peculiar gemmæ.

The family is divided into two groups: the Musci acrocarpi, the growth of whose main axis is limited and terminated by the formation of the sexual organs; and the Musci pleurocarpi, whose sporogonia are situated on special lateral shoots, while the growth of the main axis is unlimited.

A. =Acrocarpi.=

Order 1. =Weisiaceæ.= Peristome, with 16 teeth arranged in
one series, rarely wanting. Leaf with midrib. _Campylopus_,
_Dicranum_ (_D. scoparium_, common in forests), _Dicranella_,
_Cynodontium_.--_Weisia_, _Gymnostomum_ (no peristome),
_Systegium_.

Order 2. =Leucobryaceæ.= Peristome with 16 teeth. Leaves with
three or more layers of cells, of which the external ones are
air-conducting and perforated (as in the Sphagneæ), the middle
one containing chlorophyll. _Leucobryum._

Order 3. =Fissidentaceæ.= Peristome as in the preceding ones.
The leaves are arranged in two rows on the plagiotropic shoots;
in _Fissidens_ the midrib of the leaf bears wing-shaped
outgrowths. _Conomitrium, Fissidens._

Order 4. =Seligeriaceæ.= Peristome with 16 undivided teeth. Very
small Rock-mosses. _Seligeria.--Blindia._

Order 5. =Pottiaceæ.= Peristome with 16 teeth, which are
divided almost to the base, or with 32 teeth. Calyptra
hood-like.--_Barbula (B. muralis, B. ruralis), Trichostomum,
Leptotrichum.--Ceratodon purpureus.--Distichium.--Pottia._

Order 6. =Grimmiaceæ.= The leaf-cells are often papillose;
in the upper portion of the leaf, small, and of roundish
shape. The calyptra is most frequently hood-like or conical.
_Eucalypta._--_Orthotrichum_, often with short-stalked capsule,
is found on trees.--_Coscinodon._--_Hedwigia._--_Grimmia_,
_Racomitrium_.--_Cinclidotus._

Order 7. =Schistostegaceæ.= The stems are of two kinds (see
above); _Schistostega osmundacea_, in caves, has a bright
emerald protonema.

Order 8. =Splachnaceæ.= The capsule has a large, collar-like
neck (see above). _Splachnum_ (especially on manure).

Order 9. =Funariaceæ.= Capsule pear-shaped. _Funaria_ (_F.
hygrometrica_ has a very hygroscopic seta, becoming twisted
when dry, and straightening with moisture); _Physcomitrium_;
_Discelium_.

Order 10. =Bryaceæ.= The capsule is thicker towards
the apex; most frequently pendulous. _Philonotis_,
_Bartramia_.--_Aulacomnium._--_Paludella
Meesea._--_Mnium._--_Bryum_, _Webera_, _Leptobryum_.

Order 11. =Polytrichaceæ.= Single peristome, formed by 16,
32, or 64 teeth. Leaves with longitudinal lamellæ on upper
surface.--_Polytrichum_ has long, hairy calyptra. _Catharinea_
(_C. undulata_, in forests).

Order 12. =Georgiaceæ.= Peristome with 4 teeth (see above).
_Tetraphis_ (_T. pellucida_ has gemmæ).

Order 13. =Buxbaumiaceæ.= Capsule asymmetrical; double
peristome: the interior one conical, with 16 or 32 longitudinal
folds.--_Buxbaumia_ (_B. aphylla_); _Diphyscium_.

B. =Pleurocarpi.=

Order 14. =Fontinalaceæ.= Long, floating Water-Mosses.
_Fontinalis_ (_F. antipyretica_ is found in streams).
_Dichelyma._

Order 15. =Hookeriaceæ.= _Pterygophyllum._

Order 16. =Leskeaceæ.= Dull-looking Mosses, with papillose or
warted leaves.--_Thuidium_, _Thuja_-like with regularly arranged
1–3 doubly pinnate stems; _Anomodon_, _Leskea_.

Order 17. =Pterogoniaceæ.= _Pterigynandrum filiforme_, etc.

Order 18. =Fabroniaceæ.= _Anacamptodon._

Order 19. =Neckeraceæ.= Stems most frequently with flat, leafy
branches. The leaves are smooth, never with longitudinal
folds.--_Neckera._

Order 20. =Hypnaceæ.= The leaves are smooth with square,
often bladder-like, cells at the edge. _Hylocomium_ (_H.
splendens_, _H. triquetrum_); _Hypnum_; _Brachythecium_;
_Plagiothecium_.--_Eurhynchium._--_Homalothecium_, _Isothecium_,
_Orthothiecium_, _Homalia_.--_Climacium_, _Lescuræa_, _Leucodon_.

The Mosses occur all over the globe. Many are found in great
numbers, and growing thickly massed together, they form an
important feature in landscapes (for example _Sphagnum_ and
_Polytrichum_ in the Arctic Tundra). In the Northern and
Arctic regions the Mosses are very plentiful, and often form a
considerable part of the vegetation, while in the Tropics they
are insignificant.

Species of _Hypnum_ and _Polytrichum_, like _Sphagnum_, play an
important part in the formation of peat.

DIVISION III.

PTERIDOPHYTA (VASCULAR CRYPTOGAMS).

The alternation of generations is as distinct in this Division as in the Mosses, but the sexual generation consists of only a small thallus, the prothallium, which bears directly the sexual organs, _antheridia_ and _archegonia_; and the asexual generation, which arises from the fertilisation of the oosphere, is no longer a single short-lived sporangium, but a highly developed, generally perennial, plant provided with stem, leaves and _true roots_ (Ferns, Horsetails, etc.), the sporangia being borne on the leaves. In this latter generation the tissues are differentiated into epidermis, ground tissue and vascular tissue; in the last named the bundles are closed, and in the majority of cases concentric.

