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Volume 82, 1954-55
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The Structure and Development of Monoclea forsteri, Hook.

[Read before the Otago Branch. August 11, 1953; received by the Editor. August 24. 1953.]

Summary

The form and structure of the thallus is described and it is shown that growth is due to a wedge-shaped apical cell. An account is given of the development of the antheridium. Early stages in the development of the archegonium resemble Calobryum rather than typical Hepaticae. The sporogonium has a free-nuclear stage in its early development and differentiation of the main regions occurs late. Germination of the spore is described. The significance of some of these features in relation to other Hepaticae is discussed.

Introduction

The genus, Monoclea, comprises two species, M. forsteri, Hook, of New Zealand and Patagonia, and M. gottschei, Lindb. found in Central and South America, Jamaica and Japan (Stepham, 1900). Recently M. forsteri has been reported as occurring in Chile and Peru (Proskauer, 1951). Judging from the descriptions of the two species, (Stephani, 1900), M. forsteri is the stouter, heavier plant, olive green rather than pure green, with larger involucres and capsules; but apart from the size difference the two species are closely similar.

Studies dealing with the structure and development of Monoclea have been made mainly on material from Central America and Jamaica. The most comprehensive account is that by Johnson (Johnson, 1904) on material collected in Jamaica and, although he uses the name M. forsteri, the species concerned would appear to be M. gottschei as indeed Campbell has indicated (Campbell, 1940). The New Zealand species was described by Hooker (Hooker, 1820) from material collected by Forster. It has been studied by Kirk (Kirk, 1877), by Leitgeb in 1881 from alcoholic material (Johnson, 1904), and by Cavers from fresh material forwarded to him (Cavers, 1904b). More recently Proskauer has studied material from Chile, Peru and New Zealand (Proskauer, 1951).

As the literature contains a number of conflicting statements, it seemed desirable to re-examine the New Zealand form of M. forsteri and for that reason the present work was undertaken.

General Description Of Monoclea Forsteri

Monoclea forsteri is a large, thallose liverwort of a dark green or of an olive green colour. The thallus reaches a maximum length of 20 cm. and a width of 5 cm. It branches dichotomously at frequent intervals, the resulting structure being irregularly lobed with some portions growing over others. Male plants can be recognised by the antheridial clumps which bulge from the ventral surface and on the dorsal surface appear as truncated cushions with a somewhat raised posterior edge; these lie on the median line of the thallus (figs. 1 and 2). On the female plants flask-shaped cavities occur at the mid-point of each lobe and

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cause a bulge on the ventral surface (figs. 3 and 4). The cavities are 1.5 to 2 cm. long and are 3 to 5 mm. wide at the base. They narrow to a small pore opening on the dorsal surface of the thallus. Situated on a receptacle on the floor of the cavity are from six to thirteen archegonia intermixed with slender mucilage hairs. Later the ripe sporogonia project from the mouth of the cavity and in dry air the capsules open by a single slit on the ventral surface.

Occurrence In New Zealand

Monoclea frequently forms extensive mats in forest creek-beds in New Zealand. It is absent from the lowland rain-forest of Westland but is very abundant in the Metrosideros-Weinmannia forest near Waiho, South Westland, and in podocarp forest near Dunedin.

Over large areas the plants are sterile, especially in positions where they are submerged for long periods as on stones in streams and under waterfalls. Fertile plants are found on well-drained banks at the sides of streams. Near Dunedin practically all the fertile plants bear archegonia. Despite an extensive search along all the streams within a radius of thirty miles from Dunedin the writer fount only two antheridial plants. It follows, then, that fertilization must be a rare occurrence. No sporogonia were seen by the writer, nor is there any record of their being collected previously in this region. In South Westland, however, the position is different. Male and female plants frequently grow intermixed on banks and on fallen logs. Fertilization takes place freely and, as antheridia and archegonia mature throughout the year, all stages in the development both of these organs and of sporogonia can be collected by sorting through a large amount of material.

