Question

Give an illustrated account of life history of fucus

Answer 1

Occurrence of Vaucheria:

Vaucheria is represented by 54 species of which about 19 species are found in India. Vaucheria is found mostly in fresh water but about six species are marine and some are terrestrial found on moist soil.

ADVERTISEMENTS:

The terrestrial species like V. sessilis and V. terrestris form green mats on moist soil in shady places in green houses. V. amphibia is amphibous. V. jonesii was reported by Prescott (1938) in winter ice in U.S.A. The common Indian species of Vaucheria are V. amphibia, V. geminata, V. polysperma, V. sessilis and V. uncinata etc.

Thallus:

The thallus is made of long, cylindrical well branched filaments. The filament is aseptate, coenocytic structure. The thallus is attached to substratum by means of branched rhizoids or branched holdfast called the haptera. The thallus of V. mayyanadensis is differentiated in subterranean branched rhizoidal system and an erect aerial system. The filaments are rough, interwoven and appear as dark green felt like structure.

Some species like V. debaryana show calcium carbonate incrustations. The branching may be lateral or dichotomous. The filaments are non-septate, the protoplasm with many nuclei is continuous along the entire length of thallus thus the coenocytic Vaucheria thallus makes siphonaceous structure (Fig. 1A, B). The septa formation occurs only during reproduction or in Gongrosira condition or for sealing of an injury.



The thallus structure is differentiated into cell wall and protoplasm. The cell wall of thallus is thin, weak and non-elastic. The cell wall is made of two layers, the outer layer is pectic and the inner layer is cellulosic. Inner to the cell wall there is thick layer of protoplasm. A very large central vacuole filled with cell sap runs from one end of the filament to another forming a continuous canal or siphon.

In peripheral part of protoplasm are present a large number of small oval or disc shaped chloroplasts which lack pyrenoids (Fig. 1 B). Christensen (1952) reported presence of pyrenoids in chromatophores.

The chromatophores in Vaucheria contain pigments, chlorophyll a, chlorophyll e, carotenoids and an unknown xanthophyll. The pigments in Vaucheria are like those of Xanthophyceae as chlorophyll b the characteristic pigment of Chlorophyceae is absent.

Many small nuclei lie in the cytoplasm inner to the layer of chloroplasts. The arrangement of nuclei with respect to chloroplasts is reversed at the time of zoospore formation. The cytoplasm also contains other membrane bound cell organelle such as mitochondria, small vesicles and food is stored in form of oil. The growth of filament is apical, the filament increases in length by apical growth of all the branches.

Nature of Thallus:

ADVERTISEMENTS:

The thallus of Vaucheria is branched, non-septate and multinucleate structure which appears like single large cell but Vaucheria cannot be considered as single cell. As in multicellular forms mitotic divisions take place increasing the number of nuclei. The apical growth takes place. Hence the aseptate coenocytic structure of Vaucheria should be considered as acellular coenocyte.

Reproduction in Vaucheria:

Reproduction in Vaucheria takes place by vegetative, asexual and sexual methods.

(i) Vegetative Reproduction in Vaucheria:

The vegetative reproduction takes place by fragmentation. The thallus can break into small fragments due to mechanical injury or insect bites etc. A septum develops at the place of breaking to seal the injury. The broken fragment develops thick wall and later on develops into Vaucheria thallus.

(ii) Asexual Reproduction in Vaucheria:

The asexual reproduction takes place by formation of zoospores, aplanospores and akinetes

(a) By Zoospores:

The zoospores formation is the most common method of reproduction in aquatic species. In terrestrial species it takes place when the plants are flooded. Zoospore formation takes place in favourable seasons or can be induced if aquatic species are transferred from light to darkness or from running water to still water.

Zoospores are formed singly within elongated club shaped zoosporangium (Fig. 2A, B). The development of zoosporangium begins with a club shaped swelling at the tip of a side branch. A large number of nuclei and chloroplasts along with the cytoplasm move into it. A colourless protoplasmic region becomes visible at the base of cytoplasm and it is separated from rest of the cytoplasm