Part IV

and further by an involucre or perigynium (absent in Haplomitrium) which generally invests a single archegonium, and in some cases (Blasia, Fossombronia, Androcryphia) is only developed after fertilisation. In Blasia the growth of tissue subsequently to fertilisation is so active that the fertilised archegonium becomes completely sunk in the tissue of the shoot. The autheridia are borne, in all Jungermanniaceae (except Haplomitrium) on the dorsal surface of the shoot ; in Haplomitrium they are borne in three rows on the sides of the apical region. In Riella belt copliylla the antheridia are borne sunk in the tissue of the free edge of the membranous wing. As the symmetry of the plant is radial, it presents no distinction of dorsal and ventral surfaces ; still the position of the antheridia in R. helicophylla is dorsal by analogy, since in the dorsiventral R. Reuteri, the wing, which likewise bears the antheridia, is an outgrowth of the dorsal surface of the stem ; antheridia have been observed as yet only in these two species of Riella. The antheridia are shortly stalked and are in all cases provided with a pro- tection. In Metzgeria the group of antheridia is invested by an involucre which consists of the short modified ventral gametophore : in the other thalloid Anacrogynae, as also in Androcryphia, Blasia, Petalophyllum, and Kiella, each antheridium is invested by an involucre which grows up around it, so GROUP II. BRYOPHYTA : HEPATKLE. 349 that it appears to be sunk in the tissue of the shoot. In Fossombronia, Haplo- mitrium, and the Acrogynae, the antheridia are protected by the leaves : in the Acrogynaa the antheridia are borne, singly or several together, in the axils of leaves ; and in some forms (e.g. Scapania, Lejeuuia, Frullania) the upper lobe of the protecting leaf is modified in form. The distribution of the sexual organs varies even in the species of some of the genera. Among the Anacrogynae, Metzgeria, Pseudoneura, Morkia, Umbra- culum, Petalophyllum, Spliaerocarpus, Haplomitrium, Blyttia (generally), and Stella helicophylla, are dioecious; whereas Pellia, Aneura (some species), Fossombronia, Symphogyna, Androcryphia, and Itiella Reuteri, are generally monoecious. In the monoecious forms the antheridia and archegonia ate generally borne on distinct branches (diclinous), but sometimes on the pame branch (monoclinous), as in Androcryphia and occasionally in Blyttia. When a dorsiventral shoot bears only antheridia or archegonia, they are developed in the median line ; but when it bears both organs, the archegonia are median and the antheridia lateral. The Acrogynaa are generally monoecious and diclinous. The structure of the adult shoot of the Jungermanniaceaa is very simple. In thalloid forms which have no midrib, the shoot consists of parenchymatous cells forming a single layer at the margin and several layers (e.g. Pellia, Aneura, Sphaerocarpus), in the middle line of the shoot ; in those which have a midrib (e.g. M>tzgeria, Symphogyna, Blyttia, etc.), the midrib consists of several layers of cells, whereas the lamina consists of only a single layer. In Symphogyna and Blyttia the midrib is traversed by a strand of elongated prosenchymatous cells having thickened and more or less pitted walls. In Blasia the stem has an axial strand of elongated cells with delicately pitted walls. In the Acrogynaa, the stem generally consists of an axial strand of relatively thin-walled cells, surrounded by a cortex of narrow thick-walled cells; but in those forms which give rise to endogenous ventral branches (e.g. Calypogeia) the cortical cells of the ventral surface of the stem are not thick-walled. The leaves are simply single layers of similar cells, and have no midrib. The root-hairs are, in all cases, destitute of the peculiar thickenings so characteristic of the Marchantiaceae. B. The SPOKOPHITE. The course of the development of the sporophyte is, in its main features, essentially the same throughout the Jungermanniaceae. The oospore is divided by a transverse (basal) wall into two halves, epibasal and hypobasal. The epibasal cell gives rise to the capsule and its stalk (seta). It divides transversely, and the longitudinal divisions follow in both cells so that the epibasal half of the embryo consists of two tiers of each consisting of four cells. Further growth in length is effected by the cutting off, by trans- verse walls, of segments from the cells forming the apical tier ; but this apicnl growth is arrested, sooner or later, by the formation of walls parallel to the free surface (periclinal) in the apical cells, and also frequently in some of those below them, which indicate the differentiation of the capsule-wall (amphithecium) from the intprnal mass of cells (endothecium) which give rise to the spores and elaters. The cells below the capsule may, however, continue to grow and divide transversely, and by means of this intercalary growth the full length of the seta is attained. 