Some Aspects of the Life History of the Liverwort Symphyogyna hymenophyllum (Hook.) M. et N.
[Read before Otago Branch, July 10, 1948; received by the Editor, January 7, 1949.]
Symphyogyna hymenophyllum occurs all through New Zealand, but is not confined to this country, being also found in tropical and subtropical regions. It was first discovered in New Zealand at Dusky Sound by A. Menzies in 1791, when Vancouver's expedition visited this country. The original description of the species was by W. J. Hooker in Musci Exotici (1818, p. 14). Synonyms have been Jungermannia hymenophyllum Hooker, Musci Exot., p. 14; Symphyogyna flavovirens Colenso, Transact. N.Z. Inst., 1884, p. 262.
Of the thirty-nine species in the genus described by Stephani (1900, pp. 329–349) eight, are reported to be found in New Zealand.
Material And Methods
Material was collected mainly from Dunedin localities, but some was obtained from the Wellington district. Most of the killing and fixing was done in the laboratory with an air pump. The reagents used were chromacetic acid (Chamberlain, 1932, p. 274) for archegonia and sporophytes, formalin acetic alcohol for antheridia. Embryos were somewhat collapsed with both reagents. Most of the sections were of paraffin-embedded material, stained with Heidenhain's ironalum haematoxylin, Delafield's haematoxylin, with orange G and erythrosin as counter stains, and safranin and light green (in clove oil). All drawings except figures 1–5 were done with the aid of a camera lucida or projection apparatus.
Fresh sperms were placed in a drop of water on a slide and inverted over 1% osmic acid for a few minutes. The drop was dried and fixed by passing the slide through a bunsen flame. The sperms were stained with acid fuchsin which was washed off with alcohol (Chamberlain, 1932, p. 279).
Ripe spores were shaken from the split capsules on to sterile white unglazed porcelain plates and cultured in a Wardian case at room temperature (i.e., about 15°C.). During the first four months the plates were watered from beneath with boiled tap water, and after that with Knopp's Solution. After five months the young thalli were transferred to turves cut from the matted fibrous aerial rootlets at the base of a Dicksonia squarrosa trunk. These turves had been sterilised by steam under pressure and had then been sunk in soil in the garden for several weeks with only the upper surface exposed. This procedure satisfactorily prevented development of fungi, which often appear if the turves are cultured immediately after sterilisation. The gametophytes
on the turves were kept in a Wardian case and were watered from below with boiled tap water. When the gametophytes were three years old, it was noted that growth was retarded and that the plants were not very healthy, so they were transferred to a damp shady portion of the garden, where growth immediately became more normal.
General Account Of Symphyogyna
Symphyogyna species are found all through New Zealand in light subtropical forest, where they occur beside tracks, on banks and on fallen logs where there is extra light and no great depth of leaf mould. S. subsimplex is found typically on tree-fern trunks, where it forms a thick mat. Sympyogyna species frequently form pure colonies several inches to a foot in diameter. Male and female plants may be intermixed and the various species may grow intermixed.
Hymenophytum species grow in similar habitats and form similar colonies. H. flabellatum was found to be commoner than S. hymenophyllum and to form larger colonies. Pallavicinia was found to be a rarer genus in the localities where collections were made than Symphyogyna and Hymenophytum. Hooker (1867, p. 541) states that P. lyellii grows in the North Island on clay banks. The three genera, Symphyogyna, Hymenophytum and Pallavicinia, have many features in common and may be confused in a superficial examination of sterile plants. Cavers (1911, p. 75) considers that the genus Pallavicinia should be split into two genera Blyttia and Morckia. Other subdivisions of the genus have been suggested, but here for convenience Pallavicinia is used to include all subdivisions. Hymenophytum has likewise been split into two genera by some writers, e.g. Cavers (1911, p. 66) and Campbell (1939, p. 123). Hence Hymenophytum is regarded by some as the genus Podomitrium plus the genus Umbraculum. All three genera have their species in two well-defined groups, viz. the procumbent and erect dendroid types. Of the erect-growing species, there can be found, in each genus, one or more examples of a forked, fan-shaped thallus (figs. 1, 4, 5). In all three genera adventitious branches arise from the rhizome or, in prostrate species, from the midrib on the ventral side of the thallus. By this means plants of each genus can form mats consisting of numerous individuals, all probably originating from one or a few spores. A striking similarity between the genera is the way in which each has developed a strand of elongated, thick-walled, pitted cells. The pits vary in shape and orientation in the various genera and species, but the cells are essentially of the same type.