Structure of The Thallus

The vegetative thallus has no distinct midrib but is about ten cells thick in the central region and thins out to a depth of one to four cells towards the margins. Thalli bearing antheridia or archegonia are thicker, up to sixteen or eighteen cells in depth. The thallus shows little differentiation except that the surface cells are smaller than the others, and in fertile plants the cells of the central layers are elongated in the sagittal axis of the thallus (figs. 2 and 4). A mycorrhiza was not found in any of the plants examined. Chloroplasts are present mainly in the epidermal and subepidermal cells on the dorsal surface and in the epidermal cells on the ventral surface, but some chloroplasts are fount in all the cells Starch grains are found in small numbers in deeply-lying cells. The walls of all the cells have slit-like pits similar to those figured for Conocephalum (Cavers, 1904a, fig. 21). Some cells have the brown oil bodies frequently found in the Marchantiales (Rivett, 1918). In many of the superficial dorsal cells, and in cells to a depth of six or seven below, one or sometimes two large crystal aggregates (druses) of calcium oxalate occur In fresh material the crystals tend to be obscured by the chloroplasts.

Rhizoids arise over the entire undersurface of the thallus. The usual type are narrow, thick-walled rhizoids nine microns to twenty-five microns in width, some of the wider ones growing straight down into the soil, and others lying parallel to the surface of the thallus and tending to aggregate in a brown tuft along the middle line. A few have rudimentary pegs especially near the proximal

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Text-Fig. 1.
Fig. 1.—Portion of a male plant. × 1. Fig. 2.—Outline drawing of an antheridial receptacle in vertical section, showing its relation to the thallus apex. e. elongated cells. × 27. Fig. 3.—Portion of a female plant. The capsule on the left is mature; the one in the centre has opened; that on the right shows the margins curled up exposing the spores and elaters. × 1. Fig. 4.—Outline drawing of a vertical section of an archegonial cavity, in which not all the archegonia were mature. e. elongated cells, r. receptacle. × 20. Fig. 5.—Outline drawing of a vertical section of the thallus apex showing the position of the mucilage hairs relative to the apical cell. a. apical cell. × 75. Fig. 6.—Horizontal section of the thallus apex. a. apical cell. × 230.

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end, but the majority show no signs of pegs. In the central region of the thallus occur rhizoids of a different kind. Thin-walled and of diameter 25 to 35 microns, they are of the smooth type and always grow straight down into the soil.

Growth of the thallus takes place from a wedge-shaped apical cell with four cutting faces which lies at the base of an apical notch (figs. 5 and 6). On the ventral side of the notch large mucilage hairs arise and overlap one another enclosing the apex (fig. 5). These hairs wither off at a short distance posterior to the growing point.

Structure And Development Of The Antheridial Receptacle And The Antheridia

The form of the antheridial receptacle and the structure and development of the antheridium were found to correspond with the description given by Johnson (Johnson, 1904), except that in M. forsteri the receptacle is shorter, being two to three mm. long and three to four mm. wide. Sometimes a dichotomy occurs while the receptacle is forming and then two receptacles merge. In each receptacle there are twenty-five to fifty antheridia formed acropetally in radiating rows. The apex later continues the normal thallus growth leaving the receptacle as a flat cushion on the dorsal surface of the thallus (fig. 2).

In vertical sections the first indication of the development of the receptacle is that the apical depression becomes deeper than usual (fig. 7). Then, owing to more rapid growth and to repeated divisions in the ventral segments, the growing apex is for a time swung round on to the dorsal surface of the thallus, with the result that the ventral segments take up a position anterior to the apical cell and the dorsal segments become posterior. Club-shaped mucilage hairs arising from the anterior segments curve upwards and backwards over the apex (figs 2 and 7).

The antheridia arise in segments cut off posterior to the apical cell. These segments divide at first by anticlinal walls in the sagittal plane. When one of the resulting cells divides transversely and the outer densely-staining cell projects slightly, this projecting cell is recognisable as the antheridial initial (fig. 8). The cell below and surrounding cells add to the thallus. The antheridial initial divides transversely into an inner or stalk cell and an outer cell (fig. 9). The latter by transverse division forms a row of four cells which then divide by vertical and subsequently by periclinal walls as in typical Marchantiales (figs. 10–12). This results in one layer of peripheral, jacket cells enclosing the central primary spermatogenous cells. The stalk cell, meanwhile, first divides vertically; subsequent transverse divisions produce a stalk three tiers of cells high. Later, the cells of the stalk may undergo further division.