850 PART III. THE CLASSIFICATION OF PLANTS. Though the mode of growth of the epibasal half of the embryo is most com- monly that described above, it not infrequently happens that the growth of the two longitudinal halves is unequal, owing to the fact that the first longitudinal wall is not perpendicular to the basal wall, but is oblique. Consequently the apex is occupied by the two cells of the longer half, and in those cases in which the apical growth is long-continued the result is that the capsule may be developed entirely from one longitudinal half of the embryo. This mode of development has been observed in Blasia, Pellia, Fossombronia, Haplomitrium, and Symphogyna ; in Fossombronia and Symphogyna an embryo of this kind appears to grow in length, for a time at least, by means of a two-sided apical cell. In many of the Jungermanniacea? (e.g. Pellia, Lepidozia, Jungermannia, Calypogeia, Frullania) the lower end of the seta developes into a bulbous mass of cells forming a false foot, the upper margin of which grows up so as to form a sheath round the lower part of the seta in some cases. The development of the hypobasal portion of the embryo is comparatively insignificant; in most cases it is merely a small appendage to the lower end of the seta. The hypobasal cell enlarges somewhat, without undergoing any division (e.g. Radula, Madotheca, Lepidozia, often in Pellia and Symphogyna) ; or it undergoes transverse division to form a filament of two or three cells, the lowest of which becomes elongated and grows down among the cells at the base of the archegonium (e.g. Metzgeria, Aneura, Jungermannia bicuspidata) ; or it divides by a longitudinal wall into two cells which gr >w out into papillae (Frullania dilatata), or, by further division its produces four papillae (Calypogeia TrMtonumit) ; or, finally, it divides irregularly to form a small group of cells (Blasia). In some forms, however (e.g. Fossombronia, Petalophyllum, probably also Sphasrocarpus, Riella helicophyila, Notarisii, and Eeuteri), the hypobasal cell appears to give rise to a true foot, bulbous in form, comparable to that of the Marchantieae (see p. 340). In the further differentiation of the capsule, the cells of the amphithecium undergo periclinal division so that the wall eventually consists of two or more (up to six) layers of cells ; though in Haplomitrium, Sphaerocarpus, and Riella, the wall consists throughout of a single layer of cells. In the wall-cells annular thickenings are developed, which are usually transverse ; but in Haplomitrium there is a single annular thickening situated longitudinally ; the cells are un- thickened in Spheerocarpus and Riella. The planes of dehiscence of the cap- sule, except in those forms which dehisce irregularly (Riella, Sphasrocarpus), are marked out by four longitudinal rows of small-celled tissue which correspond in position with the walls between the four apical cells of the growing embryo. The archesporium, which is co-extensive with the endothecium, presents various degrees and forms of differentiation. In Riella and Sphasrocarpus it comes to consist of a number of cubical cells, some of which become the mother-cells of the spores, whereas the others persist as unaltered sterile cells. In all the other Jungermanniaceae some of the cells of the endothecium are sterile, but they develope into elaters, becoming elongated in form and spirally thickened, having sometimes two spirals (Haplomitrium, Plagiochila, Scapania, Jungermannia, Lophocolea, Lepidozia, Eadula, Fossombronia, Symphogyna, Blasia, etc.), or only one (in the Jubuleae, e.g. Lejeuuia, Frullania, and in GROUP II. BEYOPHYTA : HEPATIC^E. 351 Aneura, Metzgeria). The relative arrangement ofithe sterile and fertile ceils, dependent upon the growth of the capsule along different diameters, varies somewhat. In the lower forms, the elongated archesphorial cells are arranged more or less longitudinally, either quite straight (e.g. Frullauia, Lejeunia, Symphogyna, Umbraculum), or radiating from the apex of the capsule (Metzgeria, Aneura), or radiating from the base of the capsule (Pellia, Eadula) : whereas in the higher forms (Lepidozia, Calypogeia, Jungermaunia), these cells are placed horizontally round a central longitudinal axis, except at the apex where they radiate. In most cases the sterile and fertile archesporial cells are mingled together, but in some cases certain parts of the archesporium give rise especially to epores and others to elaters. Thus, in Pellia, the cells towards the base and those in the longitudinal axis of the capsule form only elaters, whereas in Jnngermannia the formation of elaters is confined to the cells near to the wall. Whilst the development of the embryo is taking place, growth is also proceeding in the archegonium and the adjacent tissue, giving rise eventually to the calyptra. Several of the archegonia of a group may be fertilised, but generally