In this article we will discuss about the vegetative body and reproduction of callithamnion.
Vegetative Body of Callithamnion:
The plant body is richly branched with an usual height of 2-5 cm. The branches are alternate and are arranged in definite spiral order (polystichous or distichous). Sometimes some of the lateral branches may exhibit pseudo-dichotomy. The central axis is composed of broad, thick-walled cells and generally the older parts remain enveloped by means of delicate, multicellular, corticating filaments.
The cells are uninucleate or multinucleate, and generally possess ribbon-shaped chromatophores. The outer cells of the corticating filamentous envelope may also contain chromatophores. The plant remains anchored to the substratum by means of multicellular rhizoidal filaments, coming out from the cortex as well as from the basal cells of the lower branches; these filaments sometimes unite to form a definite disc-shaped holdfast.
Reproduction in Callithamnion:
Callithamnion reproduces both by sexual and asexual methods. The plant is heterothallic. The male plants are usually smaller than the female ones. The antheridia or spermatangia occur in spherical or cushion like groups forming sori on the lateral branches. Each of these sori remains enclosed within a common gelatinous sheath and is composed of a mass of small-celled filaments, from whose tips the mother cells appear; the mother cells ultimately give rise to antheridia.
The entire protoplasmic contents of an antheridium, on maturity, are liberated as a spermatium. The procarp generally has two mother cells (pericentrals), only one of which bears the curved, 4 celled carpogonial branch. The position of the carpogonium is approximately midway between the two mother cells and it consists of a dilated basal region containing the egg nucleus inside, and a distally tapering trichogyne. A liberated spermatium, while floating on water, lodges against the trichogyne.
A dissolution takes place at the point of contact and the spermatial nucleus passes down through the opening and unites with the egg nucleus. After fertilization, each of the two mother cells, lying on the two sides of the carpogonium, cuts off an auxiliary cell. The carpogonium, at this stage, becomes broader and divides longitudinally forming two cells. Each of these cells, in its turn, cuts off a small connecting cell, which joins up with the minute protuberance developed from the adjacent auxiliary cell.
The haploid nucleus of the auxiliary cell sinks to the bottom along with one of the two diploid nuclei, while the other diploid nucleus migrates to the apical region of the auxiliary cell. The basal region of the auxiliary cell, containing degenerating auxiliary as well as the diploid nuclei, is cut off from the apical portion by means of a septum.
The apical region of the auxiliary cell then produces the gonimoblast filaments which bear carposporangia. The naked, usually spherical masses of carposporangia remain enclosed in a common gelatinous sheath. Each carposporangium liberates a single carpospore, which on germination gives rise to a diploid plant, called the tetrasporophyte. No definite cystocarp is formed.
The tetrasporophyte bears tetrasporangia developed from the fertile pericentral cells, only one pericentral cell of a transverse tier giving rise to a sporangium. The single nucleus of the tetrasporangium undergoes meiotic divisions and four tetrahedrally arranged tetraspores are ultimately formed. On liberation the tetraspores develop into the gametophytes.
There is, thus present in the life cycle of Callithamnion, a definite triphasic type of alternation of generations.
In some species, there occur polysporangia in the position of the tetrasporangia which develop polyspores within. It is believed that the polyspores are mere derivatives of the tetraspores.
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