In this article we will discuss about:- 1. Life Cycle of Ulothrix 2. Plant Body of Ulothrix 3. Cell Structure 4. Features 5. Reproduction.
Occurrence of Ulothrix:
The genus Ulothrix (Cr. oulos, woolly; thrix, hair) includes about 30 species. Most of them are found in fresh water of tanks, ponds, lakes etc. The common fresh water species are U. aequalis, U. zonata etc. Some grow in saline water (marine) such as U. pseudoflacca and U. flacca. Species like U. flaaca grows in the intertidal region.
Another member U. implexa grows as lithophytes. They prefer to grow in cold water and are available from rainy season to spring. They disappear during summer months.
Plant Body of Ulothrix:
The thallus of Ulothrix is filamentous, long, unbranched and multicellular, where the cells are arranged in a single row (i.e., uniseriate). The filaments are bright green in colour and remain attached at one end with the substratum such as stones, rocks or some other solid objects.
The plant body (Fig. 3.59A) is differentiated into three regions:
1. Basal cell,
2. Apical cell, and
3. Middle cells.
1. Basal Cell:
It is the lowermost cell of the filament, commonly long, gradually narrowed downwardly and towards the basal end it expands and forms a disc-shaped structure. The cell is hyaline or brownish in colour, which performs the function of fixation to the substratum and called holdfast or rhizoidal cell.
2. Apical Cell:
It is the topmost cell of the filament, which is dome-shaped and green in colour.
3. Middle Cells:
All the cells in-between basal and apical cells are alike. The cells are broader than its length i.e., rectangular in shape. (The cells may be quadrangular e.g., U. moniliformis or it may be much longer than its breath e.g., U. subconstricta.)
Cell Structure of Ulothrix:
All the cells except the basal holdfast can divide and their wall is composed of outer pectin and inner cellulose (Fig. 3.59B). Inner to the cell wall is the cell membrane. The cell membrane encloses the protoplast. It consists of cytoplasm, chloroplast and nucleus.
The cytoplasm forms a lining layer or primordial utricle adjacent to the cell membrane. Single nucleus remains embedded inside the cytoplasm. The central region is occupied by the vacuole, which contains cell sap.
In the peripheral region of the cytoplasm near the cell wall, single girdle or band-shaped chloroplast is present. (Other type of chioroplasts are c-shaped, collar-shaped or ring-shaped.) The chloroplast contains one (U. rorida) to many (U. zonata) pyrenoids.
Important Features of Ulothrix:
1. This, is a fresh water alga growing on substratum like sand particles, rocks etc.
2. The plant body is unbranched, filamentous and differentiated into apex and base.
3. Cells have band or girdle-shaped chloroplast containing one or more pyrenoids.
4. Reproduction takes place by vegetative, asexual and sexual means.
5. Vegetative reproduction takes place by fragmentation.
6. Asexual reproduction by takes place bi- or quadriflagellate zoospores.
7. Sexual reproduction is isogamous.
8. Zygote on germination produces 4 or more zoospores those on germination produce new filaments.
Reproduction of Ulothrix:
Ulothrix reproduces by all the three means:
Vegetative, Asexual and Sexual:
1. Vegetative Reproduction:
It takes place by fragmentation.
In this process the filament breaks up into a number of parts. Each part is capable of developing a new plant like its parent.
2. Asexual Reproduction:
Asexual reproduction takes place in winter, during its active growth. It takes place by the formation of zoospores, akinetes and palmella stage.
a. Zoospore Formation:
Zoospores are formed during favourable condition with proper growth of the plant. Any cell except holdfast is capable of producing zoospores (Fig. 3.60A). During zoospore formation the protoplast becomes slightly contracted from the cell wall.
The protoplast either develops into single zoospore (U. fimbriata) or undergoes division and form 2, 4, 8, 16 or 32 units. Each unit contains single nucleus and cytoplasm. These small units form zoospores.