In the structures surrounding the female reproductive organs and their positions on the thallus these genera differ absolutely. See table below.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Sex organs||No special branch||On a special branch||No special branch|
|Their position on thallus||dorsal||ventral||dorsal|
|Involuere||simple scale||scale (Umbraculum) or ring-like (Podomitrium)||ring-like|
Seasonal Distribution of Symphyogyna
Archegonia of all ages could be found at any time of the year. The archegonial receptacle continues to develop archegonia until one is fertilised. The season for the development of antheridia was found to be more restricted. They develop in early spring, being found abundantly by the end of August. Antheridia continue to develop until November or December and occasional late ones are found in March. Fertilisation takes place usually in spring, but sperms from occasional late antheridia may fertilise mature archegonia at other seasons. Very young embryos have been found from November to June. The development of the sporogenous tissue takes place before winter. In March and April the spore mother cells and elaters are differentiated. The spore mother cells undergo the tetrad division at the end of the winter and the spores mature just.before the setae elongate. The shedding of spores is from September to December inclusive. The times given for the seasonal distribution apply only to the Dunedin locality.
Germination of The Spore Under Culture Conditions
Germination was very slow. A few weeks after the spores were sown a few spores were seen to be noticeably larger than the freshly sown spores and chloroplasts were present in abundance, but there was no sign of rupture of the exospore. Rupture took place at the first division and the spore coat remained in position until a late stage in development. The first division of the enlarged spore is transverse (fig. 6). Divisions after this (fig. 7) may result in a globose mass of cells (fig. 8) or in a filament (fig. 9). The globose structure was found to be much more common. At this stage no rhizoids had developed. These globose masses are difficult to interpret, as only the surface cells can be seen clearly. It would seem that an apical cell is not set apart at a very early stage. Probably fig. 10 indicates the stage when an apical cell is set apart, and from then onwards development is from the apical cell. The nature of the apical cell is not known.
A procumbent Symphyogyna species whose exact identity was not determined was also cultured, and here the early stages corresponded exactly with the development of S. hymenophyllum. With the procumbent species, young gametophytes were more plentiful and more stages were observed. In fig. 12 an outline of the gametophyte shows that growth in a definite direction has set in. The posterior end of this young thallus shows the globose mass of large cells which was formed first on germination. The anterior end of the thallus consists of small cells which are undergoing active subdivision. Even at this stage, no rhizoids were observed.
Growth of S. hymenophyllum was very slow for the first six months, the thalli consisting of only 4–6 cells at the end of that time. During winter and spring growth was more rapid. When the gametophytes were 2·5 mm. long, they consisted of a posterior region still showing the original globose mass of cells, and in front of this the elongated cylindrical region from which rhizoids came off. The thallus then flattened out and the characteristic tooth-like appendages appeared at regular intervals on either margin. The thallus had forked once.
When the gametophytes were a year old they showed a procumbent thallus 5–6 mm. long which had forked twice (fig. 11). In the anterior part, where the forkings had taken place, the thallus showed clear differentiation into midrib and wings. Rhizoids had developed from the cylindrical region and also from the ventral side of the midrib. Only the apical regions of the thallus stood up from the substratum. The erect habit of growth characteristic of the species had not become evident. The procumbent species showed parallel development once more.
The gametophytes of S. hymenophyllum were cultured for three and a-half years. By this time the thalli had attained almost full size. The gametophytes were showing vegetative propagation by adventitious branches from a rhizome, but they were not growing erect as is characteristic of the mature plant. No sex organs had developed on thalli of either species of Symphyogyma under cultivation. The conducting strand within the midrib was well developed.
The rate of growth of the gametophyte under culture conditions is probably much slower than under natural conditions. At various stages of development growth was obviously retarded. Possibly the spores require a resting period after they are shed from the capsule. The late development of rhizoids is a feature. Most sporelings, e.g. Marchantia (O'Hanlon, 1926, p. 217) develop rhizoids at a very early stage. The failure of the gametophytes to grow erect after three and a-half years might possibly have been due to their sickly nature, which might account also for the absence of sex organs, as smaller plants bearing sex organs were observed growing in the field.
