Reproduction in Ulva

Ulva usually multiply bymeans of fragments which are accidentally produced from a thallus. Vegetative multiplication also takes place by means of the proliferation of perennial holdfast.

Asexual reproduction : Asexual reproduction takes place by means of quadriflagellate zoospores. The zoospores are formed in ordinary vegetative cell by the dividing up of protoplast. The divided parts of the protoplast metamorphose into zoospores, which liberate through an opening in the cell wall. The contents of any ordinary cells produce 4-8 zoospores. The zoospores are formed at first in the cells near the margin, later they are formed in other cells too which are always from the margin. The formation of zoospores continues until all the cells are used and nothing remains of the thallus but a filmy mass of empty cell wall. The liberation zoospores takes place at the time when the thalli are reflooded by incoming tides and usually during morning tides. Sometimes the zoospores are liberated in large quantities and they colour the water green. After swimming for an hour or so, a zoospore comes to rest on some substratum withdraws, its flagella and secretes a wall around it. Soon after, it divides by a transverse wall giving rise to two cells. The lower cell develops into a rhizoidal holdfast and the upper into the blade.

Sexual reproduction : The zoospores develops into sexual plant which produce gametes. The biflagellate gametes are produced at the margin of a thallus in a zone 5 to 15mm broad, of different colours from vegetative portion and a zone in which every cell forms gametes. The gametes are formed by repeated bipartition of the protoplast of a cell. The first cleavage is always parallel to the thallus surface and the second vertical to the first. Cleavage continues until 32 to 64 daughter protoplasts are formed.

Each daughter protoplast metamorphoses into a biflagellate gamete. Just before the cleavage of the protoplast each cell develops a beak like outgrowth as its outer face and it expends to the thallus surface. Later on a pore is formed at the tip of this beak, through which the gametes are liberated. The gametes are smaller than zoospores. They are priform inshape with a single chloroplast and an eye spot. The gametophytes liberate gametes at the beginning of each series offspring tide.

After fusion of the gametes quadriflagellate zygote is formed. It swims foa r short time and then comes to rest, withdraws its flagella and secretes a wal around it. Within a day or two the germination of zygote takes place. The division of the zygote nucleus is mitotic. The two daughter cells areformed by means of division of the zygote. One of the two daughter cells develops into a rhizoid and the other eventually develops into a blade. In the development of the blade first divisions are all transverse and form a filament of several cells after which both vertical and transverse cell division takes place. When these plants produce zoospores the number of chromosomes is reduced so that the zoospores have a haploid number and give rise to a sexual plant with a haploid number

Alternation of generation : There is an alternation not only of asexual plants but of asexual plants with a diploid number of chromosomes and sexual plants with a haploid numbers. . An alternation of diploid asexual generation (sporophyte) and a haploid sexual one (gametophyte). With the results of the fusion of two gametes the number of chromosomes being double and carried over to the cells of sporophyte. The reduction division takes place when the zoospores are formed. The haploid zoospores give rise to the gametophytes. Both kinds of plants are morphologically identical and therefore ulva shows an isomorphic alternation of generations.