In this article we will discuss about the classification of dictyotaceae.
Family Dictyotaceae:
Plants are of moderate to large size, simple or branched, the divisions growing from apical cells or marginal rows of apical cells, forming fan-like foliaceous, strap- shaped branching thalli, usually composed of cortical and medullary layers of cells. Sexual and asexual generations are distinct, the plants of the two phases similar in form.
Asexual reproductive elements consist of sporangia in which four or eight aplanospores (commonly called tetraspores) are produced. Sexual reproductive elements, eggs are borne singly in the superficial oogonia; and uniflagellate antherozoids, borne in the superficial antheridia.
Genus Dictyota of Dictyotaceae:
This genus is characterized by an isomorphic alternation of generations, with the growth of thalli initiated by a single apical Cell. It is an annual of the warm seas and is common along the Indian coast, usually in rocky pools between the tide marks.
The prostrate portion of the thallus is represented by cylindrical rhizome with rhizoids which affords anchorage of the flat dichotomously branched, fan-shaped upright portion of the thallus to the substratum.
The upright portion has a lower cylindrical stalk-like structure gradually expanding into dichotomously branched ribbon-like in form (Fig. 116A). A mature thallus consists of a middle layer of large colourless cells with few or no chromatophores bounded by layers of small assimilating cells, densely packed with chromatophores.
On both surfaces of the thallus there are colourless multicellular hairs which are shed during reproductive phase. Dichotomy of the upright portion is initiated by the longitudinal division of a single apical cell (Fig. 116B). The two daughter cells so formed give rise to dichotomous branching by continued independent division.
Asexual reproduction is brought about by means of tetraspores produced in superficial tetrasporangia which are scattered on both surfaces of the sporophyte, occurring singly or in small groups (Fig. 116 I & J). Each tetra-sporangium has a basal stalk cell and a spherical body.
Four tetraspores are formed in the tetrasporangium as a result of meiotic division of the large diploid nucleus followed by the cleavage of the cytoplasm into four bits corresponding to each nucleus. All the four spores germinate into new Dictyota plants—the gametophytes (male and female).
Sexual reproduction is oogamous. Both male and female gametophytes morphologically resemble the sporophyte only difference being that the gametophytes bear sexual reproductive organs, whereas the sporophytes bear sporangia. Both antheridia and oogonia are borne in sori on separate plants. The male gemetophyte bears antheridia is small groups of sori (Fig. 116C & D).
The number of antheridia in a sorus ranges from 100 to 200. Each antheridium is formed from a single epidermal cell of the thallus which by subsequent divisions forms antheridia.
A mature antheridium is a large colourless structure, the contents of which are metamorphosed into a single uniflagellate pear-shaped antherozoid (Fig. 116E), which is liberated by the gelatinization of the antheridial wall. The oogonial sorus is very similar to the antheridial sorus (Fig. 116F & G). The number of oogonia in a sorus goes even up to 100.
The oogonial sorus is developed from a superficial cell of the thallus which elongates vertically and by transverse division is differentiated into a stalk cell and a primary oogonial cell. The primary oogonial cell develops into an oogonium, contents of which develop into a single egg.
Both antherozoids and eggs are liberated out in the surrounding water and a large number of antherozoids are attracted by each ovum (Fig. 116H). Fertilization is entirely external. A single antherozoid penetrates the egg and fertilization is effected. The zygote secretes a wall and without undergoing resting period develops into a sporophytic plant which morphologically resembles the gametophyte.
In Dictyota meiosis takes place in the tetrasporangia during spore formation (Fig. 117).
Special features of Genus Dictyota:
1. Plant body differentiated into prostrate rhizome and rhizoids, and aerial dichotomously branched ribbon-like structure.
2. Internal differentiation of tissue of the plant body for the division of labour.
3. Isomorphic alternation of generations.
4. Reduction division during the development of tetraspores in the tetrasporangia borne on the sporophyte.
5. Gametophytic plant body heterothallic.
6. Sexual reproduction oogamous.
7. Fertilization of ovum outside the plant body in the open water.