The young antheridium early separates laterally from the adjoining cells and for a short time projects above the thallus surface, but due to repeated divisions in the surrounding cells there is formed an upward-growing platform of tissue which gradually rises above the antheridia leaving them sunk in flask-like pits (figs. 2, 11 and 12). This platform, like the antheridia, develops acropetally. The walls of the antheridial pits are lined with mucilage hairs of a more slender type than the club-shaped hairs associated with the thallus apex, and the older cavities are filled with mucilage. A half-grown antheridium is illustrated in Fig. 13. The mature antheridium is of similar conical shape with a short stalk which becomes compressed as the enlarging antheridium fills the pit. The jacket

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Text-Fig. 2
Fig. 7.—Vertical section of a young antheridial receptacle, showing the apical cell, mucilage hairs, and a young antheridium. × 230. Fig. 8.—Vertical section of a young antheridial receptacle showing the apical cell and antheridial initial. × 250. Fig. 9.—Vertical section of a two-celled antheridium; the lower cell is the stalk initial. × 250. Fig. 10.—Vertical section of an antheridial receptacle showing the apical cell and two young antheridia. × 250. Fig. 11.—Vertical section of a young antheridium showing vertical walls in the upper cells and in the stalk region. × 250. Fig. 12.—V.S. of a young antheridium, showing its location in a flask-shaped pit, and the development of mucilage hairs. The stalk is three tiers high. × 250. Fig. 13.—V.S. of an antheridium about half-grown. × 185. Fig. 14.—V.S. of a young archegonial cavity, showing mucilage hairs arising from the floor and from the ventral lip. × 230.

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remains uniseriate except in the multicellular, tapering cap. The central region is occupied by a mass of spermatogenous cells, in the formation of which the last division is a diagonal one.

Structure Of The Archegonial Cavity And The Archegonia

The archegonial cavity is shaped like a flask flattened in a dorsiventral plane. Situated on its floor is the receptacle on which the archegonia arise. Typically, the cavity lies at the mid-point of a lobe of the thallus, midway between two growing points which are responsible for further vegetative growth of the thallus. Very occasionally only one new growing point develops and the cavity then appears to be in a lateral position.

The wall of the cavity has been called an involucre (Stephani, 1900), but it would seem preferable to reserve the term involucre for protecting flaps or sheaths such as occur in Targionia and Marchantia. In Monoclea the wall of the cavity is formed from the surrounding thallus, the ventral portion from the ventral half of the thallus and the dorsal portion from the dorsal half. At the base of the cavity the ventral and dorsal walls are each half the thickness of the thallus immediately posterior.

The formation of the archegonial cavity is first indicated in vertical sections by a depression slightly deeper than the apical depression (fig. 14). Arising from the ventral lip and also from the floor of the very young cavity are club-shaped mucilage hairs similar to those usually associated with the thallus apex. The whole cavity is filled with mucilage. All the cells at the base of the young cavity stain densely and divide repeatedly, forming a cushion-shaped receptacle of small cells which widens the base of the cavity as it enlarges (fig 4). No apical cell could be located amongst them, but they have the appearance of the actively-dividing derivatives of an original apical cell. The neck of the cavity, on the other hand, does not widen and the pore always remains so small as to be scarcely discernible. When the cavity has deepened a little, there arise on the receptacle slender mucilage hairs of a different type to those originally present. The original club-shaped hairs in the cavity and on the ventral lip now wither off.

Archegonia first make an appearance when the cavity has reached a length on 0.2 to 0.5 mm. They arise from the floor amongst the mucilage hairs in no definite order and at first cannot be distinguished from developing hairs. The initial cell of the archegonium divides at first by transverse walls. Then the terminal cell divides by inclined vertical walls which meet in such a way as to cut out an axial cell surrounded by jacket cells (fig. 15). The axial cell apparently functions as a central cell (fig. 16). Such a condition is found also in Calobryum blumei (Campbell, 1920) but is not typical of Hepaticae as a whole. An older stage in development is illustrated in fig. 17.

