In this article we will discuss about the vegetative body and reproduction of ulva.
Vegetative Body of Ulva:
The vegetative body is an expanded thalloid sheet of two-celled thickness. From the lower end of the thallus, some cells give rise to rhizoidal outgrowths, which help to fix the plant to the substratum.
These outgrowths finally become closely attached to one another, and give rise to a pseudo-parenchymatous holdfast, which is perennial in nature and bears new blades every year during the spring.
In cross-sectional view, the cells of the thallus are more or less isodiametric or somewhat vertically elongated to the surface of the thallus, and their walls are somewhat fused with one another forming a matrix, which is rather thick and gelatinous in consistency.
Each cell contains towards its inner face a solitary nucleus, and towards its outer face there is a single laminate to cup-shaped chloroplast with one pyrenoid only. New cells are formed by divisions of pre-existing cells in perpendicular directions to the surface of the thallus.
Reproduction in Ulva:
Ulva reproduces vegetatively, asexually as well as sexually. Vegetative reproduction in Ulva takes place by accidental fragmentation of the thallus growing usually in quiet estuarine waters.
Asexual reproduction takes place with the help of quadriflagellate zoospores, produced within the vegetative cells of the thallus. At first the cells which are near the margin of the thallus produce zoospores, and then the remoter ones. The production of zoospores goes on until, practically speaking; all the cells have behaved like zoosporangia. The zoospores come out through a pore in the cell wall, swim for a very short duration, come to rest, and then secrete a wall. A zoospore germinates to give rise to a new sexual plant.
The majority of the species of Ulva are heterothallic. The gametes are generally isogametes, but U. lobata appears to be anisogamic. The protoplast of a vegetative cell undergoes repeated cleavages until sixteen or thirty-two daughter protoplasts are formed within the parent cell.
Finally, each of these protoplasts becomes metamorphozed into a single biflagellate gamete. The gametes come out through a pore developed on the cell wall. The discharge of the gametes is sometimes so very copious that the water turns green in colour.
Each gamete possesses a single chloroplast and a prominent eyespot, and is pyriform in shape. Gametes unite in pairs to form a zygote. The zygote is a first quadriflagellate; it swims for a short while, comes to rest by withdrawing the flagella, and secretes a wall around itself.
The zygote undergoes germination within 24-48 hours after rest, and an equational division of the zygote nucleus takes place. As a result, ultimately new diploid plants are produced. In some cases haploid thalli are developed parthenogenetically from the gametes.
It is of interest to note that in the life-history cycle of Ulva an isomorphic alternation of generations can be traced. Morphologically the two types of plants, the sporophyte and the gametophyte, are identical. The sporophytic thallus produces the haploid zoospores, which develop into gametophytes. The zygotes, produced by the union of gametes developed on these gametophytes, give rise to new diploid thalli.
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