The =sexual generation=, =gametophyte=, or =prothallium=, is _always a thallus_, although not always green and leaf-like (Figs. 205, 215, 222, 229, 235, etc.) It is very small, even in cases where it attains the greatest development, and consists only of parenchymatous cells. The prothallium is nourished by hair-like roots (rhizoids) and has only a transitory existence, dying soon after the fertilisation of its oosphere.

The ANTHERIDIA exhibit great variations in structure which, however, must be considered as modifications of the fundamental type which is found in the Mosses. These modifications will be mentioned under the various families. The _spermatozoids_ are always spirally-coiled, self-motile, protoplasmic bodies, with most frequently a large number of fine cilia on the anterior end (Figs. 206, 223, 234). They are formed principally from the nucleus of the mother-cell, and portions of the cytoplasm often remain for a time attached to their posterior end.

The ARCHEGONIA are more uniform throughout the entire Division, and more closely resemble those of the Mosses. They are, as in the previous Division, principally flask-shaped; but the central portion, which encloses the oosphere, is always embedded in the tissue of the prothallium, so that the neck, which is formed of 4 rows of cells, projects above the surface (Figs. 201 ^3, 222 _h_). The development of the archegonium in a Fern is seen in the accompanying figure (Fig. 201). The archegonium is developed from a surface cell, which divides into three cells by two walls in a direction parallel to the surface of the prothallium (Fig. 201). The most internal cell becomes the ventral portion of the archegonium. The external one (_b_) divides perpendicularly to the surface of the prothallium into four cells, which again divide parallel to the surface and form the neck (_b_, in 2 and 3). The intermediate cell projects upwards into the neck and divides into two, the lower one, after the separation of the ventral canal-cell, becoming the _oosphere_, and the upper one the _neck-canal-cell_ (_c_, in 2 and 3).

As in the Mosses, the divisional walls of the neck-canal-cells become mucilaginous, causing the rupture of the neck of the archegonium. Fertilisation takes place as in the Mosses, and the passage of the spermatozoids, along the neck, to the oosphere, has been observed. Water (rain or dew) is similarly necessary for the movements of the spermatozoids, and hence for fertilisation. The other classes of the Division chiefly deviate from the Ferns in having the archegonium sunk deeper into the prothallium, and the neck reduced in length (compare Fig. 201 with Figs. 216, 222, 235, 236).

According to the nature of the spores, the three classes of the Vascular Cryptogams are each divided into isosporous and heterosporous groups.

I. The =isosporous= Vascular Cryptogams have _only one kind of spore_. The prothallium developed from this is in some cases monœcious, bearing both antheridia and archegonia; but in others there is a distinct tendency for each prothallium to bear only antheridia or archegonia (diœcious)--true Ferns and _Lycopodium_.

In _Equisetum_ there is only one kind of spore, but two kinds of prothallia are developed, one of which bears only antheridia (male), the other only archegonia (female); but the one that bears antheridia may be transformed into the one that bears archegonia and vice versa.

II. In the higher group, =heterosporous= Vascular Cryptogams (_Selaginella_ and _Isoëtes_, etc.), there are two distinct kinds of spores, the _small_, microspores, and the _large_, macrospores. The _microspores_ are male, and produce prothallia which bear only antheridia. The _macrospores_ are female, and produce prothallia which bear only archegonia.

Corresponding to this difference in the spores, there is also found a difference in the development of the prothallium. In the Isosporeæ the prothallium is large, and either green, leaf-like, and provided with rhizoids (most of the Ferns, Horsetails, etc.), or subterranean, pale-coloured, and globular (_Ophioglossum_, _Lycopodium_). It lives vegetatively for a fairly long time, and generally produces a large and varying number of archegonia and antheridia. The prothallium in the Heterosporeæ is gradually more and more reduced, its independent and vegetative life becomes of less and less importance, it becomes more dependent on the mother-plant, and projects from the spore very slightly, or not at all. The antheridia and archegonia become reduced in number to one, and also degenerate in point of development.

It may here be remarked that the gradual development of the asexual generation, the development of the two kinds of spores, and the progressive reduction of the prothallium and sexual organs which is found in this Division, is continued to the Gymnosperms and Angiosperms. The microspores are in these called pollen-grains, and the male prothallium is very rudimentary. The macrospores are termed embryo-sacs, and the female prothallium, the endosperm.

The =asexual generation=, =sporophyte=. When the oosphere, which in this case as in all others is a primordial cell, is fertilised, it surrounds itself with a cell-wall and commences to divide into a number of cells, to form the embryo.

The first dividing wall (basal wall) is nearly horizontal, and
in the direction of the longitudinal axis of the archegonium.
The next wall is vertical, and the next perpendicular to the
other two. The oosphere, therefore, is now divided into eight
octants by these three walls. The basal wall divides the
embryo into a hypobasal and an epibasal half. From the first
one, by continued divisions, the first root is developed; from
the latter, the stem and leaves. After the formation of the
octants the development proceeds in somewhat different ways in
the various classes. In addition to the stem, leaf, and root,
a “foot” is developed from the hypobasal half which remains
enclosed in the prothallium, and conveys nourishment from the
prothallium to the young plant until it is able to sustain
itself (Fig. 202). The formation of these members in the embryo
depends on the position of the oosphere in the archegonium and
prothallium, and is independent of gravity.

