Classification of Anthocerotales | Bryophyta

In this article we will discuss about the classification of anthocerotales.

The class Anthocerotopsida (also called Anthocerotae) is a small but very distinct group of Hepatics differing from the others, specially, in its sporophyte which is of indefinite growth because of a characteristic intercalary meristem at the base of the capsule. The sporogenous cells are amphithecial in origin.

The cells of the gametophytes, usually, have large single chloroplasts with a conspicuous pyrenoid each. The sex organs are embedded in the gametophyte.

The Class has a single order Anthocerotales in which four genera, Anthoceros, Mega- ceros, Dendroceros and Notothylas, are universally recognised. But, the largest genus Anthoceros has now been divided into three Stephani (1916) first took out 55 species to form the genus Aspiromitus and, lately, Proskauer (1951) has proposed the creation of a third genus Phaeoceros out of Anthoceros.

Thus, the total number of genera are now six and, all these used to be included within the single family Anthocerotaceae.

But, Muller (1940) proposed that the genus Notothylas should be separated to form a second family Notothylaceae. He is supported by Proskauer (1951) but Reimers (1954), while accepting the two families, has retained only the four original genera. There are about 300 species within Anthocerotales and these are distributed throughout the world, spe­cially in tropical to temperate regions.

The Anthocerotales are represented in India by the following:

Family Anthocerotaceae

Aspiromitus (4 spp.), Antheroceros (including Phaeoceros- 25 spp.,), Mega-ceros (1 sp.) and

Dendroceros (1 sp.).

Family Notothylaceae

Notothylas (3 spp.).

Of these, two genera, Anthoceros and Notothylas belonging to the two families, are described below.

Family Anthocerotaceae Genus Anthoceros:

Anthoceros is the largest genus of the Anthocerotopsida comprising about 200 of the 300 species included within the Class. About 25 species of these are reported from the tropical, subtropical and temperate hilly areas all over India—the Himalayas, Chhotanagpur, the Ghats, Nilgiri, etc.

Of these, mention may be made of Anthoceros erectus Kashyap (Fig. 439C & D) and A. himalayensis Kashyap (Fig. 439E & F), found all over the Himalayas, in South- India and even in the Gangetic plains.

They grow in very moist, shady places by clay banks and ditches or in the crevices of rocks. Campbell (1939) noted that the species of Anthoceros fall into two groups, with black spores and with yellow spores, the two groups also showing other morphological differences. Proskauer (1951) has placed the yellow spored ones into the new genus Phaeoceros.

Gametophyte:

The gametophyte is a very simple, small, prostrate, dorsiventral thallus of a greasy dark-green color. The thallus is scarcely branched though usually dichotomously lobed (Fig. 439B), often sub-orbicular (Fig. 439A). There is no definite midrib although the middle part of the thallus is always many layers thick. Sometimes, as in A. erectus (Fig. 439C & D), the thallus is raised on a thick ascending stalk.

Again, in A. hallii, the thallus is long and pinnately lobed. The dorsal surface of the thallus may be smooth or rough with ridges or even with some lamellar growths. The lower ventral surface always bears numerous rhizoids which are only of the smooth-walled type. Scales, tuberculate rhizoids or mucilage hairs are absent in Anthoceros.

The thallus tissue (Fig. 440A) shows little or no differentiation. The thallus is composed of uniform, thin-walled, parenchymatous cells which are many cells deep in the middle (from 6 to 8 cells in A. laevis to 30-40 cells deep in A. crispulus). In some species, the upper epidermal layer may differ in being formed of smaller, compact cells with larger chloroplasts.

Each cell of the thallus (Fig. 440B) shows a single oval and flattened cliloroplast with a central pyrenoid resembling some green alga like Coleochaete. The nucleus remains very near the chloroplast. Such a chloroplast with pyrenoid is unknown in the whole group of Embryophyta except in Selaginella.

But, this ‘pyrenoid’ is now known to be different from those in the algae as it is actually an aggregate of 25 to 300 disc- or spindle-shaped bodies each of which is capable of being transformed into a rudimentary starch grain.

In several species of Anthoceros, the deeper layers of the thallus show two or more chloroplasts instead of the normal one. There are no air chambers or pores in the tissue of the thallus but the lower ventral surface of the thallus shows small, rounded, opaque bluish green spots which may be detected with a hand lens.

These are intercellular cavities filled with mucilage and opening on the ventral side by means of narrow slits (Fig. 440A & C). These mucilage cavities always contain colonies of endophytic Nostoc. It was previously assumed that this Nostoc was of no use to Anthoceros but now it has been claimed that nitrogen fixed by this Nostoc may be utilised by the latter.

In certain ether species of Anthoceros, in addition to these mucilage ducts, there are schizogenous tubular cavities behind the growing point and running longitudinally through the thallus and also commonly, filled with mucilage. In older parts this mucilage may be replaced by gaseous contents. In a few species, again, there may be some enlarged cells filled with mucilage.

