In this article we will discuss about the vegetative body and reproduction of ulothrix.
Vegetative Body of Ulothrix:
The vegetative body is a thallus which is an unbranched filament of indefinite length, consisting of a single row of short, cylindrical cells joined end on end. The filament is differentiated into base and apex, since its basal cell is colourless, elongated and gradually narrowed to form a holdfast, which helps in its attachment to the substratum.
Each cell possesses either a thick or a thin cell wall, which often becomes stratified. The protoplast of each cell is always uninucleate and contains a single girdle-shaped chloroplast, which encircles it partially or completely. The chloroplast contains one to several pyrenoids.
Reproduction in Ulothrix:
Ulothrix reproduces in a variety of ways mentioned below:
Vegetative reproduction takes place usually by the accidental breaking of the filament into many fragments with a few ceils in each. Asexual reproduction takes place by the formation of zoospores, which develop from the protoplasts of ordinary vegetative cells excepting the holdfast. According to species, the number of zoospores produced from each protoplast may be 1, 2, 4, 8, 16, or 32.
In any case, during the formation of zoospores there is a slight contraction of the protoplast followed by nuclear division and subsequent cleavage of the cytoplasm forming two daughter protoplasts, and this process may continue until 32 daughter protoplasts are formed.
Each daughter protoplast is then usually metamorphozed into a quadriflagellate zoospore. When a single zoospore is formed the protoplast of the entire vegetative cell is directly metamorphozed into it.
Each zoospore is pyriform in shape with a stellate chloroplast at the posterior end and a prominent eyespot and is either- (a) Large, 4-flagellate, with an anterior eyespot, called macrozoospore, or (b) Small, 4- or 2-flagellate, with a median eyespot, called microzoospore.
The zoospores are liberated through a pore on the lateral wall of the parent cell. They swim actively in water from one to several days, but ultimately cease their movement and attach themselves to some objects in water by the flagellate end. Finally, the flagella disappear, a wall is secreted around each and by elongation of the cell and subsequent divisions a new plant is formed.
When zoospores are not liberated from the parent cells, each of them secretes a cell wall around it and forms a thin-walled aplanospore. The aplanospores may germinate even before liberation from the parent cell. In some cases, the protoplast of a vegetative cell may be directly metamorphozed into a single thick-walled aplanospore which ultimately germinates into a new plant.
The sexual reproduction is isogamous, which takes place by the union of two flagellate gametes of equal size. But the fusing gametes come from different plants. Hence, the plant is dioecious (heterothallic).
The formation of gametes, as well as their mode of liberation from the parent cell, takes place in the same manner as the zoospores. The number of gametes produced may be 8, 16, 32, or 64. Each gamete is biflagellate, pyriform, with a conspicuous eyespot and smaller in size than a zoospore.
After gametic union a quadriflagellate zygote is formed, which remains motile for a time but ultimately comes to rest, secretes a thick wall around it and after accumulation of considerable amount of food-reserve passes into a resting period.
At the end of the resting period, the zygote nucleus undergoes reduction division and forms 4 to 16 daughter nuclei, from which either zoospores or aplanospores are produced. The zoospores or aplanospores under favourable conditions develop into new plants.
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