In the Mosses the asexual generation is the sporogonium, which is limited in its development and in a great measure dependent upon the sexual generation, upon which it is situated; but in the Pteridophyta this generation is an independent and highly developed plant, provided with stem, leaf, and true roots, and has in many instances an unlimited development. The Pteridophyta are the lowest Division with _true roots_. The root which is first formed is very similar in nature to the primary root of the Monocotyledons; it very soon dies and is replaced by others which are more permanent, and developed upon the stem (adventitious roots); roots are wanting in _Salvinia_, _Psilotum_, and some Hymenophyllaceæ. The differentiation is, however, not so complete as in the Flowering-plants, and so many leafy forms are not found. The various members of these plants are anatomically much higher than in the Mosses, having an epidermis, a ground tissue with variously differentiated cells, and a highly developed vascular system. The vascular bundles, like those in the Monocotyledons, are without cambium, and closed; they are therefore incapable of any increase in thickness. In general the bundles are concentric, with the bast round the wood (Fig. 203). The wood is almost entirely made up of scalariform tracheides.

In _Isoëtes_ a secondary thickening takes place by a cambium,
which is formed inside the cortex, constructing secondary
cortex to the exterior, and secondary wood towards the
interior.--_Botrychium_ has also a thickening growth. Collateral
vascular bundles occur in _Osmundaceæ_, _Equisetaceæ_, and the
leaves of many _Polypodiaceæ_, etc.

It is a point of special interest, that the gigantic forms of Ferns, Equisetums, and Club-Mosses (which flourished in earlier geological periods, when these classes attained their highest development) possessed some means of increasing in thickness.

The _sporangia_ are in all cases _capsule-like_, and burst open when ripe to eject the spores. They are nearly always situated on the leaves (in _Lycopodiaceæ_, in the axils of the leaves, or above these, on the stems themselves). In some forms (LEPTOSPORANGIATÆ), the sporangia are developed from a single epidermal cell; in others (EUSPORANGIATÆ), from a group of epidermal cells, or from cells which lie beneath the epidermis. In the first group a primitive mother-cell (archesporium) is formed, which divides commonly into sixteen special mother-cells. In the latter group, on the other hand, a number of primitive spore-mother-cells are developed. In each sporangium three different tissues are generally developed; an innermost _sporogenous_ one (_s_ in Fig. 204 _A_), which arises from the archesporangium; an outermost one, which forms the _wall_ (_a_), and may be one or, more rarely, several layers in thickness; and an intermediate one, the _tapetum_ (Fig. 204 _A_, _B_, _b t_), which is rich in protoplasm, and whose cells are dissolved so that the spores float freely in the fluid thus provided. The spores arise as in the Mosses (in tetrads), by the cross-division of the special mother-cells, and according to the manner in which they are arranged in the mother-cell have either a tetrahedral form, with a large base resembling a segment of a ball, or are oblong (bilateral spores). Their construction is the same as in the Mosses (p. 187).

The spore-formation in its earliest commencement takes place in the same way in the Isosporous and the Heterosporous Vascular Cryptogams; but from a certain point, after the tetrahedral division, a difference occurs with regard to the macrosporangia. All the spores formed in the microsporangium may complete their development; but those which are formed in the macrosporangium are generally aborted, with the exception of one or four, and these consequently attain a much larger size (see Fig. 239.--The series to the left are microsporangia; those to the right, macrosporangia).

APOGAMY. In some Ferns (_Pteris cretica_; _Aspidium filix mas_,
var. _cristatum_; _A. falcatum_; _Todea africana_) the young
plant is not developed as a consequence of fertilisation, but as
a bud from the prothallium. This is known as apogamy, or loss of
the power of sexual reproduction. The antheridia are generally
more or less developed; archegonia are entirely wanting in _Asp.
filix mas_, var. _cristatum_. This variety has probably only
become apogamous through cultivation. Many specimens of _Isoëtes
lacustris_, in a lake in the Vosges mountains, produce in the
place where the sporangia are usually found, a vegetative shoot
which grows into a new plant, so that the sexual generation is
wanting in this case. Some specimens have sporangia on some
leaves, and shoots on others.

Apospory, or the formation of prothallia instead of sporangia
and spores on the leaves, is found in _Athyrium filix
femina_, var. _clarissimum_. In this case the development
of the sporangia proceeds only to a certain point, and from
these arrested sporangia the prothallia are produced. Normal
sporangia are entirely wanting in this variety, and in _Aspidium
angulare_, var. _pulcherrimum_, sporangia are completely
wanting. Compare the Mosses (page 188).

The Vascular Cryptogams are divided into _three large classes_, in each of which a progressive development can be traced from the isosporous to the heterosporous forms, but some of these are now only known as fossils.

Class 1. =Filicinæ= (=Ferns=).--The stem is small in comparison with the leaves, and branches only seldom, and then by lateral shoots. The leaves are scattered, large, often deeply divided, and of various highly developed forms. The undeveloped leaves are rolled up in the bud, having what is termed circinate venation. The sporangia are situated on the edge or on the lower side of the leaves, those on which the sporangia are borne (_sporophylls_) being often the ordinary foliage-leaves; but in a few cases the fertile differ from the barren ones (a higher stage in development). The fertile leaves are not confined to definite parts of the shoot, and do not limit its growth. The archesporium is most frequently unicellular.

_A_. =Isosporous=: Sub-Class 1. Filices (True Ferns).

_B_. =Heterosporous=: Sub-Class 2. Hydropterideæ (Water Ferns).

Class 2. =Equisetinæ= (=Horsetails=), in its widest meaning.--The leaves in this class are small in comparison with the stem. They are arranged in whorls, and unite to form a sheath. The sporangia are situated on specially modified, shield-like leaves, which are closely packed together and form a “cone.” The cone is borne terminally, and limits the growth of the shoot. The sporangia are developed from a large group of epidermal cells, the archesporium being unicellular. The branches are arranged in whorls, and develope acropetally.

_A_. =Isosporous=: Sub-Class 1. Equisetaceæ. Existing forms.

_B_. =Heterosporous=: Sub-Class 2. Extinct forms.

