In this article we will discuss about the asexual and sexual modes of reproduction in anthoceros with the help of diagrams.
Asexual Reproduction in Anthoceros:
By progressive death and decay of older parts of thallus vegetative reproduction is not so common in Anthoceros as in other liverworts, Instead the thallus is capable of producing many tubers, which can withstand unfavourable conditions especially the drought to which the thallus is intolerant.
These tubers readily regenerate on return of favourable conditions and thus make the thallus perennial. On the periphery of a tuber are 2-3 layers of corky hyaline cells. This layer surrounds the inner cells which contain starch, oil globules and aleurone grains.
Sexual Reproduction in Anthoceros:
Most of the hornworts are monoecious such as A. fusiformis, A. punctatus and A. himalayensis. The hornwort thalli are protandrous; and archegonia formation begins when antherozoids are mature. This is to prevent self-fertilization.
Some species of Anthoceros are also dioecious and heterothallic; the female plants have broader fertile lobes than male plants. The sex determination is genotypic, two spores of a tetrad develop into male plants and other two into female plants.
These plants grow in close proximity. Dioecious species are A. erectus, A. laevis, A. khandalensis and A. gemmulosus. Both types of sex organs are initiated just behind the growing point, and are embedded in dorsal region of thallus. At a place, there is usually one archegonium embedded in the thallus.
Contrary to it the antheridia are formed in groups, enclosed in chambers, visible as discrete areas known as androecia (Fig. 3.1C). The place of an archegonium can be identified by the presence of a mucilage mound (Fig. 3.1D, 3.2E) on the thallus.
The androecia occur in acropetal succession close behind the growing point, embedded in the thallus, externally appearing as scooped areas; as seen in A. erectus. However, in A. gemmulosus the androecia are hemispherical elevations on the thallus (Fig. 3.2C).
Inside a chamber, many antheridia (Fig. 3.2C) appear in acropetal succession. Normally there are four or less antheridia in a chamber but their number may go up to as many as 30 as in A. gemmulosus and 22 in A. erectus.
A fundamental difference between Anthocerotae and Hepaticeae is that the antheridia] initial is the inner daughter cell (Fig. 3.2C).
Presence of many antheridia in an antheridial chamber is possible due to:
(a) Budding at the base of an old antheridium, which are secondary antheridia and
(b) Antheridial initial by its repeated vertical divisions form many initials, all of which form antheridia, these are all primary antheridia. Both these modes (a and b) are seen in A. erectus.
A mature antheridium is an orange-coloured club-shaped structure (Fig. 3.2D) with a multicellular stalk and a cylindrical body. The dehiscence of a mature antheridium is due to its pressing against narrow opening of the roof
An archegonium arises from a dorsal segment of an apical cell in its close proximity, and remains embedded in the thallus. New archegonia arise in acropetal succession.
A mature archegonium has 2-4 cover cells, a jacket enclosing 4-6 neck canal cells, a venter canal cell and an egg. At the region of venter the jacket cells divide peripherally to form bistratose jacket. The place of an archegonium on the thallus can be identified by the presence of a mucilage-mound (Fig. 3.2E).
At the time of fertilization the cover cells are thrown off, the venter canal cells and cells of neck disintegrate releasing a mucilaginous mass which becomes continuous with the mucilage-mound on the thallus surface. The sperms caught in mucilage reach the egg. Prior to fusion, the egg enlarges and fills the venter. After entry into the egg, the male nucleus, prior to fusion, can be seen along with the female nucleus. Their fusion results into a zygote.
The first division of zygote is longitudinal (Fig. 3.2F); in this way hornworts differ from the rest of bryophytes. Subsequent divisions result in a quadrant and then into an octant embryo. The lower cells of this are destined to form the foot (Fig. 3.2F) while the upper ones form the rest of sporophyte, which is a capsule without a seta.
Early in the development of an embryo there is differentiation of amphithecium and endothecium, whereby the endothecium contributes to the formation of central sterile region (columella) and the amphithecium produces sporogenous tissue and jacket.
At the base of capsule, early in development, differentiates an intercalary meristem, which contributes towards the continuous growth of capsule. In these features the sporophyte of Anthoceros differs from the rest of liverworts.
A protective sheath or involucre encloses the developing sporophyte. The upper part of involucre is derived from the venter of archegonium and the lower part from the cells of thallus. With the elongation of sporophyte, the involucre is pierced through its apex. A part of involucre remains at the apex of sporophyte—it is reminiscent of calyptra of mosses—and the lower part forms a collar around the base of sporophyte (Fig. 3.3A).
This collar helps to retain moisture, which is needed at the base of capsule around the intercalary menstem. In this manner continued growth and differentiation of cells is possible in the sporophyte as long as conditions permit. These conditions are favourable weather as well as vigour of thallus. With age, the thallus becomes old, it tends to decompose and sporophytes are isolated.
The sporophyte is an elongated structure, giving the name ‘hornwort’ to plants with a bulging foot that penetrates thallus. The outer layer of the foot is a superficial lining of elongated cells (Fig. 3.3A). These cells often produce short haustorial projections that push amongst the cells of thallus.
The inner part of the foot is made up of irregularly arranged vacuolated cells. The jacket of sporophyte is multistratose (Fig. 3.3B) and has stomata. A stoma is typical, with a linear aperture and two elongated guard cells (Fig. 3.3C). The jacket is normally 4- to 5-cell thick. The epidermal cells of jacket are thick-walled. The epidermal cells have 2-4 shallow grooves, which mark the lines of dehiscence.
In the central region of sporophyte is columella of elongate cells, which assist in internal conduction.
The young sporophyte has an archesporium (Fig. 3.3A) of a single layer of cells which remains single-layered up to maturity in A. erectus and A. crispulus. However, in A. gemmulosus the archesporium becomes multi-layered as the capsule advances to maturity. The archesporial cells can be identified by their small size and dense contents. Higher up in the capsule these cells differentiate (Fig. 3.3A) in an equal ratio, into spherical-to-oval spore-mother-cells and elliptical to rectangular elater-mother-cells.
Each spore-mother-cell on undergoing meiosis forms a spore tetrad. An elater-mother-cell forms a four-celled elater. Each elater is a compound structure homologous to a spore tetrad. The mature elaters are vermiform four-celled structures with tapering ends. Walls of elaters have irregularly arranged thick and thin areas.
The dehiscence of a capsule (Fig. 3.1F, G) is usually by two longitudinal lines, occasionally it is by a single line and rarely by four lines. In A. crispulus the blackened mature sporophyte splits near the tip, first along one line of dehiscence which then extends backwards.
It is followed by splitting along other lines of dehiscence forming two valves; still attached at the tip and exposing the columella, mass of spores and elaters. The elaters, as they dry out, tend to twist. In this way elaters help to loosen the spores and allow them to disperse. Air currents also play a role in the dispersal of spores.
The spores are semi-circular with a conspicuous triradiate mark (Fig. 3.3D). They tend to remain as tetrads. Spores are unicellular as well as multicellular. The spores are variously ornamented—vermicular, ridged to spiny.
Spores are viable for a long period and germinate readily to form thalli. The sporelings are of two types. In Anthoceros, on a short uniseriate germ-tube differentiates an apical cell that initiates the growth of thallus (Fig. 3.3E).