In the mature archegonium the venter, in places at least, is two cell-layers thick (fig. 18). The neck is exceptionally long, almost one mm. very much spirally twisted and often curved especially as the archegonium becomes older; it is formed of six rows of cells. The axial canal is filled by the egg, ventral canal cell, and 14 to 16 neck canal cells.

Fertilization takes place when the cavity is two to three mm. long or when further enlargement has occurred. Increase in size of the cavity is independent of fertilization for large cavities are found containing only old unfertilized archegonia.

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Text-FIG 3
Figs. 15 and 16.—Vertical sections of young archegonia. × 225. Fig. 17.—Vertical section of an older archegonium. × 225. Fig. 18.—Longitudinal section of the basal portion of a mature archegonium. × 225. Fig. 19.—A. B. C. Vertical sections of a young embryo with four free nuclei B is the median section; the distance from A to B is 1/50 mm. and from B to C 1/25 mm. × 250. Fig. 20.—V.S. of an embryo showing six of the twenty-six free nuclei. × 250. Fig. 21.—V. S. of an embryo after walls have formed. × 250. Fig. 22.—V.S of an older embryo. f. approximate location of foot, s. approximate location of seta. × 250.

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Development Of The Sporogonium

The longitudinal diameter of the egg at the time of fertilization is 0.125 mm. This is considerably greater than that of other liverworts for which measurements we e available, the range being from 0.015 mm calculated from figures of Cyathodium (Lang, 1905) to 0.063 mm. in Marchantia. Calculating from the figures given by Johnson (Johnson, 1904), the size in Monoclea gottschei is 0.081 mm.

The early stage in the embryo development is a free-nuclear stage (figs. 19 and 20). Over 40 embryos at this stage were sectioned, the number of nuclei ranging from four to twenty-six. Round the embryo is a distinct membranous wall. This is lined by a thin film of cytoplasm in which the nuclei he more or less evenly spaced. While the free nuclei are forming, the venter increases in length and in thickness to form the calyptra. It is 3 to 4 layers thick at the four-nucleate stage and 7 layers thick at the 26-nucleate stage. The archegonial neck does not wither but is carried up by the enlarging calyptra; chloroplasts appear in the lower half.

About the time when there are twenty-six free nuclei, cytoplasm accumulates in the foot region and cytoplasmic strands appear throughout the embryo. In these strands walls are laid down, tending to take the form of a vertical wall and transverse walls although there is no regularity in their position (fig. 21). By the time walls are formed the embryo has twenty-nine to thirty nuclei and the venter is eight to ten cells thick.

From now on the cells of the embryo divide rapidly with no apparent regularity (figs. 22 and 23). The capsule region gradually becomes recognisable by its smaller cells with denser contents. If the seta region is slightly narrower then the foot and the capsule as shown in fig. 23, it can be distinguished from the foot, but, if it is of the same diameter, the boundary between the two regions cannot be determined with certainty. Later the cells of the seta region divide almost altogether by transverse walls and regularity of cell arrangement is characteristic of this region, marking it off from both the foot and the capsule. In the capsule there soon develops a fairly regular layer of peripheral cells but, whereas anticlinal divisions are frequent, preiclinal divisions also take place for some time, so that these peripheral cells cannot be regarded as jacket cells. They have dense cytoplasm, similar to other cells of the capsule region. The jacket is clearly differentiated when the peripheral cells remain cubical with less dense cytoplasm whereas the central cells assume a fusiform shape (fig. 24). These spindle-shaped cells gradually lengthen and then separate from one another Some remain one-celled and will form elaters; others divide to give a row of eight spore mother cells, each file being of fusiform shape (fig. 25). The elaters as noted by other writers, become extremely long, from 400 to 700 microns, with a diameter of 7.5 microns at the widest part and with slightly tapering ends Brown thickening in the form of a double spiral band is laid down on the wall as the elater matures (fig. 28). The spore mother cells round off and separate from one another. Just before the capsule is ripe the nucleus of the spore mother cell divides into four and the cytoplasm cleaves into four areas each of which secretes a firm wall around itself. As a result tetrads of spores are formed. The cytoplasm of the spore mother cell was not found to become lobed in outline prior to the nuclear division (fig. 26), although this feature which occurs in Junger-manniales has been reported for Monoclea (Johnson, 1904). Slightly later stages.