Class 3. =Lycopodinæ= (=Club-Mosses=).--Roots generally branching dichotomously. The leaves are scattered or opposite, and in proportion to the stem very small, undivided, and simple. They are scale-like and triangular, tapering from a broad base to a point. The sporangia are situated singly (except in _Psilotaceæ_), and almost in every case on the upper side of the leaf or in the axil of a leaf; but in some cases they are borne on the stem, just above the leaf-axil. The sporangia arise from groups of epidermal cells. The sporophylls are often modified, and differ from the foliage-leaves; they are then arranged in cones placed terminally on branches, thus limiting their growth.

_A_. =Isosporous=: Sub-Class 1. Lycopodieæ.

_B._ =Heterosporous=: Sub-Class 2. Selaginelleæ.

Class 1. =Filicinæ= (=Ferns=).

The characteristics of this class have already been given on page 204.

The class is divided into two sub-classes:--

1. The TRUE FERNS, FILICES, have one kind of spore which generally developes monœcious prothallia, relatively large and green. The sporangia are most frequently situated in groups (_sori_), which are often covered but not enclosed by an _indusium_.

2. WATER FERNS, HYDROPTERIDÆ, have microsporangia with many (4 × 16) microspores, and _macrosporangia, each with one macrospore_. The prothallium is small, and projects but slightly from the germinating spore. The sporangia are situated in groups (_sori_), which are either enclosed by an indusium, or enveloped in a portion of a leaf, to form “fruits” termed _sporocarps_.

The old name for the Hydropterideæ, “Rhizocarpeæ,” _i.e._ the
“root-fruited,” originated from the erroneous supposition that
the sporocarps were borne on the roots.

Sub-Class 1. =Filices= (=the True Ferns=).

Of the eight orders (with about 4,000 species) comprised in this sub-class, the Polypodiaceæ is the largest (having about 2,800 species) and the most familiar; for this reason it will be taken as typical.

=The sexual generation.= When the spore germinates, the external covering (exospore) is ruptured, as in the Mosses. The internal cell-wall (endospore) grows out as a filament, which soon divides and gives rise to the prothallium, a flat, cellular expansion resembling the thallus of a Liverwort. In its fully developed state the prothallium is generally heart-shaped, dark green, and provided with root-hairs, and it attains a diameter of about one centimetre (Fig. 205). It is formed of one layer of cells, except along the central line near the anterior depression, where it becomes several layers of cells in thickness, forming the “cushion,” on the lower side of which the archegonia are developed. The antheridia are first formed; they are thus found on the oldest parts of the prothallium, on its edge, or among the root-hairs. The archegonia are developed later, and are therefore found near the apex. Several tropical Ferns have prothallia[18] deviating from this typical form; _Trichomanes_ (Order _Hymenophyllaceæ_) has filamentous, branched prothallia, which resemble the protonema of a Moss. Others, again, have strap-shaped prothallia, which resemble the thallus of certain Liverworts.

The ARCHEGONIA have been already mentioned (p. 199, Fig. 201). The ANTHERIDIA are hemispherical or slightly conical bodies (Fig. 206). They consist, as in the Mosses, of a wall formed by one layer of cells, which encloses a number of spermatozoid-mother-cells (_A_ and _B_). The antheridia when ripe absorb water, and are ruptured, and the spirally-coiled spermatozoids liberated (Fig. 206 _S_). The spermatozoids have been observed to pass down the neck of the archegonium, and to fuse with the oosphere.

=The asexual generation.= The first leaf, the “cotyledon,” of the embryo developed from the oospore (Figs. 202, 205) is always small, and has a very simple shape. The leaves which occur later become more perfect, stage by stage, until the permanent form of leaf has been attained.--The STEM is most frequently a subterranean or a semi-aerial rhizome; it is only in the tropical, palm-like Tree-Ferns, that the stem raises itself high in the air and resembles that of a tree, with leaf-scars or with the remains of leaves attached (Figs. 207, 203); in certain species the stem is encased in a thick mat of aerial roots (_Dicksonia antarctica_). When the rhizome is horizontal the internodes are frequently elongated, and the leaves are arranged in two rows, as in _Polypodium vulgare_ and in the Bracken-Fern (_Pteridium aquilinum_), etc.; it is also generally _dorsiventral_, having a dorsal side on which the leaves are situated, and a ventral side, different from the former, on which the roots are borne. When the stem ascends in an oblique direction, or is nearly vertical, its internodes are extremely short, and the leaves are arranged in a spiral line with a complicated phyllotaxis, _e.g._ in _Athyrium filix-fœmina_, _Aspidium filix-mas_, etc. The BRANCHING upon the whole is extremely slight, and is generally confined to the petiole (_e.g. Aspid. filix-mas_), or to the stem near the insertion of the leaves. Several species normally form buds on different parts of the lamina. The buds which are formed on the stem are not confined to the leaf-axil as in the higher plants. The Tree-Ferns, generally, do not branch at all.

The VASCULAR BUNDLES are _concentric_, with the wood surrounded by the soft bast. In transverse section they are seen as circles or irregularly-shaped figures (Fig. 203), the name of “King Charles and the Oak” (Bracken-Fern) having originated from the appearance which the bundles present in oblique section. In _Osmunda_ they are collateral and resemble those of the Flowering-plants. Round each individual bundle is often a sheath of thick-walled, hard, brown, sclerenchymatous cells, which act as a mechanical tissue; similar strands are also found in other parts of the stem.

The LEAVES in nearly all species are only foliage-leaves, borne in a spiral. They have an apical growth which continues for a long time, and some require several years for their complete development. In the buds they are rolled up (_circinate_); not only the midrib, but also all the lateral veins, and even the terminal portions of a leaf are sometimes rolled up together, the tissues of the leaf being already fully developed and only waiting to expand. The leaves are often excessively divided and compound, with pinnate branches, and have an epidermis with stomata and a well-developed system of venation. Stipules are only found in _Marattiaceæ_ and _Ophioglossaceæ_.

