Part 18

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.