only one gives rise to a fully developed sporogonium, and itself takes part in the formation of the calyptra. The calyptra is sometimes developed from the venter of the archegonium alone (e.g. generally in the Acrogynaa, and in Metzgeria, Fossouibronia) ; in the Anacrogynas the adjacent tissue of the shoot frequently takes part in its formation, as is shown by the fact that the unfertilised archegonia of the original group are found on the sides, or even on the top of the calyptra (e.g. Aneura, Svmphogyna). The wall of the catyptra consists of one or more layers of cells, and keeps pace with the growth of the embryo which it encloses until the spores are mature. The cells of the seta then rapidly elongate, causing the rupture of the calyptra, and the capsule is exposed. The capsule then dehisces, generally into four valves, eometimes irregularly, and the spores are set free. The Jungermanniacea3 may be classified as follows : Series I. ANA.CROGYN;E : growth in length not necessarily arrested by the development of archegonia. Section A. Anelatereae : the sterile cells in the capsule do not develope into elaters. Fam. 1. Riellete : including the two genera Kiella (foliose) and Sphaerocarpus (thalloid) ; further characterised by the absence of annular thickenings in the cells of the wall of the capsule, by the irregular dehiscence of the capsule, and (probably) by the development of a true (hypo- basal) foot. Section B. Elatereae : the sterile cells in the capsule develope into elaters. a. Thalloid Forms. Fam. 2. Metzgeriece : genera, Aneura and Pseudoneura (Riccardia), Metzgeria. ,, 3. Diplomitriftf : Blyttia and Morkia (Pallavicinia), Umbraculum (Hymenophyton), Pellia, Symphogyna. Foliose Forms. /3. Fam. 4. Codoniece : Fossombronia, Androcryphia (Noteroclada), Petalophyllum, Blasia. ,, 5. Haplomitriece : Haplomitrium, Calobryum. 352 PART IIT. THE CLASSIFICATION OF PLANTS. Series II. ACKOGYN.E : growth in length arrested by the development of archegonia ; all foliose. Fam. 6. Gymnomitriece : Gymnomitrium, Sarcoscyphus (Marsupella) , Alicularia (Nardia), etc. 7. Jungermanniea; : Plagiochila, Jungermannia, Lophoco'ea, Scapania, etc. 8. Geocalycece : Saccogyna, Calypogeia, etc. 9. Trichomauoidece : Lepidozia, Mastigobryum, etc. 10. Ptilidea : Tricbocolea, Ptilidiurn, Seudtnera, etc. 11. Platyphyllea : Kadula, Madotheca. 12. Jubulece : Phragmicoma, Lejeunia, Frullania, etc. Order III. Anthocerotaceae. A. The GAMETOPHYTE. The protonema developed from the germinating spore is a flattened plate of cells; in Authoceros the formation of the flattened plate is sometimes preceded by the outgrowth of the contents of the spore, covered by the endosporium, into a filament at the apex of which the plate of cells is developed ; in Dendroceros the first cell-divisions connected with the formation of the protenema frequently take place widiin the spores before they are shed from the sporogonium. The adult shoot is developed as a lateral out-growth from the flattened protonema. The Morphology of the adult shoot. The adult shoot is thalloid, and its symmetry is dorsi ventral. It is semi circular, or nearly circular, in outline in Anthoceros and Notothylas; but in Dendroceros it is elot'gated and branched, in which genus it has a FIG. 246. Anthoceros Icevis K (nat. size). The capsules, some as yet unopened. strongly developed midrib which projects especially on the ventral (under) surface. There are no ventral scales on the under surface, but numerous uni- cellular root-hairs. The growth of the shoot is effected, in Anthoceros and Notothylas, by a series of marginal growing-points ; in Deudroceros, by a growing-point at the apex of the main shoot and of each of its branches. In the growing-point there is a row of initial cells, each of wbich acts as an apical cell ; their form is wedge- shaped in Anthoceros, dorsal and ventral segments being alternately cut off by the formation of oblique walls; in Dendroceros their form is nearly hemispheral, the base being internal, and segments are cut off by the successive formation of transverse walls at right angles to the long ax : s of the shoot. Branching, or at least the formation of new growing-points, takes place in the manner described for the Marchantiaceae (p. 184). The sexual organs are developed from the dorsal segments formed in the growing-point, and are situated in the middle line behind each growing-point in Anthoceros and Notothylas. on the midrib in Dendroceros. The antheridia, which have a long stalk in Dendroceros, are developed endogenously, and remain enclosed in the tissue until maturity ; they are developed either singly (some species of Anthoceros, Dendroceros) or in groups. The archegonia are sunk in the tissue, the apex of the neck reaching to the dorsal surface of the shoot. The shoots are monoecious ; the sexual organs are sometimes intermingled in the same group (frequently in Anthoceros). GROUP II. BRYOPHYTA : HEPATIC.E. 353 