Very often peculiar hairs or scales (_paleæ_, _ramenta_), dry, brown, flat and broad, are found on stem and leaf.

The SPORANGIA are small, round capsules, which, in a very large number of Ferns, are formed on the back, but more rarely on the edge of the ordinary foliage-leaves. It is very seldom that there is any difference in form between the barren foliage-leaves and the fertile leaves, as is found for example in _Blechnum spicant_ or _Struthiopteris_; or that the fertile part of the leaf is differently constructed from the barren portion of the same leaf, as in the Royal-Fern (_Osmunda_). In such instances the mesophyll of the fertile parts is poorly developed.

The sporangia in the _Polypodiaceæ_ are lens-shaped, with long stalk (Fig. 211 _D_): their wall consists of one cell-layer on which a single row of cells, passing vertically over the top (that is along the edge of the sporangium), is developed into the “ring” (annulus). The cells of the annulus are very much thickened on the inner and side walls, and are yellowish-brown. The thickened cells, however, do not entirely encircle the sporangium, and on one side, near the stalk, they pass over into large, flat, thin-walled cells. These form a weak point in the wall, and it is here that the sporangium is opened diagonally by the elongation of the annulus. The sporangium of the Polypodiaceæ opens as it dries. The cells of the annulus are very hygroscopic, and in straightening, the annulus bends back with a jerk, thus ejecting the spores to considerable distances. The cells of the annulus absorb water with great readiness. [The sporangium arises as a single epidermal cell, from which a basal stalk-cell is cut off. Three oblique cell-walls, intersecting near the base, are next formed in the upper cell, and a fourth between these and parallel to the free surface; an inner tetrahedral cell enclosed by four others is thus formed, the outer cells become the wall of the sporangium, while the inner cell, by a series of walls, parallel to its sides, cuts off a layer of cells which eventually form the tapetum, the remaining central cell constituting the archesporium.]

The SPORES are either oblong and bilateral, or they are tetrahedric with curved sides, depending upon the way in which the tetrad division has taken place.

The sporangia are almost always situated on the nerves and gathered into groups, _sori_, which differ in form in the various genera. The sori, in many genera, may be covered by a scale-like structure, the _indusium_ (Figs. 211 _B_, 212).

In the majority of cases, each sorus is situated on a small papilla (_placenta_, or _receptacle_), which is supplied by a small vascular bundle. Between the sporangia, hairs (_paraphyses_) are often situated, which spring either from the placenta or from the stalks of the sporangia.

=Systematic Division.= The Ferns may be divided into two groups, characterized by the structure and development of the sporangia. The sporangia in the EUSPORANGIATÆ take their origin from a group of epidermal cells, and their walls are formed by several layers of cells. The archesporium is the (not tetrahedric) hypodermal terminal cell of the axial row of cells which give rise to the sporangium. In the LEPTOSPORANGIATÆ the sporangia are developed from single epidermal cells, and their walls are uni-layered. The archesporium is a central, often tetrahedric cell, from which sixteen spore-mother-cells are developed.[19] It is difficult to say which form is the oldest (according to Prantl, those which have the sori on the nerve-endings); however, the Eusporangiatæ would seem to have made their appearance long before the others, and also well defined Marattiaceæ and Ophioglossaceæ occur in the Kulm and Coal period, before the true Polypodiaceæ.

About 4,000 species of Ferns are now existing, and they are found especially in tropical and sub-tropical forests.

Family 1. =Eusporangiatæ.=

Order 1. =Ophioglossaceæ.= The prothallium differs from that of all other Ferns in being _subterranean_, _free from chlorophyll_, _pale_ and _tuberous_. The stem is extremely short, with short internodes, most frequently unbranched, vertical, and entirely buried in the ground (Fig. 208 _st_). In several species (among which are the native ones) one leaf is produced every year, which has taken three to four years for its development. In _Botrychium_ a closed, sheath-like basal part of each leaf covers the subsequent leaves during their development. In _Ophioglossum_ and others each leaf has at its base an intrapetiolar, cap-like sheath, which protects the succeeding leaf. The leaves are of two kinds: (_a_) foliage, which in _Ophioglossum vulgatum_ are lanceolate and entire, but in _Botrychium_ however, are pinnate (_b_ in Fig. 208 _A_, _B_); and (_b_) fertile, which are found facing the upper side of the foliage-leaves. These latter in _Ophioglossum_ are undivided and spike-like (Fig. 209 _A_), but pinnate in _Botrychium_ (Fig. 208 _B_). Each foliage and fertile leaf are branches from the same petiole. The large sporangia are placed laterally, and open by two valves. No annulus is formed (Fig. 209).--_Ophioglossum_ reproduces vegetatively by adventitious buds on the roots.

Three genera with about twelve species.

Order 2. =Marattiaceæ= are tropical Ferns, whose gigantic leaves resemble those of the Polypodiaceæ, but have stipules in addition. The sporangia are grouped in sori, situated on the lower side of the leaves, the sporangia in each sorus being arranged either in two rows or in a ring. In _Angiopteris_ they are isolated (Fig. 210 _A_), but in the other species (_Kaulfussia_, _Danæa_, _Marattia_), they are united, and form “synangia” divided into a number of chambers corresponding to the sporangia. These open by clefts or pores. _Marattia_ presents the highest development, as its sporangia are completely united in a capsule-like synangium, which is closed until maturity, and then opens by two valves. In each valve there is a row of three to eleven sporangia, each opening by a slit towards the inside (Fig. 210 _B_, _C_). An indusium encloses the sorus, except in _Kaulfussia_; it is formed of flat and lobed hairs, which resemble the hairs of the other portions of the leaves. In _Angiopteris_ and _Marattia_ the indusium is very rudimentary; in _Danæa_ it forms a kind of cupule.