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Text-Fig. 4.
Fig. 23.—V. S. of a young sporogonium before any regions are sharply demarcated. × 250. Fig. 24.—Diamond-Shaped cells from the initial region of the capsule. × 400. Fig. 25.— Two files of spore mother cells and an elater cell. × 400. Fig. 26.—Two spore mother cells after meiosis, and part of an elater cell. × 400. Fig. 27.—Spore tetrads within the spore mother cell wall. × 400. Fig. 28.—A mature elater and two ripe spores. × 450. Fig. 29.— V. S. of the top of a ripe capsule. × 55. Figs. 30–36.—Stages in germination of the spore. × 225.

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present a lobed appearance due to the arrangement of the spores within the spore mother cell wall (fig. 27) but soon the spores become separate and free. The ripe spore is oval to almost spherical in shape, green in colour with a brown wall; its diameter is twenty microns by ten to fifteen microns.

All the time the capsule is developing it is bright green in colour due to the chloroplasts in the sporogenous cells. A few chloroplasts are also present in the jacket of the capsule, in the seta tissue and in the calyptra. The foot has very few chloroplasts; it remains short, about one millimetre in length, and is quite loosely attached to the gametophytic tissue. When the sporogomum is fully developed the foot can be recognised by its small, irregularly arranged cells containing starch grains.

The jacket of the capsule is uniseriate even at the top (fig 29). This is in contrast with the capsules described by Johnson which had a two-layered cap (Johnson, 1904, fig. 48). The jacket cells at the top of the capsule are approximately isodiametric; elsewhere they are elongated lengthwise. The cells on the dorsal side are wider radially than those on the ventral side About the time that the spore tetrads are forming the jacket undergoes change. Bars of thickening are laid down on all the cell walls except the outer one. On the inner walls the bars form a network and they tend to form a network on the lateral walls. Two layers of cells across the base of the capsule also have thickened bands. The thickenings become brown in colour and the capsule as a whole appears brown.

Up to the stage at which the capsule is mature and brown in colour it is still enclosed in the calyptra, but the seta now elongates rapidly carrying the capsule out of the calyptra and out through the pore of the cavity in twenty-four to forty-eight hours. The seta is about forty cells in height, with the length of the individual cells ranging from 20 to 35 microns. The rapid elongation of the cells to a length of 280 to 600 microns results in extension of the seta from one mm. to a length of 14 to 24 mm. The calyptra forms a colourless, fleshy sheath about 10 mm. long round the base of the sporogonium. The seta, as it elongates, is positively phototropic. If the thallus is erect the seta is straight and stands vertically; more often the thallus is growing horizontally and the seta then curves so that the capsule is brought into a vertical position, if the thallus is growing downwards, the seta curves strongly back on itself. That phototropism and not geotropism is involved is shown by the position assumed by the seta under unilateral illumination.

The ripe capsule is five mm. long and one mm. wide and in transverse section is oval. It opens on a dry day by a single longitudinal slit on the ventral surface, the slit commencing a little below the tip where the isodiametric cells meet the elongated cells. Dehiscence is due to tension as the capsule dries, combined with the shape of the cells and the arrangement of the thickened bands. As the slit extends downwards, the two edges curve outwards and the capsule assumes a spoon shape as seen from the dorsal side. The spores are gradually freed by the twisting of the elaters.

Commonly three sporogonia mature in each cavity, sometimes four, and less frequently two or one. They may mature simultaneously or one may have shed its spores while the others of the same cavity are still immature. Where there are several sporogonia of the same age they have difficulty in emerging through the narrow pore; some may become wedged between others and remain within the cavity.