(A) Vegetative Structure
The gametophyte of S. hymcnophyllum is differentiated into a colourless, wingless, creeping rhizome and a green, winged, upright, fan-shaped thallus. It is the erect habit of growth which distinguishes this species from other Symphyogyna species found in New Zealand which are procumbent. The rhizome is covered with rhizoids which are thin-walled cells typical of Jungermanniales. In the fan-like part of the thallus there is a distinct midrib which is prominent on the ventral side and tapers quickly into one-layered wings. At fairly regular intervals along the margin of the wings are characteristic appendages in the form of slender multicellular teeth. Some Symphyogyna. species, e.g. S. sinuata and S. brogniartii, have a lobed thallus of varying degrees of lobing. Goebel (1905, p. 37) considers that these may show stages of development of leaves within the genus. Using this argument the appendages of S. hymenophyllum would be regarded as a very early stage in the development of leaves in the genus as a whole.
Growth of the subaerial thallus of S. hymenophyllum is from a single wedge-shaped apical coll cutting off segments dorsally and ventrally. This apical cell lies at the base of a sinus caused by rapid growth of the segments, and is surrounded by numerous two-celled mucilage hairs which protect the growing apex. The apical coll comes to lie more to the ventral than the dorsal side of the thallus.
Symphyogyna species, in common with other liverworts, e.g. Pallavicinia and Hymenophytum, have a method of vegetative propagation. Fig. 1 shows one S. hymenophyllum plant from the rhizome of which adventitious branches have arisen. An apical cell is cut out from a surface cell of the rhizome and growth of the new branch continues from this apical cell. A cylindrical branch grows out and later turns upward and broadens out into the winged assimilating part of the thallus. Behind the apex, the young branch develops its own conducting strand, which never joins on to the strand of the parent rhizome (fig. 15). The fact that the two sets of conducting tissue do not meet explains how new branches thus formed can break away easily from the parent plant and become established as independent plants.
The rhizome and the subaerial part of the thallus are traversed by a strand of tracheid-like elements. The walls of the strand cells stain bright red with safranin. When tested for lignin with the following reagents: aniline sulphate, phloroglucin, iodine and sulphuric acid, and chlor-zinc-iodine, the walls gave negative results. The middle lamella of the walls of the strand of S. aspera is stated by McCormick (1914, p. 405) to give reactions for pectose. She does not comment on the chemical nature of the thickenings on the walls. Tansley and Chick (1901, p. 7) found that P. hibernica var. β wilsoniana had two strands of thin-walled but lignified cells. In P. lyellii they state that the walls of the cells of the strand are thick and supplied with pits. They describe the shape and orientation of the pits and the colour of the walls when fresh. They do not, however, state whether or not they found the walls to be lignified.
Once the strand cells are differentiated and the thickenings are laid down on the walls, they lose their contents and become dead cells. The thickened walls have simple round or oval pits scattered irregularly over them (figs. 13, 14). In S. aspera (1914, p. 404) the pits were found to be spirally arranged and were elongated and slit-like in shape. In P. decipiens Tansley and Chick (1901, fig. 4) show oval or elongated pits often arranged spirally. They state that the pits in P. lyellii (1901, p. 9) are spirally arranged, being circular or clongated in the direction of the spiral. Their distribution over the walls is stated to be irregular.
Experiments with living S. hymenophyllum plants were made to see if the strands conducted water. The stalk of the thallus was placed in a saturated aqueous solution of eosin. The eosin solution travelled 1·5 cm. above the level of the solution in 7–15 minutes. During this period there was very little diffusion in the parenchymatous tissue of the stalk. These conduction experiments were similar to those which had already been carried out by Tansley and Chick (1901, pp. 9–10), who obtained similar results using P. lyellii. The writer was not troubled, as they were, by the eosin travelling up the parenchymatous cells of the stalk, thus obscuring the view of the strand.