la exceptional cases the autheridia of Anthoceros may be developed at the surface, i.e. exogenously. The structure of the adult shoot. The adult shoot, in Anthoceros an<i Notothylas, consists of several layers of cells in the middle line, thinning out to a single layer of cells at the margins. The tissue in the middle line consists of longitudinally elongated cells, the walls of which, especially in the older parts of the shoot, frequently present reticulate or even spiral thickening. In Dendroceros there is a prominent midrib, on each side of which is attached a laminar portion, consisting of a single layer of cells ; in some species there are no intercel'ular spaces, but in others there are air-chambers in the mid- rib, which give to its surface the areolated appearance so well-marked in the Marchantiaceaa (see p. 338). A characteristic structural feature is the presence of apertures of the nature of pores in the superficial layer of those parts of the shoot which consist of several layers of cells. They occur generally only on the under (ventral) sur- face of the shoot, but in some species of Dendroceros (D. javan'cus, crispatus, Breutelii) they are present on the dorsal surface also. The pore is bounded by two guard-cells, formed by the division of one of the superficial cells. It leads into a cavity which is, from the first, filled with mucilage. Into this cavity the Alga Nostoc obtains access through the pore, and there grows and multiplies. The growth of the Nostoc in the cavity is accompanied by active growth of the cells of the surrounding tissue, so that the whole mass projects from the surface of the shoot, and the guard-cells grow and divide, so that the cavity becomes completely closed. At the same time, filamentous septate out- growths arise from the cells of the wall of the cavity, which ramify throughout the mucilaginous mass (comp. Blasia, p. 347). The chloroplastids of the Anthocerotaceae are peculiar, on account of their relatively large size, and of the fact that they occur singly in the cells and con- tain a pyrenoid (see p. 100). B. The SPOBOPHYTE. The early stages in the development of the sporophyte of the Anthocerotacea appear to be much the same as in the case of other Liverworts (Fig. 239 C). The oospore divides transversely into an epibasal and a hypobasal half : and each of these divides by two perpendicular walls so that the embryo consists at this stage of eight cells. The cells of the epibasal half divide transversely several times, and then further apical growth in length is arrested by the formation of periclinal walls, marking the differentiation of amphithecium and endothecium, first in the four apical cells, and subsequently in those below them. By the repeated formation of periclinal walls, the amphi- thecium comes to consist of several layers of cells. The hypobasal cells under- go but few divisions, giving rise to a bulbous foot, the superficial cells of which grow out into papillae and penetrate between the cells of the adjacent tissue of the gametophyte. The details of the differentiation of the epibasal portion of the embryo vary considerably. In Anthoceros and Dendroceros (Figs. 238, 239) the archesporium is developed from the innermost layer of cells of the amphithecium, a peculi- arity, the only other instance of which, in the Muscmeaa, is to be found in the Sphagnaceae (p. 331) : the endothecium gives rise to an axial strand of sterile tissue, termed the columella, which is completely invested (except at the base, V. S. B. AA 354 PART III. THE CLASSIFICATION OF PLANTS. where it is continuous with the tissue of the foot) by the archesporium. In Notothylas. the archesporium is commonly co-extensive with the endothecium ; but in some forms the archesporium is developed from only the external layer of the endothecium, the rest of the endothecium constituting a columella (as in the Bryineae among the Musci). In all three genera some of the cells derived from the archesporium are sterile. In Dendroceros these cells develope into elaters with spiral thicken- ings, each elater consisting of a row of several cells with an apparently con- tinuous spiral band. In some species of Anthoceros (e.g. vicentianus, giganteus, etc., constituting the subgenus Anthocerites) there are elaters quite similar to those of Dendroceros : in other species (e.g. tuberculatus, glandulosus) the elaters have the same form, but they have no spiral band ; in others (Icevis, punctatus) the sterile cells do not form distinct elaters, but a network of sbort cells, with spiral thickening, in the meshes of which lie the mother- cells of the spores. In Notothylas the sterile cells do not develope into elaters ; they are so arranged as to form chambers, in which lie the mother-cells of the spores. The sporogonium of Anthoceros and Dendroceros has no seta ; when the apical growth has ceased, the capsule continues to elongate by basal growth, and hence does not ever