AlthoughMarchantia polymorpha L. is widespread as a weed in gardens of the northern hemisphere, particularly on damp walls and paths and on burnt-over ground, it is not known to be established in New Zealand in its typical form. However, within recent years a variety resembling aquatica Nees has become a nuisance in humid glass-houses and shade-houses in the North Island and in some cases is even more troublesome than Lunularia. Its initial establishment as far as can be determined is to be associated with the use of imported peat in potting mixtures and of imported peat pots together with the maintenance of high humidity. But it may have arrived in New Zealand in various ways and on more than one occasion.

Ever since M. polymorpha was recognised as a species, it has been known to comprise a number of varieties and forms. Nees (1838) considered var. aquatica to be a habitat form whereas Müller (1951-8) termed it a variety. Burgeff (1943) on the basis of his numerous culture and breeding experiments considered it to be sufficiently well-defined to merit the rank of a separate species. In the present paper it is being treated as a variety but the fact that it has been considered to be a species indicates that morphologically it is a fairly constant form. It may also be biochemically distinct, for Tucker (1963) reported that the thallus of var. aquatica contains a specific antigenic protein in contrast to that found in the typical form of M. polymorpha.

A description of typical M. polymorpha can be obtained in many of the textbooks of botany and bryophyte floras which have been written in the northern hemisphere, for example Smith (1955) and Müller (1951-8). However, since var. aquatica differs from the page 180

PLATE 1: (Top) Plants with archegoniophores x 2. (Below) Thallus showing cupules and dark centre line x 1 ½. (Photos by D. J. Scott).

page 181 type in some respects and it alone appears to occur in New Zealand, it is the variety as found in New Zealand which is described here.

The membranous thallus is up to 8 cm long and up to 1.5 cm in width with the margin as a rule somewhat crinkled. Forking occurs at a wide angle, at intervals of from 1 to 3 cm, and either the two branches grow equally so that the plant tends to form a rosette or one overtops the other and dominates the growth. The upper surface is translucent green or yellowish-green in colour and non-glossy, with the centre line, where air-chambers are lacking, appearing colourless near the apex and dark and sunken further back. Under a lens the air-chambers are clearly delineated and are as a rule rhomboidal to hexagonal in shape but towards the mid-line of the thallus they are much extended in a lengthwise direction. The under surface is green at first but in older parts becomes magenta or brown, particularly near the centre line. It carries rhizoids of several types including smooth-walled ones of diameter 24 to 45/mic., and tuberculate ones of diameter 7 to 42/mic. with walls either straight or crenulate and sometimes tinged with magenta. The internal thickening of the wall may be in the form of rounded pegs, jagged pegs or a helical band. The rhizoids as a rule do not grow directly downwards into the soil but tend to collect into a slender, magenta-brown strand at the mid-line and then to pass backwards parallel to the thallus for some distance. On the under surface, too, are 3 series of scales which are usually colourless and difficult to see (Fig. 1.). The marginal scales tend to project beyond the edge of the thallus; they have at their tips a zone of cells with colourless, mucilaginous contents which later may turn brown; the cell-walls throughout the scale may turn brown also. The laminar scales may be little evident or may occur in an irregular double row. The median scales carry a broadly cordate appendage, 0.3 to 0.4 mm in width, whose cells gradually increase in size towards the centre (Fig. 4). Cells on the edge tend to project giving the appendage an irregularly dentate margin, and some 4 to 9 deeper cells each contain a large oil body.

Cupules (gemma-cups) containing gemmae lie on the upper surface along the centre line of many thalli, being formed particularly when the thallus is young (Plate 1). They are 2.5 mm in diameter and 2 mm high, cut on the rim into triangular lobes with ciliate margins (Fig. 6). On the outer surface of the cupule there are projecting papillae. Müller (1951-8) notes the absence of gemmacups in var. aquatica. This occurs in New Zealand chiefly in established plants bearing archegoniophores.

Anatomy of the thallus

In transverse section the thallus is up to 0.15 mm deep at the midline, and outwards from this increases temporarily to a depth of up to 0.16 mm, and then for most of the distance to the margin has a page 182 depth of only 0.09 mm or less. The epidermis on the upper surface consists of thin-walled cells with chloroplasts, and very rarely near the margin with projecting papillae. It is interrupted by barrel-shaped air-pores enclosed by 4 or 5 tiers, each of 4 cells (Fig. 3). Lining the pore is a conspicuous deposit of resin. The photosynthetic layer which is present except at the centre line (Figs. 2, 3) consists of a single series of low air-chambers, up to 0.03 mm in height, occupied by green filaments 2-3 cells high. The side walls and floor of the chambers contain chloroplasts also. The compact ventral tissue, up to 10 cells in height near the centre line, rapidly becomes thinner outwards and for most of the width of the thallus may be only 2 cells high; it consists of thin-walled cells, indistinctly pitted. In a few of the cells of the thallus a large, brown oil-body is present.

Gametophores

These are said to be produced infrequently (Burgeff, 1943). This may be due partly to the fact that thalli rarely get the chance to become sufficiently established to enable them to pass beyond the juvenile stage, for on a number of plants observed in glasshouses in New Zealand there were gametophores. As in other Marchantia species the plants are dioicous.

Antheridiophores were present from February until April, at which time new ones were still forming. Each has a green or brown stalk up to 2 cm high and a flat-topped head, up to 1 cm in diameter, with 8 lobes green in colour except for a broad, colourless margin. Short projections from cells on the sides of the lobes make the margin here denticulate. On the underside of the head are colourless scales.

Archegoniophores were found from February until April, appearing in a lateral position on the main thallus terminating a short arrested lobe; for after their initiation the other branch of the fork continues to grow forward strongly. They are delicate, dainty structures (Plate 1). Each has a slender stalk, up to 4.5 cm high, which at first is green and later dull-brown, and a head up to 1 cm in diameter with 9 narrow, terete rays, of diameter 0.6 to 0.8 mm, slightly decurved. Papillae are abundantly present on the surface of the rays. On the under side of the head between the rays are 8 receptacles, each with several archegonia at first and with sporogonia later, and each enclosed by 2 boat-shaped valves which are deeply lobed and fimbriated on the edge (Fig. 5). The sporogonia when ripe have bright yellow capsules containing smooth, yellow spores of diameter 8 to 12 mic. untermixed with bispiral elaters.

Aids in identification

In most cases M. polymorpha var. aquatica will be found in the vegetative state. Then the dark centre-line is particularly distinctive page 183

FIG. 1: Underside of the thallus showing the position of the scales. I — laminar scale; m — median scale; mg. — marginal scale. x 2.
FIG. 2: Transverse section of part of the thallus where air-chambers are lacking. x 80.
FIG. 3: Transverse section of part of the thallus with air-chambers. x 80.
FIG. 4: Appendage of a median scale. x 80.
FIG. 5: Edge of the involucre. x 75.
FIG. 6: Part of the cupule rim. x 75.

page 184 in comparison with the thallus of the Marchantia species indigenous to New Zealand (Campbell, 1965). In addition, it is readily distinguished from the thallus of M. berteroana or M. foliacea by the thin texture, by the non-glossy upper surface and by the projecting marginal scales. The appendages of the median scales lack the border of small, thin-walled marginal cells as found in M. berteroana. The third indigenous species, M. macropora, with its rough surface and elevated air-pores is easily recognised. Gemmacups and antheridiophores of M. polymorpha var. aquatica resemble those of M. berteroana and alone are not distinctive. The archegoniophores somewhat resemble those of M. berteroana but are distinguishable by their slender form and the papillate surface of the rays.