(B) Reproductive Structure
The gametophytes of S. hymenophyllum are strictly dioecious. The male and female plants are similar in size and quite indistinguishable until the sex organs develop. The antheridia occur singly in several longitudinal rows over the midrib (fig. 2). The archegonia
are borne directly over the midrib, being arranged in groups on cushions (fig. 23). There is usually a group of archegonia immediately over a fork in the thallus, but they are not restricted to this position. When a dichotomy occurs during the development of antheridia, they continue to develop along both arms of the thallus. Each antheridium and each group of archegonia is protected by a single scale-like involucre which is inserted transversely on the thallus immediately posterior to the antherridium or cushion of archegonia.
Development of Antheridium
Antheridia arise in acropetal succession behind the apex, but come to lie in several rows further back on the thallus (fig. 2). Normally each antheridium has its own involucre, but sometimes when antheridia develop very close together they are found under the same involucre (fig. 16). The antheridium develops from a surface cell of the thallus which projects as a papilla. The series of divisions which go on in this cell is typical of the development of the antheridium in Jungermanniales (fig. 17). The mature antheridium consists of a globose head and a short slender stalk. In the head of the antheridium the mass of spermatogenous tissue is surrounded by a single-layered wall in the cells of which are chloroplasts. The stalk is two cells in diameter and never more than two cells high.
The broad outlines of spermatogenesis are as follows. The sperm mother cell is comparatively large. It is filled with densely staining cytoplasm and has a large nucleus (fig. 18). Before the final division of the sperm mother cell takes place, its contents round off to a certain extent from the cell wall (fig. 19). This rounding off process continues until all the cytoplasm is located in a narrow zone round the large, densely staining nucleus. The cells also lose their polygonal shape and come to lie more or less freely in the autheridial cavity, their outlines being on the whole circular. This loss of polygonal shape would naturally follow the cleaving of the protoplasts away from the cell wall.
The nucleus from this stage onwards forms the greater part of the protoplast. The sperm mother cell divides into two spermatocytes. It is impossible to say that the division is in a diagonal plane, as the sperm mother cells are round rather than polygonal and the spindles point in all possible directions. The two young spermatocytes point in opposite directions from each other and are somewhat flattened against each other. The major part of their bulk is made up by the nucleus, but there is a little cytoplasm attached, more being found at one end of the nucleus than at the other. At this stage there is no dividing membrane between the spermatocytes (fig. 20). Later there forms between the two spermatocytes a membrane which stains as deeply as the wall of the sperm mother cell (fig. 21).
In stained sections the cilia are not seen clearly, but the change in shape of the nucleus can be seen. The membrane formed between the two spermatocytes disappears early. The nucleus elongates and begins to curl round inside the cell. The sperm mother cell wall finally breaks down and the sperms are ready to be shed from the antheridium. A fresh sperm, free from the antheridium and treated as already described, showed an elongated spirally coiled nucleus with two long, fine eilia inserted at one end (fig. 22).
The temporary membrane between the two spermatocytes is an interesting feature. In Fossombronia longiseta, Humphrey (1906, p. 96) states that no wall is formed between the two spermatocytes. In Plagiochila adiantoides, Johnson (1929, p. 51) mentions the formation of a cell plate which is not a definite cell wall, and Clapp (1912, pp. 181–182) speaks of a membrane in Aneura pinguis which stains as deeply as does the cell wall of the sperm mother cell.
Development of Archegonium
The cushion on which the archegonia develop is formed by the tissue below the first-formed archegonia becoming meristematic. The youngest stages in the development of these cushions were not observed in the material studied. The apical growth of the thallus is not checked by the development of archegonia and there may be several cushions with archegonia along the midrib with sterile tissue between them. On the youngest cushion nearest the apex there will be only a few archegonia, but on older cushions near the base of the thallus as many as thirty-five archegonia have been observed. Mucilage hairs are produced in abundance with the archegonia.
The archegonium arises as a papillate projection from a surface cell of the cushion or area which will give rise to a cushion. This is cut off by a transverse wall. The basal cell resulting remains within the thallus and the outer cell which projects beyond the thallus undergoes a series of divisions typical of the development of the archegonium in the Jungermanniales. The division of the primary ventral cell into ventral canal and egg cells does not necessarily take place at a late stage. It may take place when only four neck canal cells have been formed.