become fully mature throughout. In Notothylas the sporogonium has a short seta, and elongates by intercalary growth, the grow- ing-point being situated at the junction of the capsule with the seta ; but the intercalary growth is of limited duration, so that the whole capsule becomes mature throughout, and frequently becomes disconnected from the seta. The pod-shaped capsule of Anthoceros and of Dendroceros splits from the apex into two valves (Fig. 246). Stomata occur in the epidermis of the capsule in most species of Anthoceros, but they appear to be wanting in the other genera. Since the archegonia are sunk in the tissue of the shoot, the calyptra, which invests the developing embryo, is developed mainly from the surrounding tissue, and only to a small extent from the wall of the arcliegonium. CLASS IV. MUSCI (Mosses). A. The GAMETOPHYTE. The protonema is more conspicuous in the Musci than in the Hepaticae : it sometimes persists until the sporogonia are developed and the spores are ripe (e.g. Ephemerum), and in many cases the subterranean portion persists from year to year. It is generally filamentous and much branched ; but in some forms it is a flattened expansion (e.g. Sphagnum, Andreaea), or cylindrical branched and shrubby (Andreaea), or it bears lateral flattened expansions which are assimilatory organs (e.g. Andreaea, Tetraphis, etc.). The filamentous protonema consists of a sub- aerial and of a subterranean portion, which differ in that the cells of the former contain chloroplastids, their walls are colourless, and the septa are transverse ; whereas those of the latter do not GROUP II. BEYOPHYTA : MUSCI. 355 contain chloroplastids, and their walls are brown and their septa oblique. The protonema presents, in fact, a certain differentiation into shoot and root, the term rhizoids being applied to the root-like filaments. This differentiation is, however, of little morphological value, since the differences between the shoot- and root-filaments depend entirely on external conditions : thus, if the rhizoids be exposed to light they assume the characters of the subaerial filaments. In Andreeea the differentiation is altogether wanting. The flattened protonema of Sphagnum bears rhizoids on its margins and under surface. The growth in length of the protonemal filaments is apical : the terminal cell behaves as an apical cell from which segments are successively cut off by transverse or oblique walls. The Adult Shoot arises as a lateral bud on the subaerial portion of the protonema. In some cases (Bryineee) the subterranean portion gives rise to lateral buds : these are small, spherical or lenticular, multicellular bodies of a brown colour, filled with reserve materials, and are termed bulbils', when they are brought to the surface they give rise to adult shoots, either directly or with the interven- tion of protonema. The adult shoot is in all cases differentiated into stem and leaves, and may be branched or unbranched. Its symmetry is commonly radial or isobilateral, less commonly dorsiventral. In the former case it is attached to the soil by rhizoids springing from its basal portion ; in the latter, by rhizoids developed on its under surface. In Sphagnum, rhizoids occur only on young shoots. The growth of the adult shoot (and its branches) is affected by means of an apical growing-point with a single apical cell which is generally a three-sided pyramid : in Fissidens, however, though the subterranean shoots, and in some species the subaerial lateral shoots, have a three- sided apical cell, yet, under the influence of light, the apical cell eventually becomes two-sided. Each segment cut off from the apical cell gives rise to a leaf : hence the arrangement of the leaves, and the symmetry of the shoot, is generally determined by the form of the apical cell. Thus in Fissidens, the leaves are arranged in two rows and the symmetry of the shoot is isobilateral : in other cases (e.g. Fontinalis) the leaves are in three rows, and the symmetry of the shoot is radial. But to this rule there are exceptions. Thus, in the sterile shoots of Schistostega, although the apical cell is three- 356 PART III. THE CLASSIFICATION OF PLANTS. sided, the leaves are arranged, in consequence of subsequent dis- placement, in two rows, and the symmetry of the shoot is isobi- lateral. Again, in Sphagnum, Polytrichum, Andreasa, etc., tho leaves are not arranged in three rows with a divergence of -|-, but spirally with divergences of f, -f, etc., because the walls of the segments cut off successively from any one side of the three-sided apical cell are not parallel to each other, but are inclined at an angle. The symmetry of the shoot is, however, radial. Branching is confined to perennial shoots, and is lateral, never dichotomous. When the growth of the main shoot is arrested by the formation of sexual organs at the apex (acrocarpous), one (or more) of the lateral branches (termed innovations) close