The thallus usually grows by a single apical cell with two cutting faces, one on the dorsal and the other on the ventral side. But, A. erectus and A. himalayensis have been shown to grew by a group of apical cells. The apical cell may split into two giving rise to the dichotomous lobes.

Vegetative Reproduction:

Vegetative reproduction may be by the decay of older parts and growth. But this method is not so common in Anthoceros as in the other Hapatics.

Under favourable conditions, many species of Anthoceros are known to produce tubers by the thickening of marginal patches of the thallus tissue. Such a tuber is pro­tected by a layer of cork cells and may resist desiccation. This germinates into a thallus on the return of favourable condi­tions. Such stalked tubers are known in A. himalayensis (Fig. 439E).

Gemmae are known to develop on the margin of the thallus of some species of Anthoceros. Sometime they are stalked and developed mucilage pores even before they are detached from the parent plant. Theses gemmae germinate into new gametophytes as in Marchantia.

Sexual Organs:

Anthoceros is dioecious or monoecious. A. erectus is a dioecious species while A himalayensis is monoecious. Monoecious species are usually protandrous. Both antheri­dia and archegonia are embedded in the dorsal surface of the thallus. They are ini­tiated just behind the growing points where they develop in continuous rows.

As opposed to the Hepaticopsida, the antheridium develops from a hypodermal cell A superficial dorsal cell usually divides by a periclinal wall (Fig. 441 A). The upper daughter cell becomes the roof initial which divides and redivides forming a multi- celled roof (one or more layers thick) over the antheridium below.

The lower cell is the antheridial initial which may develop a single antheridium or may divide to give rise to several antheridia.

Sometimes younger antheridia may also develop from the base of the older antheridium so that there may be antheridia of different ages in a cluster and the number, which is originally 1, 2 or 4, may go up to 25 or even more within a single antheridial chamber which is gradually formed below the roof with the accumu­lation of mucilage in the intervening space (Fig, 441B).

The antheridial initial divides transversely to form a primary stalk cell below and a primary antheridial cell above.

Further development of the antheridium and spermatogenesis is as in the Hepaticopsida A mature antheridium (Fig. 441B) shows a more or less slender stalk (four to more rows of cells) bearing a club-shaped or spherical antheridium which has a jacket one or more layers thick containing the androcytes inside.

Each jacket cell contains a plastid which develops chlorophyll and becomes green as the antheridium matures. In the ripe antheridium these plastids remain green or become orange. Each androcyte forms a biflagellate antherozoid (Fig. 441C).

The archegonia are produced acroprtally from superficial dorsal cells close to the apical growing point. In case of monoe­cious species, the archegonia develop later in the same thallus which had developed the antheridia. The archegonia develop singly and are completely embedded in the gametophyte in direct contact with the surrounding vegetative cells.

The archegonial initial (Fig. 442A) functions directly as the primary archegonial cell, there being no stalk. Three vertical walls (Fig. 442B & C) cut off three outer jacket initial cells and a primary axial cell. This axial cell now divides trans­versely (Fig. 442D), the lower one becoming the primary ventral cell and the upper divides again (Fig. 442E) forming a top cover initial and a lower primary neck canal cell.

The deve­lopment of the jacket is difficult to follow as it is indistinguishable from the surrounding vegetative cells. The cover initial forms a rosette of four cover cells, the primary neck canal cell forms a vertical row of 4 to 6 or more neck canal cells and the primary ventral cell forms the ventral canal cell and the egg (Fig. 442F).

The neck wall is formed by six vertical rows of cells. There is usually a mucilage mound (Fig. 442G) covering the growing archegonium.

Fertilisation:

At maturity, the roof of the antheridial chamber .breaks down exposing the antheridia which, on absorbing water, rupture by apical apertures. The androcytes are now extruded out into the covering film of water where the antherozoids soon get liberated. The mature archegonium, only whose cover cells are protruding (Fig, 442G), also bursts.

The neck canal cells and the ventral cell disintegrate and are ex­truded into the mucilage mound while the cover cells are thrown off. The egg now becomes directly exposed. Fertilisation then takes place as in the Marchantiales.

Sporophyte:

The zygote (fertilised egg) increases in size and completely fills up the venter. Then it secretes a cellulose wall round itself and becomes four-celled (Fig. 443A) by two successive walls at right angles to one another. Another series of vertical walls brings about the 8-celled octant stage composed of two tiers of 4 cells each. The lower tier of four cells forms the sterile foot after repeated divisions.

The lowermost cells of the foot glow short rhizoid-like projections increasing the absorptive surface for sucking food from the gametophyte. The upper tier of cells divides and its lower cells usually form an intermediate part which, later on, develops an intercalary meristematic tissue.

Very soon, periclinal divisions in the upper cells separate out the inner endothecium and the outer amphithecium (Fig. 443B). The endothecium gives rise to the sterile columella (Fig. 443C) which is composed to 16 vertical rows (the number of rows is even more in some species) of cells as is seen in the t.s. of a mature sporophyte (Fig. 444B).