The numerous fossil Marattiaceæ (15 genera, with 98 species)
present similar differences to those now living, but more
various forms are found, for example, with solitary free
sporangia. Those now living are the last small remnant (4 genera
with only 23 species) of a once dominant family, which existed
from very early times, and whose culminating point was reached
in the Kulm and Coal periods.

The Ophioglossaceæ appear also in the Kulm and Coal periods,
and were about as numerous as at the present time (presumably 2
genera, with 19 species). Leptosporangiate Ferns appear however
to have occurred first of all in the Trias-formation.

Family 2. =Leptosporangiatæ.=

Order 1. =Polypodiaceæ.= Sporangia on the lower side of the leaves, _stalked_ and provided with a _vertical_, incomplete annulus; dehiscing by a transverse cleft (Fig. 211 _D_).--The genera are distinguished by the form of the indusium and the position of the sori, etc.

1. The sporangia cover the entire lower surface of the leaf (Tropical America and Asia). _Acrostichum_, _Platycerium._

2. Sori without indusia, circular or oval. _Polypodium_ (Fig. 211 _A_). The leaves are most frequently situated in two rows on the dorsal side of the creeping rhizome, and fall off leaving a smooth scar behind.--_P. vulgare_, common in woods, on stones. (_Phegopteris_ also has no indusium; see page 214).

3. The sporangia are situated in continuous lines just inside the margin of the leaf.--_Pteris_[20]: the sporangia form a continuous line along the entire margin of the leaf (Fig. 211 _C_), which bends over and covers the sporangia, forming a “false-indusium.” _Pteridium_ has linear sori situated on a marginal vascular bundle, covered by two linear basal indusia, of which the outer is bent over like the edge of a leaf.--_P. aquilinum_ (Bracken) has a wide-spreading rhizome with large alternate leaves, placed on opposite sides, at some distance apart. Only one leaf is developed from each branch every year.

_Adiantum_ (Maiden-hair): sori on the underside of small
portions of the edge of the leaf, which are bent over (false
indusium). _Cryptogramme_ (_Allosorus_), _Cheilanthes_.

4. The sori are oval or linear, situated on one side of the vascular bundle.--_Asplenium_ (Fig. 212 _A_): sori linear; indusium with one of its edges attached at the external side. _A. ruta muraria_ (Wall-Rue); _A. septentrionale_; _A. trichomanes_.--_Athyrium_: sori linear or curved; _A. filix-fœmina_ (Lady-Fern).--_Scolopendrium_ (Fig. 212 _B_): sori as in _Asplenium_, but situated in pairs across the lanceolate, entire leaves. Each sorus is covered on the external side by an indusium, whose free edges are parallel and approach each other. _S. vulgare_ (Hart’s-tongue).--~_Blechnum_ (_B. spicant_, Hard Fern; the fertile leaves differ from the barren, the pinnæ being narrower, while the underside is almost entirely covered with sori, and hence they are of a much darker brownish hue than the barren ones).--_Ceterach_: indusium rudimentary or absent.~

5. Sori circular and covered by a shield-like, or reniform indusium.--_Aspidium_ (Fig. 211 _B_); the leaves wither away and leave no scar upon the root-stock. _A. filix-mas_ (Male-Fern); _A. spinulosum_.--_Phegopteris_ has no indusium, the withered bases of the leaf-stalks are persistent; _P. dryopteris_ and _P. polypodioides_.

6. The indusium is situated below the sori, and has the shape of a one-sided scale (_Cystopteris_, _Struthiopteris_), or of a cup or cupule, which in _Woodsia_ is sometimes fimbriate (Fig. 212 _C_, _D_).

7. The sori are situated on the margin of the leaf, and at the end of a vascular bundle. Indusium, semi-cupular. _Davallia._ Principally tropical species. 1 in S. Europe.

This order is the greatest, comprising about 2,800 species, the majority being perennial plants. A few are large, and known as Tree-Ferns.

As plants in conservatories and rooms the following are
cultivated: species of _Gymnogramme_ (tropical America),
_Lomaria_, _Nephrolepis_, _Pteris_ (_P. serrulata_, _cretica_).

Officinal. _Aspidium filix-mas_, rhizome and the withered
petioles.--Species of _Alsophila_ and _Cibotium_ give Penghawar
Djambi. The rhizome of _Pteridium aquilinum_, var. _esculentum_,
contains so much starch that it is used as food.

The other orders of true Ferns deviate from the Polypodiaceæ,
especially in the formation of the annulus, the bursting of the
sporangium and its mode of attachment and development, and in
the differences in the formation of the prothallium, etc. The
principal are:--

Order 2. =Hymenophyllaceæ.= To this order belong the lowest and most Moss-like Ferns; the leaves, with the exception of the veins, are most frequently formed of _only one layer of cells_, and consequently stomata are wanting; the formation of the prothallium also somewhat resembles the Mosses. Sori marginal, on the _extremities of the vascular bundles_, and surrounded by a _cupular indusium_. The sporangia are sessile, with equatorial annulus. _Hymenophyllum_ (_H. tunbridgense_, European). _Trichomanes_ (_T. speciosum_, European). Species about 200, which live especially on rocks and trees in damp and shady tropical forests. Some have no roots.

Order 3. =Cyatheaceæ.= Annulus _complete_ and oblique. To this order belong, principally, the tree-like Ferns with palm-like habit. The number of species is about 200, they are all tropical and form forests in some regions of Australia. _Cibotium_ and _Dicksonia_ have marginal sori, with cupular, basal indusium. (The stem of _D. antarctica_ is covered with aerial roots.) _Alsophila_ (without indusium); _Cyathea_ with cupular, inferior indusium (Fig. 212 _E_).

Order 4. =Gleicheniaceæ.= Sporangia with equatorial annulus, and longitudinal dehiscence, most frequently groups of 3–4 in sori without indusium (Fig. 213). _Gleichenia_: the apical growth of the leaves continues for a long time.

