Tuatara: Volume 25, Issue 1, July 1981
Notes on Some Anthocerotae of New Zealand
Notes on Some Anthocerotae of New Zealand
(a) Position of Anthocerotae in the Plant Kingdom
The Anthocerotae (hornworts) is one class of the Division Bryophyta (bryophytes) and consists of a single order, Anthocerotales, and a single family, Anthocerotaceae. The better known classes of the Bryophyta are the Musci (mosses) and Hepaticae (liverworts). All bryophytes show the same type of life cycle which possibly arose independently in different groups. (Clarke & Duckett, 1979).
(b) Characteristic features of Anthocerotae
The Anthocerotae are readily distinguished from other bryophytes by certain characteristic features. The thallus is of homogeneous structure, with simple rhizoids and without ventral scales. Cavities occupied by blue-green algae are normally present. Antheridia are situated in chambers sunk in the thallus. Archegonia are also immersed and their jacket cells are indistinguishable from the surrounding tissue. The sporophyte is long and narrow, with a central columella of sterile tissue and a basal meristem. Its only protection is a basal collar (involucre) formed of tissue originating mainly in the thallus.
(c) Representation in New Zealand
In the family Anthocerotaceae, as represented in New Zealand, two main groups may be distinguished. In one the capsule wall has pores (stomata) (fig 3b) and the elaters (here usually termed pseudoelaters) lack a helical band of thickening. The other shows characters which are opposite of these. For members of the first group, which alone is known in Europe, the genus Anthoceros was established by Micheli as early as 1729 and the name was later adopted by Linnaeus (1753).
However, over the years, as explained by Schuster (1963), much nomenclatural confusion has arisen. It appears that the simplest solution is to retain the genus name Anthoceros and use it, as Linnaeus did, for both yellow-spored and black-spored taxa. This course is followed in the account below. The genus names, Phaeoceros and Aspiromitus, then become redundant. Within the second group confusion of nomenclature has arisen also. Two genera, Megaceros and Dendroceros, are usually recognised, but the boundary between them is ill-defined. Proskauer (1953) concludes that the only difference between the genera appears to be that Megaceros has broadly radiating thalli and Dendroceros has strap-shaped ones. Both genera are represented in New Zealand.
(d) Key to the genera in New Zealand
|1.||Pores present in the capsule wall; pseudoelaters lacking a thickening band.||Anthoceros|
|Pores lacking in the capsule wall; elaters with a helical thickened band…2|
|2.||Thallus strap-shaped, consisting of a broad midrib region and a sharply defined unistratose wing; chloroplasts usually one per cell.||Dendroceros|
|Thallus broadly radiate; chloroplasts usually more than one in epidermal cells, 212 in hypodermal cells.||Megaceros|
II The Genus Anthoceros
|(i)||Anthoceros laevis L.|
Anthoceros laevis is distributed world-wide in suitable habitats. In New Zealand it is found throughout, usually on the side of ditches or wet banks.
Although references to A. laevis are made in many botanical works, to Proskauer (1948a, 1948b, 1951, 1954a, 1954b, 1958a) we are indebted for the assembly of much information based on his detailed studies of material collected at first in England and later throughout the world. More recently Hassel de Menendez (1962) has reported on the species in Argentina and Paton (1973) has provided notes on the species in Britain.
Morphology of the Gametophye.
The thallus is typically perennial and morphologically is exceedingly variable. It grows as dark green, irregular rosettes, 1-3 cm or more in page 9
Anatomy of the Thallus.
The thallus in transverse section shows an epidermis composed of small, thin-walled cells and a compact ground tissue 5-8 layers deep. Each chlorophyllous cell contains a single large chloroplast. Situated in page 10 the ground tissue are mucilage cavities opening by pores on the ventral surface and occupied by colonies of the blue-green alga, Nostoc (fig. 3a). The pores arise close to the thallus apex where morphologically, although always open, they closely resemble stomata (fig. 3c). Further back the aperture becomes either more circular or 4-angled (fig. 3d). Goebel (1905), following on the earlier accounts by Janczewski in 1872 and by Leitgeb in 1874-81, describes the cavities in detail. Should a hormogonium of Nostoc penetrate the pore, it multiplies in the small substomatal chamber to form a globular colony and in so doing gradually enlarges the cavity. Meanwhile surrounding cells of the thallus grow into the cavity as filaments interwoven with the Nostoc trichomes and the pore becomes closed by what appears to be a mass of loose parenchyma. It has been shown that the blue-green alga present in A. punctatus can fix nitrogen and that an interchange of nitrogen and carbon takes place between the alga and the hornwort (Rodgers and Stewart 1977; Stewart and Rodgers 1977).