The amphithecial cells divide periclinally and its outer layers form the jacket which is 4 to 6 cells in thickness in the mature sporophyte (Fig. 444A & B). The outermost layer of the jacket becomes an epidermis with cutinised outer walls. Regular stomata with guard cells (Fig. 444C) occur on this epidermis.

The inner cells of the jacket are parenchymatous and, usually, contain two chloroplastids each in place of the single chloroplastids of the gametophytic cells—but this number may vary.

The inner cells of the amphithecium become the archesporium (Fig. 443C) which overarches the rounded apex of the columella and may or may not extend to the base of the columella. This tissue may become up to 4 layers in thickness. Alternate trans­verse tiers of the archesporium become spore mother cells and sterile cells.

The spore mother cells form the spore tetrads in the usual way, while the sterile cells join end to end forming simple or branched pseudoelaters of 3 to 5 cells (Fig. 444A & B) which lose their proto­plasmic contents when mature and remain thin-walled or are variously thickened.

The mature sporophyte (Fig. 444A) shows a bulbous foot, the lowermost cells of which are haustorial, though often sharply delimited by the gametophytic tissue. Above the foot is the slender, erect, smooth, cylindrical capsule standing out like a bristle (or a ‘horn’—hence the English name ‘hornwort’) which projects out 2 to 15 cms in some cases.

The base of the capsule is en-sheathed by an involucre to a certain height. This involucre is formed by the development of the gametophytic tissue immediately surrounding the base of the growing capsule.

Between the foot and the capsule there is no seta but a short intermediate zone which is meristematic and is regenerating the capsule from the base. Thus, the capsule is always in different stages of growth: it is in the embryonic stage at the base and very mature at the tip. The center of the capsule is taken up by the columella and round it is the jacket with a regular epidermis and stomata (Fig. 444C) below which is the green paren­chymatous photosynthetic tissue.

The columella is not 3 regular conducting tissue although it may represent the first tendency towards it. In between the jacket and the columella is the cylindrical archesporial zone which is young and one-layered at the base, gradually showing differentiation into spore mother cells and pseudoelaiers upwards.

At the top there are regular spores and pseudoelaters. When mature, the top of the capsule becomes black or brown. A split appears below the tip and gradually widens upwards. Hygroscopic movement of the pseudoelaters releases the mature spores at the top and the split into two halves progresses downwards.

The tip of the columella projects out like a flagellum. The top of the capsule looks twisted at this stage (Fig. 444D).

New Gametophyte:

The spore (Fig. 445A) germinates on a suitable substratum. The exospore bursts and the endospore comes out as a germinal tube (Fig. 445B) at the tip of which the new gametophyte is formed by the development of a thallus.

Figure 446 on shows the life cycle of Anthoceros.

Family Notothylaceae Genus Notothylas:

Notothylas is a common genus in India growing on shady moist soil or rock in tropical and subtropical regions. Three Indian species were reported previously: N. indica, N. levieri and N. chaudhurii. To these Udar & Singh have added two more species: N. anaporata and N. pandei.

Because of its peculiarities, Midler (1940), Proskauer (1951) and Reimers (1954) are of opinion that this genus should be taken out of the Anthocerotaceae and placed to form a new family Notothylaceae.

The gametophytic thallus (Fig. 447A) is yellow-green forming an orbicular or sub- orbicular rosette which has got a characteristic pleated and not flat appearance. Anatomically, the tissue generally resembles that of Anthoceros, not showing any diff­erentiation. It is 6 to 8 cells deep in the middle, thinning out to 1 to 3 cells in the edges.

Notothylas may be dioecious but all the Indian species are monoecious and protandrous. The antheridia (Fig. 447B) and the archegonia (Fig. 447C) generally resemble those of Anthoceros, only the neck canal cells are wider.

The sporophyte differs in different species. In N. indica (Fig. 448A), the endo­thecium forms the columella and the amphithecium forms the archesporium on the inside and the jacket on the outside.

In some species, the ardhesporium is developed partly from the amphithecium and partly from the outer endothecium. In N. levieri (Figs. 448B & C) and N. chaudhurii, there is no columella, the entire endothecium forming the archesporium while the amphithecium forms the jacket.

The archesporium develops from the endothecium in the Hepaticopsida and from the inner amphithecium in Anthoceros. Thus, the position of Notothylas is clearly between the two. In the cell wall thickening of the capsule wall also some Notothylas species approaches the Jungermanniales of Hepaticopsida.

The mature sporophyte is only 2 to 3 mm long, tapering at both ends and lies flat (horizontal) on the thallus, scarcely projecting beyond the thallus margin (Figs. 447A and 448C). It is usually completely en-sheathed by the thin, membranous involucre. The foot is much smaller than in Anthoceros although the haustorial out­growths are well developed.

The intermediate meristematic zone also is much less deve­loped, the meristematic development being negligible so that the capsule does not grow much.

The pseudoelaters are unicellular, of irregular form, and have thickenings on the walls. The dehiscence is not as prominent as in Anthoceros. N. indica and N. levieri dehisce along one suture like a follicle. Spores are often liberated by the decay of the capsule wall.