During development and growth of the axial row the wall cells divide transversely to keep pace in length. There are more wall cells in a vertical row than there are neck canal cells.
The axial row of the mature archegonium consists of the egg cell, the ventral canal cell and a number of neck canal cells of which the maximum number seen in longitudinal section was eight, although curving archegonia were observed which must have had more than eight.
The number of neck canal cells seems to vary considerably in the Jungermanniales. Thirteen were observed in S. aspera (McCormick, 1914, p. 407) and eighteen in P. lyellii (Haupt, 1918). For the Jungermanniales in general Smith (1938, p. 47) gives the number 4–16, and Campbell (1939, pp. 110–111) gives it as 5–6. The mature archegonium has its wall cells arranged in five vertical rows. Externally the mature archegonium is seen to consist of a long, curved, slender, spirally twisted neck, a narrow venter consisting typically of one layer, and a short stalk. There are chloroplasts in the wall cells of the archegonium.
The ventral canal cell and neck canal cells break down, the cover cells come apart from one another and the egg is ready for fertilisation. If fertilisation takes place, the one-layered venter undergoes periclinal and anticlinal divisions. These divisions still take place, but to a lesser degree if fertilisation fails to occur.
Fig. 1–S. hymenophyllum. Female plant with young sporogonia. Nat. size.
Fig. 2–Old male plant with antheridia. Nat. size.
Fig. 3–Female plant with mature sporogonium. Nat. size.
Fig. 4–Hymenophytum flabellatum. Female plant with sporogonia. Adventitious branching also shown. Nat. size.
Fig. 5–Pallavicinia! connivens. Female plant showing adventitious branching. Nat. size.
Figs. 6–10-Stages of germination of spore of S. hymenophyllum. X 290.
Fig. 11–Young gametophyte, one year old—g., original globular mass of cells; r., rhizoids. Length is 5–6 mm. X 14 ½
Fig. 12–Procumbent Symphyogyna species. Surface view of young gametophyte. X 290.
Fig. 13–Section of strand cell showing the simple nature of the pits. X 2,000.
Fig. 14–Portion of strand cell in face view showing irregularly scattered pits. X 2,000.
Fig. 15–Young adventitious branch arising from rhizome. X 14.
Fig. 16–L.S. thallus showing two antheridia under one involucre. X 3G.
Fig. 17–Young stage in development of antheridium. X 290.
Figs. 18–21–Stages in formation of sperms. X 685.
Figs.22–Fresh sperm. X 685.
Fig. 23–L.S. thallus (diagrammatic) showing cushion (c.) with archegonia and involucre (inv.). X 40.
Fig. 24–L.S. three-celled embryo in situ. X 200.
Fig. 25–L.S. four-celled embryo. X 290.
Fig. 26–L.S. apex of 16–celled embryo showing apical cell. X 290.
Fig. 7–L.S. apex of older embryo with apical cell. X 290.
Fig. 28–L.S. older embryo. Apical region of small cells not suitable for drawing. X 157 ½.
Fig. 29–L.S. young sporogonium showing withered neck (n.) of archegonium and old archegonia carried up on calyptra (cal.); m.h., mucilage hairs; f., foot; inv., involucre. X 36.
Fig. 30–L.S. apex of young sporogonium. X 157 ½.
Fig. 31–L.S. apex of older sporogonium. X 157 ½.
Figs. 32–33–L.S. two stages in development of sporogenous tissue. X 290.
Fig. 34–Lobed spore mother cells in rows. No thickenings on elaters. X 290.
Fig. 35–End of elater showing double spiral. X 685.
Fig. 36–L.S. seta before elongation. X 68.
Fig. 37–L.S. seta after elongation. X 68.
Fig. 38–Capsule split into four valves adhering at the apex. X 14 ½.
Fig. 39–Face view of unsplit capsule wall showing dehiscence line (d.l.) of thinwalled cells. X 157 ½.
Only one sporogonium was found to develop from an archegonial group. If more than one egg was fertilised, only one developed into a sporogonium.
