In this article we will discuss about the vegetative body and reproduction of ectocarpus.
Vegetative Body of Ectocarpus:
The thallus is differentiated into a prostrate, irregularly and more or less profusely-branched portion by means of which it remains attached to the substratum, and an erect portion made up of tufts of branched filaments, whose cells are joined end to end in a single series and are generally narrowed towards the apices of the branches.
In some species, the older portions of the main branches are corticated by the development of a sheath formed by the descending rhizoidal branches. The protoplast of each cell is uninucleate and contains either many disc-shaped chloroplasts or a few band-shaped ones with irregular margins.
Reproduction in Ectocarpus:
Ectocarpus reproduces both asexually and sexually.
The asexual reproduction takes place by the formation of biflagellate zoospores produced within unilocular or multilocular zoosporangium borne by the sporophytic plant, usually terminally and singly on lateral branchlets.
The terminal cell of a branchlet enlarges considerably, becomes more or less ellipsoidal in shape and forms a unilocular sporangium.
As it increases in size, the number of chloroplasts also increases considerably. The nucleus first divides by reduction division and is subsequently followed by several mitoses into 32-64 daughter nuclei. Then, there follows a progressive cleavage of the entire protoplast forming several uninucleate daughter protoplasts, each with a single chloroplast.
Each daughter protoplast is finally metamorphosed into a zoospore with two laterally inserted flagella of unequal lengths. The whole mass of zoospores, thus formed, is extruded through a small opening at the apex of the zoosporangium, and they begin to swim freely in water in all directions. Each zoospore, after a period of activity, comes to rest and develops into a new gametophytic plant.
The development of a multicellular sporangium also takes place from the terminal cell of a lateral branchlet. This cell, by repeated transverse and vertical divisions, forms hundreds of small cubical cells, arranged in 20-40 transverse tiers. The protoplast of each cell is then metamorphosed directly into a single biflagellate zoospore. These zoospores are liberated through a small pore, either terminal or lateral, on the wall. Each zoospore, after a period of activity, comes to rest and gives rise to a sporophytic plant, i.e. a plant of the same generation from which it has developed.
It is to be noted that the unilocular sporangia are sporangial in nature in which reduction division takes place prior to the formation of haploid zoospores, and these are borne on the sporophytic plant (diploid). The multilocular sporangia borne by the sporophyte are also diploid. The zoospores produced by these structures are diploid, since there is no reduction division preceding their formation.
In order to differentiate the multilocular sporangia from the multilocular gametangia, which are morphologically alike but are borne on the gametophytic plant, these have been designated as neutral sporangia and the zoospores, neutral zoospores, as they give rise to the same generation (sporophyte) instead of the alternate generation (gametophyte).
Sexual reproduction takes place by the fusion of physiologically different isogametes produced within multilocular gametangia borne by the gametophytes.
The structure and mode of development of these gametangia are similar to those of the neutral sporangia borne by the sporophytic plant. Each protoplast of a gametangium develops into a biflagellate gamete. The gametes are similar to zoospores and fusion occurs only between the gametes from separate gametophytic plants (heterothallic), which are often interpreted as sexually distinct. The diploid zygote, thus formed as a result of union of gametes, germinates and forms the sporophytic plant (diploid).
In the life cycle of an Ectocarpus, there is an alternation of a distinct haploid generation of sexual plants bearing male and female gametes respectively and a generation of diploid asexual plant producing the zoospores.
Sometimes the gametes may develop parthenospores which give rise to new gametophytes.
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