The zoospores are of different types:
Types of Zoospores:
Species of Ulothrix with narrow cells produce quadriflagellate zoospores of one kind. But species with broader cells like U. zonata produces 3 types of zoospores (Fig. 3.60B, C and D).
i. Quadriflagellate macrozoospores- usually 4 per cell,
ii. Quadriflagellate microzoospores- usually 8 per cell, and
iii. Biflagellate micro- zoospores- usually 16-32 per cell.
The zoospores are morphologically almost alike but differ in size, number of flagella, and position of the eye-spot. They also vary in their swimming period. The macrozoospores can swim for few to 24 hours, but the micro- zoospores (both types) can swim for 2-6 days. The macrozoospores are pear-shaped with tapering at posterior end, while the microzoospores are narrowly ovoid with rounded posterior end.
Germination of Zoospores:
After maturation the zoospores are liberated through a pore developed on the side wall and initially remain in a delicate mucilage vesicle (Fig. 3.60A). The vesicle soon dissolves and zoospores are liberated. The zoospores can swim for different period of time (few hours to even 6 days).
Coming in contact with suitable substratum they lose their flagella and get attached. The cell divides horizontally. The lower attached cell develops into holdfast and the upper cell forms the filament (Fig. 3.61) through repeated divisions.
With the sudden change of environment towards unfavourable condition during zoospore formation the protoplast units do not form flagella and remain inside the mother cell as non-motile units. These unicellular; uninucleate, thin walled non-motile units are called aplanospores (Fig. 3.62A). During favourable condition they germinate after or before liberation from parent cell.
During drought, sometimes the entire protoplast of a cell may round up and forms a single thick walled structure, the hypnospore. During favourable condition it germinates and develops into a new filament.
Akinetes are formed during extreme unfavourable condition. During this process the cell becomes enlarged, protoplast accumulates food material and then forms thick wall around itself. This thick walled resting vegetative cell is called akinetes (Fig. 3.62B). It is found in U. zonata, U.acrorhiza, U. oscillarina etc.
e. Palmella Stage:
Sometimes the wall of the aplanospore mother cell becomes mucilaginous. Consequently the wall of aplanospores also gets enveloped by mucilaginous substance. These coverings protect the aplanospores against desiccation.
In this way -many green round bodies become enclosed in a mucilaginous mass, called palmella stage (Fig. 3.62C). During favourable condition these green bodies come out by the dissolution of the mucilage covering and germinate into new plants.
3. Sexual Reproduction:
Towards the end of growing season they reproduce by sexual means. The sexual reproduction is isogamous i.e., the union between similar gametes (Fig. 3.63).
The fusion takes place between the gametes developed in different filaments i.e., they are heterothallic or dioecious (U. rorida is monoecious). Morphologically gametes are similar to the microzoospores. They are produced in the gametangium (Fig. 3.63A) similar to zoosporangium.
Depending on species the number of gametes may be 8, 16, 32 or 64. Like microzoospores they are uninucleate and biflagellate but smaller in size. Though the gametes are morphologically similar, they are physiologically different and designated as + and – strains.
Liberating from the gametangium, the gametes swim for some time. The gametes of opposite strain (+ and -) fuse and form a quadriflagellate spindle shaped zygote (Fig. 3.63B, C). Initially the zygote keeps on swimming but later it settles down on some substratum, withdraws its flagella and becomes round off.
The zygote takes rest for about 5-9 months. After rest the nucleus of zygote (2n) undergoes meiosis and forms 4 haploid nuclei of different strains (2+ and 2- type). The mitosis may follow the meiosis and forms 8 (Fig. 3.63E) to 16 haploid nuclei.
The nuclei along with some cytoplasm form spores, called meiospores. The meiospores are haploid and quadriflagellate. On germination they develop into haploid Ulothrix filaments either + or – type.
Sometimes, the gametes fail to fuse, lose their flagella, and secrete a thick wall around them and are called parthenospores or azygospores. After rest, each parthenospore germinates directly into a new plant.
Ulothrix zonata, U. tenerrima, U. aequalis, U. variabilis, U. oscillarina etc.
Life Cycle of Ulothrix:
Fig. 3.64 and 3.65 depict life cycle of Ulothrix.