The following description of the development of the embryo is incomplete, as only a small number of really young embryos were obtained which were suitable for study. The first division of the fertilised egg is by a transverse wall into a hypobasal and an epibasal cell. The earliest stage in the material studied showed two transverse walls making a filament of three cells. As mitotic figures were lacking it was impossible to say whether it was the epibasal or hypobasal cell which had divided again. At the three-celled stage the cells were of equal size (fig. 24).
It seems to be a general rule in Hepaticae that the first division of the fertilised egg is by a transverse wall. In Aneura pinguis at the three-celled stage, Clapp (1912, p. 183) states that the cells were much elongated, which was not the case with S. hymenophyllum. In some Hepaticae, Smith (1938, p. 47) states that the two original transverse divisions divide the young embryo into three cells which give rise respectively to capsule, seta and foot. In others, he states that the epibasal cell resulting from the first transverse division gives rise to all three sections, the hypobasal cell acting merely as a haustorium. It is likely that S. hymenophyllum belongs to this latter group, as S. aspera is stated to be of this type.
A filament of four cells is formed by another transverse wall. At this stage elongation had started and the basal cell of the filament had elongated more than the other cells (fig. 25). No embryos were found of an age just older than the four-celled filamentous stage. At the next stage observed the embryo consisted of approximately sixteen cells. The embryo was elongated and slender, appearing to be about two cells in diameter in longitudinal section. At this stage it appeared, from the arrangement of the walls in the apical region, that growth had taken place from an apical cell (fig. 26). Just when this apical cell was cut out the writer cannot say, but it appears to have been at an early stage in development. The nature of the apical cell is not known. A slightly older and more massive embryo seen in fig. 27 also showed segments cut from an apical cell. At this early stage the foot is becoming differentiated and its cells somewhat embedded in gametophytic tissue which is disintegrating to a certain extent.
In Riccardia (Smith, 1938, p. 53), Aneura (Clapp, 1912, p. 184) and S. aspera (McCormick, 1914, pp. 408–409) periclinal walls in the capsule are described which separate a one-layered wall from the sporogenous tissue. The wall and sporogenous tissue subsequently subdivide. In fig. 27 the embryo of S. hymenophyllum was beyond the stage at which the periclinal walls in the other genera were stated to be laid down. As stages just younger than this were not obtained, it is impossible to say if the walls subdividing the segments are periclinal walls of this nature. The apical cell is still functioning at this stage.
The next stage of development observed was a much elongated embryo (fig. 28). Here the basal part of the embryo had ceased active cell division. The haustorial cells of the foot are rounding off and assuming that curious bulbous effect common to them. The major part of the embryo consists of large cells which would not appear to be capable of further division. The apical region consists of small cells with dense cytoplasm and large conspicuous nuclei. This is the region which will give rise to the seta and capsule.
The sporogenous tissue is finally set aside as shown in fig. 30. This stage is similar to that indicated in fig. 29, which shows the extent of the sporogenous tissue in comparison with the rest of the sporogonium. The apical cap is already developing and the wall of the capsule is two or three cells thick. The tissue immediately below the fertile tissue is compact and regular and undergoing further subdivision. This region will give rise to the seta. The tissue below the seta region is composed of large cells in a permanent condition derived from the basal part of the embryo as seen in fig. 28. The lower part of this region is termed the foot. It tapers off gradually and becomes bent over. The sporogenous tissue continues to subdivide until a large mass of fertile tissue is formed.
While the embryo is growing and developing changes are taking place in the gametophytic tissue surrounding it. The cells of the venter of the archegonium undergo periclinal and anticlinal subdivision until the venter is up to eight cells thick. This region later becomes part of the calyptra. But the venter is not the only gametophytic tissue to become meristematic. The cells of the cushion on which the fertilised archegonium was situated subdivide, with the result that the cushion elongates and carries all the unfertilised archegonia up with it to the apex of the calyptra. That it is the cushion also and not the venter only which is elongating can be seen by the mucilage hairs which are carried up on its side (fig. 29). The venter of the archegonium does not bear mucilage hairs on its outer side. The venter and cushion continue to keep pace with the growing sporogonium until a massive calyptra about 6 mm. long is formed.
