In this article we will discuss about the classification of ulvales.
Plants are usually tubular or parenchymatous thalloid, capillary to broad, occasionally reduced to one or two rows of cells; attached or becoming free-floating; one or two cells in thickness. Cells possess one or two large lateral chromatophores with pyrenoids and single nucleus. Sexual and asexual plants are morphologically indistinguishable exhibiting isomorphic alternation of generaions.
Reproductive cells of the thallus are unaltered or slightly enlarged.
Ulva, commonly known as sea lettuce, is rather a marine alga with the exception of a few species that grow in brackish water. The alga is a very interesting one for, besides having advanced parenchymatous thalloid type of vegetative body (Fig. 43A), it exhibits in its life cycle an isomorphic alternation of generations which has great resemblance with higher plants.
The expanded sheet of thalloid body is composed of two layers of cells which are developed from a single uniseriate filament expanding by lateral divisions. The basal part of the thallus is narrowed into a short stalk which ultimately develops into a holdfast of multinucleate rhizoids growing out from the uninucleate lower cells, with which the alga remains attached to its substrate.
Each cell of the thallus is uninucleate, and contains a single laminate to cup-shaped chloroplast with a single pyrenoid (Fig. 43B). Cell division may take place at any part in the thallus.
Vegetative multiplication is practically unknown except in places where the alga grows in quiet water.
Asexual reproduction takes place by means of quadriflagellate zoospores (Fig. 43C) that are formed from the cells of a diploid thallus—sporophytic generation. These zoospores germinate to produce haploid thalli—gametophytic generation, whose cells again produce biflagellate gametes (Fig. 43D to J). The thalli are heterothallic. Both sporophytic and gametophytic generations are morphologically indistinguishable.
The method of production of zoospores and gametes is identical except that during zoospore formation the division of the nucleus is reductional. The isogametes fuse to form a zygote which loses its flagella and germinates immediately and by division and re- division develops into a diploid thallus (Fig. 43K to T). Most species are isogamous, but a few are anisogamous.
The nature of alternation of generations in Ulva is isomorphic (Fig. 44).
Some Indian species: Ulva lactuca Linn.; U. lobata Kg.; U. reticulata Forsk.
1. Advanced parenchymatous expanded sheet of thalloid vegetative ‘body with holdfast.
2. Laminate to cup-shaped chloroplast.
3. Asexual reproduction by quadriflagellate zoospores formed from the cells of a diploid thallus (sporophyte).
4. Meiosis during zoospore formation
5. Sexual reproduction is usually isogamous, in a few species anisogamous; gametes developed from the cells of the haploid thallus (gametophyte)
6. Exhibits an isomorphic alternation of generations which has great resemblance with higher plants.