Dictyota: Vegetative Body and Reproduction | Algae | Botany

In this article we will discuss about the vegetative body and reproduction of dictyota.

Vegetative Body of Dictyota:

The plant body is a regularly, dichotomously branched, flat, membranous, frond-like thallus, which spreads out in a more or less fan-shaped fashion from a comparatively narrow, cylindrical base with a holdfast; the holdfast remains attached to the substra­tum with the help of rhizoids. The entire plant grows by means of a single apical cell.

Internally, a thallus consists of three layers, a central one (me­dulla) composed of large-sized cells, and an upper and a lower epidermal layers, which are made up of smaller cells containing chromatophores; groups of mucilage hairs arise from these epider­mal layers.

Reproduction in Dictyota:

Dictyota reproduced by vegetative, asexual and sexual methods.

Vegetative reproduction may take place by the decay of the older parts of the thallus, followed by subsequent fragmentation. In some cases, specially modified structures, called gemmae or brood buds, are developed on the body of the thallus for the pur­pose.

Asexual reproduction takes place with the help of large, non- motile spores, called tetraspores. The tetraspores are produced inside tetrasporangia, which are usually grouped together forming ill-defined sori on both the sides of the thallus.

The tetrasporangia are without any involucral covering and are produced on diploid asexual plants only. The solitary nucleus of a tetrasporangium un­dergoes a reduction division and gives rise to four, slightly elon­gated, naked tetraspores, each having a haploid set of chromo­somes.

On maturity, the tetraspores come out through an apical opening on the tetrasporangial wall and germinate directly giving rise to male and female sexual plants in equal proportions, which are morphologically alike with the asexual plants. In some cases, a tetrasporangium may fail to produce tetraspores, and then it ger­minates quickly and reduplicates the same diploid generation.

Sexual reproduction is oogamous. The gametophytes are nor­mally heterothallic. The sex organs are differentiated in groups of sori on the male plants, each sorus being enclosed by a well-defined involucre (a sterile jacket layer of cells).

Each white and glistening antheridial sorus consists of about 100-200 antheridia, and each antheridium forms about 1500 uniflagellate sperms. The oogonial sori are deep brown in colour and are also invested by individual rudimentary involucre.

Each oogonial sorus contains about 25-50 oogonia, each producing a single egg. On maturity, the apex of the oogonium becomes gelatinized and dehisces, thereby liberating the egg. There is a marked periodicity, usually correlated with the fort­nightly sequence of spring and neap tides of the lunar month, in the development of sex organs and gametes.

When sperms and eggs are discharged in sea-water, the latter secrete a substance which attracts the sperms and fertilization is effected very soon (often in course of half an hour’s time). The fertilized egg or oospore readily germinates into a diploid asexual plant. In rare cases, an unferti­lized egg secretes a wall around it and germinates parthenogenetically.

It is to be noted that Dictyota possesses an isomorphic alter­nation of generations.