Without yet mentioning the state of the sporogenous tissue the structure of the sporogonium and surrounding tissues at the stage just before elongation of the seta may be described. The foot tapers considerably, the bottom part being usually bent over in the form of a sharp crook. Only one or two sporogonia were seen in which the foot had a different form, and here it was anchor-like. The cells of the basal part of the foot are large and somewhat altered. The surrounding tissue of the gametophyte is partially disintegrated. The cells above this bent basal part are large and vacuolated with an abnormal appearance. The foot is embedded deeply in the gametophytic tissue.
The seta consists of a zone of compact isodiametric cells, forming a very regular tissue (fig. 36). It passes abruptly into the sporogenous tissue. The wall of the capsule at maturity consists of three layers. The outer layer is made up of large cells with band-like thickenings on the radial walls. The cells of the two inner layers are thin-walled, narrow and elongated. Situated at even distances round the periphery
of the mature capsule wall are four vertical zones of thin-walled cells (fig. 39). They indicate the lines of dehiscence and do not extend to the apex of the capsule, which takes the form of a tapering cap several cells deep. This beak-like region remains intact when the walls of the capsule split at dehiscence and spore dispersal. The oval mature capsule is about 4 mm. long.
At the stage indicated in fig. 30 the sporogenous tissue is fairly regular and compact. The cytoplasm is densely staining and the nuclei are large and conspicuous. Cell walls are clearly defined. As growth and subdivision continue some of the cells appear longer than others (fig. 31). The tissue loses its regular appearance as most of the cells now exhibit a more irregular polygonal shape. The elongated cells will give rise to elaters, the others to spore mother cells. From this time onwards the walls in the sporogenous tissue undergo some change and do not take up the stain. Hence the developing spore mother cells and elaters appear as isolated protoplasts in an indefinite matrix (figs. 32, 33). The cells which develop into spore mother cells are of very irregular shapes, as indicated in fig. 33. The tissue at this stage in development is very hard to kill and fix without plasmolysis. The irregular spore mother cells are gradually transformed into the lobed vacuolar spore mother cell typical of the Jungermanniales (fig. 34). The spore mother cell divides to give a tetrad of spores. The reduction division was not observed, so whether a quadripolar spindle was formed, as in S. aspera (McCormick, 1914, p. 414) or two successive divisions took place, is not known. The developing spores are densely filled with chloroplasts. The spores and elaters develop their thickened walls practically simultaneously. The brown outer coat of the spore is very thick and sculptured in the form of an irregular reticulum. The inner wall does not show up clearly. The elaters are long, slender structures with thickenings laid down in the form of a double spiral (fig. 35). All the elaters are free within the capsule and there is no elaterophore in Symphyogyna.
After the spores and elaters have matured, the seta elongates rapidly by elongation of the cells (figs. 36, 37), and pushes the capsule through and beyond the calyptra. When elongation is complete (fig. 3), the capsule splits into four valves (fig. 38) along the dehiscence lines. The elaters are hygroscopic and their movement effects a gradual dispersal of spores from the capsule. After the spores are shed the cells of the seta lose their turgidity and the seta withers. The rate of elongation of the cells of the seta is very rapid. Numerous setae were measured before and during elongation, and it was found that on an average they elongate from 0·2 cm. to their final length of 2 cm. in three days.
1. A brief account of the ecology and seasonal distribution of Symphyogyna hymenophyllum is given, and comparisons are made with related genera.
2.Germination of the spore under culture conditions is slow; a globular mass of cells is formed before growth from an apical cell sets in. Rhizoids develop late.
3. The vegetative structure of the thallus is described with special reference to the conducting strand.
4. The sex organs, which are borne on the dorsal side of the thallus of separate male and female plants, are typical of the Jungermanniales. A membrane is formed between the two spermatocytes.
5. The young embryo develops from an apical cell.
6. The cushion and venter elongate to give a massive calyptra. Elongation of the seta is by elongation not subdivision of its cells. Developing spores contain chlorophyll.
The writer is greatly indebted to the late Dr J. E. Holloway, F.R.S., for helpful criticism and kindly advice during the course of the investigation, which was carried out under his direction at the Department of Botany, University of Otago, as subject for a thesis for the Master of Science degree. The writer is grateful also to Mrs E. A. Hodgson, Wairoa, for dried herbarium specimens of Symphyogyna, Hymenophytum and Pallavicinia.
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