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Germination Of The Spore

Spores were sown in wardian cases on sterilized bricks moistened with Knop's solution. Spores sown fresh or after storing for two months germinated but no germination took place after eight months' storage. The spore enlarges without any rupturing of the wall to about twice its original diameter—that is, to about forty microns (fig. 30). After sixteen to thirty days it divides by a transverse wall (fig. 31). As a rule each cell now divides by a wall meeting the first wall at right angles, so producing a quadrant stage (fig. 33), although if a wall is formed in only one of the cells, a three-celled stage results (fig. 32). An apical cell with two cutting faces is delimited by the formation of an inclined wall in one of the cells at the quadrant or at the three-celled stage (fig. 34). Growth now takes place as a result of the activity of the apical cell (figs. 35 and 36). The other cells of the quadrant divide a few times. A narrow rhizoid is formed at the one-cell or two-cell stage (fig 32), or if moisture is plentiful, it does not appear till much later. It arises from no definite place. Additional rhizoids develop when the sporeling has grown considerably. The original apical cell soon becomes a wedge-shaped apical cell, and later becomes sunk in an apical notch For a time the thallus is ribbon-shaped and two to three cells thick but when about twelve mm. long it branches. The characteristic brown oil bodies make an appearance when the length of the thallus is no greater than 0.5 mm.

Discussion

Monoclea has been classified either amongst the Marchantiales or amongst the Jungermanniales. Typical of present-day opinion is the statement by Campbell that “possibly it should be placed at the beginning of the Marchantiales, near the point where the Marchantiales and Jungermanniales diverged from some common stock” (Campbell, 1936, p. 60); and the classification by Evans who places Monoclea in an advanced position amongst the Jungermanniales (Evans, 1939). Evans' classification is evaluated by Fulford in the following terms:— “this is without doubt the most up-to-date list that we have and will furnish the basic outline of arrangement of genera for a long time to come” (Fulford, 1948, p. 171). Points of affinity between Monoclea on the one hand and Jungermanniales and Marchantiales on the other are set out by Smith (Smith, 1938, p. 29).

The present paper supplies some additional evidence in favour of Campbell's view, some evidence of affinity with Calobryum, and some information which at present has no bearing on the problem of relationships Calobryum is classified either in the Jungermanniales (Evans, 1939), or in a separate order, Calobryales (Smith, 1938).

The presence at the thallus apex of a single wedge-shaped apical cell with four cutting faces indicates affinity with the Marchantiales for several of which a similar apical cell has been described (Haupt, 1926, 1929). Jungermanniales, except for Pellia calycina (Hutchinson, 1915), have an apical cell with either two or three cutting faces. It has been reported previously (Cavers, 1904b) that Monoclea has a group of apical cells, but by careful examination of median sections it can be determined that there is but a single one.

The method of formation of the spores from the spore mother cell shows affinity with Marchantiales rather than with the Jungermanniales where deep lobing of the cytoplasm of the spore mother cell occurs prior to nuclear division (Haupt, 1918). The appearance in Monoclea has been misinterpreted in the past.

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Affinity with Calobryum is indicated by the development of the archegonium (figs. 15 and 16). Johnson gives only one figure of a young archegonium (Johnson, 1904), and this shows segmentation of the type usual in Hepaticae. In Calobryum both types of segmentation are found (Campbell, 1920) and possibly both occur in Monoclea. Further collection and sectioning of material would be necessary in order to elucidate this point. The early ontogeny of the archegonium in Calobryum is regarded by Smith as “the most primitive of any in the Hepaticae” (Smith, 1938) and Evans classifies the genus in the lowest suborder of the Jungermanniales (Evans, 1939). From this point of view archegonial development in Monoclea would have to be regarded as a primitive feature. There are other points of resemblance between Monoclea and Calobryum—the long, twisted archegonial neck, the large number of neck canal cells, and the general appearance of the sporogonium. In Calobryum. blumei the capsule opens by a single slit much as it does in Monoclea (Campbell, 1920, Pl I, fig. 5).

The early embryogeny of Monoclea has no parallel amongst other Hepaticae and consequently throws no light on relationships Johnson states that he “did not find the earliest stages” (Johnson, 1904). Although Cavers gives a series of figures he states that “he has not yet been able to follow in detail the development of the sporogonium” (Cavers, 1904b). Campbell's figures (Campbell, 1940, fig. 59G. after Johnson) resemble those given by Cavers (Cavers, 1904b) but do not correspond with any of those included by Johnson in his paper (Johnson, 1904). Possibly the free nuclear division in the young embryo may have some connection with the low cytoplasm content and large size of the fertilized egg. The longitudinal diameter of the latter (0.125 mm.) is greater than that found in any other liverwort for which measurements could be obtained.

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