Fig. 3. (a) Underside of young thalli showing pores. Scanning electron micrograph by A. Craig. (b) A pore on the capsule wall. Nomaski interference microscopy. Photo by A. Craig. (c) A pore on the gametophyte (early stage). Nomaski interference microscopy. Photo by A. Craig. (d) Later stage of a pore. A diatom and a Nostoc trichome are visible within the pore. Nomaski interference microscopy. Photo by A. Craig.
Morphology of the Sporophyte.
Dehiscing sporophytes are usually 3-7cm tall and fall within the range of 0.7-9cm determined by Proskauer (1948a). Opening takes place by one or two slits which do not reach the apex and in dry air the valves twist spirally (fig. 4). More detail is given by Proskauer (1948b). The ripe spores are a translucent yellow colour. They were found to have a maximum diameter of 40-50 microns so falling within the much wider range given by Proskauer (1958a). The markings of the spore coat are very varied but generally there is a fine granulation on all the faces and some larger conical projections, up to 2.5 microns high, mainly on the spherical face (fig. 5). However in some specimens the spore coat is almost smooth (Proskauer, 1958a, Hassel de Menendez, 1962). The pseudoelaters are either unbranched or slightly branched filaments, made of 1 to 5 cells, and show no helical thickening bands.
A Note on Sex Distribution.
In regard to the sexual condition Proskauer (1951) created two species, one dioecious and restricted to Europe and the other monoecious and essentially world-wide in distribution. He found also a difference in chromosome morphology. Later he reduced the species to subspecies page 12
Certainly, for New Zealand material, any attempt to allocate specimens to subspecies is for practical purposes too time-consuming page 13 to be of value. Cultured samples of several populations and some single-spore cultures have been studied for up to three years and compared with plants in nature. Three examples only of the many different patterns are mentioned here. The only population in the grounds of Massey University, which exclusively occupied an area of 50cm2. on a seepage bank, had archegonia only in July and did not form sporophytes. In October the plants produced numerous bulbils which persisted when the older parts of the thallus dried up in summer. A population from the nearby Tiritea Valley had abundant antheridia in July, so much so that plants grown in full light appeared reddish whereas those in shade were yellowish. In October archegonia were forming on some of the plants and in December there were sporophytes. Still later bulbils made their appearance. In a population collected at Huka Falls there was again a different pattern with one branch of a fork bearing archegonia and the other antheridia.
Comments on Synonymy.
There is no doubt that A. laevis is an exceedingly variable species. But it does not seem possible to split it into smaller, consistently uniform taxa. This situation is reflected in the number of synonyms which appear in the literature, a number which Proskauer (1958a) calculated to be some 200. Undoubtedly there is difficulty at times in distinguishing A. laevis from other species of Anthocerotales in New Zealand especially when using herbarium material and in the absence of reproductive structures.
Clarke, G.C.S. and Duckett, J. G. (Editors) 1979: Bryophyte Systematics. Academic Press, London.
Goebel, K. 1905: Organography of Plants Part II. Special Organography. English edition. Oxford University Press.
Hassel de Menendez, G. G. 1962: Estudio de las Anthocerotales y Marchantiales de la Argentina. Opera Lilloana 7 : 1-297.
Linnaeus, K. V. 1753: Species Plantarum II, Stockholm.
Paton, Jean A. 1973: Phaeoceros laevis (L.) Prosk. subsp. carolinianus (Michaux) Prosk. in Britain. Journal of Bryology 7 : 541-543.
Proskauer, J. 1948a: Studies on the morphology of Anthoceros 1. Annals of Botany N.S. 12 : 237-265.
—— 1948b: Studies on the morphology of Anthoceros. II. Annals of Botany N.S. 12 : 427-439.
—— 1951: Studies on Anthocerotales. III. The genera Anthoceros and Phaeoceros. Bulletin Torrey Botanical Club 78 : 331-349.
—— 1953: Studies on Anthocerotales IV. Bulletin Torrey Botanical Club 80 : 65-75.
—— 1958a: Studies on Anthocerotales. V. Phytomorphology 7 : 113-115.
—— 1958b: Nachtrag zur Familie Anthocerotaceae. In K. Muller, Die Lebermoose Europas, Rabenhorst's Kryptogamen-Flora 3rd ed., 6 : 1303-1319.
Rodgers, G. A. and Stewart, W. D. P. 1977: The cyanophyte-hepatic symbiosis 1. Morphology and physiology. New Phytologist 78 : 441-458.
Schuster, R. M. 1963: Studies on antipodal Hepaticae I. Annotated keys to the genera of antipodal Hepaticae with special reference to New Zealand and Tasmania. Journal Hattori Botanical Laboratory 26 : 185-309.
Stewart, W. D. P. and Rodgers, G. A. 1977: The cyanophyte-hepatic symbiosis II. Nitroge fixaton and the interchange of nitrogen and carbon. New Phytologist 78 : 459-471.