Order 5. =Schizæaceæ.= Annulus apical. To this order belongs _Aneimia_, which is so commonly cultivated in conservatories. The two lowest pinnæ are metamorphosed, having no leaf parenchyma and being covered with sporangia. _Schizæa. Mohria. Lygodium_, a climber, whose leaves have unlimited growth and attain a length of several metres. About 70 species. Tropical.

Order 6. =Osmundaceæ.= The sporangia have at the apex a lateral group of strongly thickened cells, which gradually pass over into the ordinary cells. The sporangia open by a longitudinal cleft. Indusium wanting. _Osmunda_ bears the sporangia upon peculiar, branched pinnæ, without parenchyma (the uppermost in the leaf). _O. regalis_ (Royal-Fern): European.

Sub-Class 2. =Hydropterideæ= (formerly Rhizocarpeæ), =Water Ferns=.

The following further characteristics must be added to those given on page 205:--

=Sexual generation.= The MICROSPORES produce an extremely rudimentary prothallium, formed of only a single cell, and having also a very much reduced bicellular antheridium with a small number of spermatozoid mother-cells in each cell (in _Salvinia_ 4, in _Marsilia_ and _Pilularia_ 16). In _Salvinia_ the microspores remain embedded in a hard mucilaginous mass (at first frothy) which fills up the cavity of the sporangium. The prothallium must therefore grow out through this slime and also through the wall of the sporangium (Fig. 214), and it thus terminates in a relatively long cell.

In _Marsilia_ the microspores are set free from the microsporangium, and the prothallia, with the antheridia, remain in them until the spermatozoids are liberated. The latter are spirally-twisted threads.

The MACROSPORES, on germination, give rise to a very reduced prothallium, which in _Salvinia_ bears 3 archegonia; but, if these are not fertilised, the prothallium may continue to grow and become a fairly large, green body with several archegonia (Fig. 215 _A_, _B_). In _Marsilia_ the prothallium is still more reduced, it is enclosed in the macrospore, and only bears one archegonium. The archegonia are similar in structure to those of the Ferns, but are smaller, and sunk more deeply in the tissue of the prothallium.

=The asexual generation= is developed from the fertilised egg-cell. It is a dorsiventral, horizontal shoot. In _Salvinia_ it bears at first a shield-like leaf, the scutiform leaf (Fig. 215 _C_, _a_), which is succeeded by the ordinary foliage-leaves. The young plants of _Marsilia_, likewise, have less perfect leaves in the very early stage.

The formation of the sporangium is the same as in the Leptosporangiate Ferns. (The 16 spore-mother-cells originate from one central, tetrahedric archesporium.)

The Hydropterideæ are divided into 2 orders, the chief differences between them being found in the asexual generation.

Order 1. =Salviniaceæ.= This order more nearly approaches the true Ferns, especially so on account of the form of the indusium. Only one species is found in Europe, _Salvinia natans_ (Fig. 217). This is a small, floating, annual, aquatic plant, entirely _destitute of roots_. The dorsiventral, horizontal stem bears two kinds of leaves, which are arranged in whorls of three. Two of these which turn upwards are oval, entire, “_aerial foliage-leaves_” (Fig. 217 B, _b^2_-_b^3_); the third, the “_water-leaf_” (_b^1_) is submerged and divided into a number of hair-like segments, similar to the submerged leaves in many aquatic plants, for instance, Water-buttercup (see also Fig. 215 _C_). The whorls of leaves alternate with each other; there are thus 4 rows of dorsally-placed aerial leaves, and two rows of ventrally-placed submerged leaves. The sporangia are situated in sori, each sorus being borne on a small column (receptacle or placenta) and enveloped by a _cupular_, but _entirely closed indusium_ (Fig. 218). _The sori are situated on the submerged leaves_ (Fig. 217 _B_, _s-s_) _and are unisexual_, _i.e._ each sorus contains microsporangia only, or macrosporangia.

_Azolla_ belongs to this order. It is a very small, floating,
tropical water-plant (America and East India), with horizontal,
root-bearing stem. The stem branches profusely by lateral buds,
and bears the two rows of leaves on its dorsal side, the roots
on the ventral side. Each leaf is bifid, and divided into an
upper dorsal, and a lower ventral portion. The upper segments
float on the surface of the water and are arranged like tiles
on a roof, each one overlapping its neighbour. In each floating
segment a large cavity is found, in which _Anabæna_ is always
present. The lower segments are submerged.

Order 2. =Marsiliaceæ.= The characteristic feature of this order, and one not possessed by other Fern-like plants, is that the sori (2–many) are enveloped _in leaf-segments_ which _close round them_ and form a “sporocarp,” just in the same manner as the carpels, in the Angiospermous Flowering-plants, close round the ovules and form ovaries. The sori contain both micro-and macrosporangia. When the spores are ripe, the sporocarp opens in order to disperse the spores (Fig. 220).

The two genera (with 57 species, Temperate, Tropics) are land-and marsh-plants, whose dorsiventral, creeping stem bears roots on the under surface, and the leaves in two rows on the upper side (Figs. 219, 221). The leaves of _Marsilia_ are compound, and divided into four small leaflets springing from the apex of the petiole (Fig. 219), and resemble the leaves of _Oxalis_. In the bud the leaves are circinate (Fig. 219 _b_), and at night they exhibit the well-known sleep-movements. The sporocarps are borne on the petioles of the fertile leaves, near their bases (Fig. 219 _f_); they are oblong and resemble small beans, the outer cells being hard and sclerenchymatous, while the inner ones are divided into a number of loculi arranged in two rows. On germination, water is absorbed, the two sides separate slightly, as valves (Fig. 220 _A_), and a long vermiform mass of gelatinous, parenchymatous cells (Fig. 220), swollen by the water, emerges, bearing a large number of sori arranged pinnately. Each sorus (_sr_) is covered by a thin indusium. (The thin covering may be considered an indusium physiologically, though not morphologically).

_Marsilia quadrifolia_, in Europe. Many species are found in
Australia. The nutritious sporocarps of _M. salvatrix_ were
the means of saving the Burke expedition in the interior of
Australia, and hence this species has earned its specific name.

_Pilularia_ has linear leaves, without lamina. The sporocarps are spheroid (Fig. 221), brown and hard, and situated near the base of the leaves. They are 2–4 chambered and open by a corresponding number of valves.

Class 2. =Equisetinæ (Horsetails.)=

The characteristics of this class have been described on page 204.

It is divided into two sub-classes:--

1. THE ISOSPOROUS EQUISETINÆ. To this sub-class belong, with certainty, only the EQUISETACEÆ now existent, which are represented by only one genus, _Equisetum_.

2. THE HETEROSPOROUS EQUISETINÆ. Forms which are now extinct.

Sub-Class 1. =Isosporous Equisetinæ.=

Order. =Equisetaceæ (Horsetails).=

=The sexual generation.= The prothallium is green and leaf-like, as in the majority of Ferns, but irregularly branched and curled. It is often unisexual. The male prothallia bear antheridia only, and are smaller and less branched (Fig. 222 _A_) than the female; the latter may attain a diameter of ½ an inch, and bear archegonia only (Fig. 222 _B_). The antheridia and the archegonia resemble those of the Ferns, but the spermatozoids (Fig. 223) are larger and less twisted. On the last curve is situated a more or less elongated appendage of cytoplasm (Fig. 223 _c_).

=The asexual generation.= The embryo is similar to that of the Ferns. The fully developed _Equisetum_ is a perennial herb, with widely creeping (in some species tuberous) rhizome, from which extend erect, aerial, most frequently annual shoots.

The vegetative aerial STEMS are divided into a number of internodes by the whorls of leaves (Fig. 224). The internodes are hollow, the cavities being separated from each other by the transverse partitions of the solid nodes. The lower portion of the internode, which is encased by the leaves, has much thinner and softer cell-walls, so that the stem is easily separated into segments just above the nodes. Each internode has a large number of ridges and furrows, and bears at its apex a whorl of leaves whose number and position correspond to the ridges of the internode. As in the case of other verticillate plants, the whorls are placed alternately, one above the other; the same arrangement is also found in the ridges on two successive internodes. In addition to the large air-cavity in the centre of each internode (the central cavity), a whorl of tubular air-passages is found in the cortex of the stems, opposite the furrows (vallecular canals). There is also a similar air-passage (carinal canals) in each of the vascular bundles, which are placed in a ring, one opposite each ridge, and therefore alternating with the vallecular canals. The vascular bundles are _collateral_ as in the majority of Flowering-plants, but poorly developed. The xylem of each bundle consists of two groups of annular or spiral vessels, close to the outer border of the carinal canal, and two groups of scalariform tracheides, each placed on a radius passing through a group of spiral vessels. The phloëm is placed between these four groups, each of which has only a few vessels. The stiffness of the stems is mainly due to the large amount of silica in the cell-walls of the epidermis, and to the sclerenchymatous cells of the ridges.

All LEAVES are situated in _whorls_. The VEGETATIVE are simple, undivided, 1-nerved, and are united into toothed sheaths (Fig. 224 _a_, _b_). The branching of the stems in some species (_E. arvense_) is very abundant. The branches break through the base of the leaf-sheaths (Fig. 224 _b_), and generally _alternate with the teeth_ (leaves).

The FERTILE LEAVES (_sporophylls_) are different from the barren ones. They are _free, shield-like_, each one having a short stalk bearing usually an hexagonal plate (Fig. 224 _d_), and closely compressed into an ear or cone (Fig. 224 _a_, _c_). The _Equisetums_ thus present an advance in development distinctly beyond that of the Ferns, which is further emphasized by the circumstance that a transition from the sheath-leaves to the fertile-leaves is found in the involucre or annulus, a “collar” of specially modified leaves situated at the base of the cone (Fig. 224 _a_ and _c_). The cone may be considered as a very rudimentary flower, and the annulus may be regarded as a very early stage in the formation of a flower (perianth). See page 235.

The SPORANGIA are situated on the underside of the sporophylls, one at each angle; they are sac-like, and open inwardly by a longitudinal cleft (Fig. 224 _d_). An annulus is wanting; but in the wall of the sporangium, as in the pollen-sacs of the Flowering-plants, a layer of cells, with annular or spiral thickenings, is developed, which assists in the dehiscence of the sporangium.

The SPORES are green; the walls composed of four distinct layers, of which the outer is gradually separated, except at one point, and becomes split into four long bands (_elaters_) (Fig. 225). The elaters are extremely hygroscopic, coiling round the spore when moistened, and expanding as soon as dry, presenting a most lively object under the microscope when breathed upon and allowed to dry. The second layer, when germination commences, becomes detached from the inner wall, which is formed of the exospore and endospore.

The order has become much reduced, and at the present time includes only one genus, _Equisetum_, with about twenty-five species, which are distributed over the entire globe, particularly in damp situations. In SOME SPECIES the barren shoots are green and very much branched, but the fertile ones are unbranched, pale brown, and possess no chlorophyll (_E. arvense_, Field-Horsetail, Fig. 224, and _E. maximum_). IN OTHERS the fertile and barren shoots are alike green, and either both unbranched (_E. hiemale_), or branched (_E. palustre_, _E. limosum_, etc). The fertile shoots of _E. silvaticum_, up to maturity, resemble those without chylorophyll of _E. arvense_, but after that period they produce green branches, and thus resemble the barren ones.