In this article we will discuss about:- 1. Occurrence of Vaucheria 2. Thallus Structure of Vaucheria 3. Affinities.
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.
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 Structure of Vaucheria:
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:
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.
Affinities of Vaucheria:
The position of Vaucheria in algae has always been debatable. Fritsch (1935) placed it in the order Siphonales of the class Chlorophyceae, it was also supported by Iyengar (1951). Chadefaud transferred it to Xanthophyceae and Smith (1959) placed it in order Heterosiphonales of the class Xanthophyceae. This view was supported by Chapman (1962), Taylor, Prescott (1969) and Morris (1968).
A. Affinities of Vaucheria with Xanthophyceae:
(i) Siphonaceous, acellular thallus.
(ii) Predominance of carotenoids, over chlorophylls. Absence of chlorophyll b from Vaucheria which is a characteristic pigment of Chlorophyceae.
(iii) Chloroplasts without pyrenoids.
(iv) Reserve food material is oil, instead of starch.
(v) In Vaucheria antherozoids flagellation is heterokontic. There are two lateral, unequal flagella. The anterior flagellum is tinsel type and the posterior as whiplash type.
B. Affinities of Vaucheria with Chlorophyceae:
(i) Multinucleate, aseptate, coenocytic thallus.
(ii) Sexual reproduction being advanced oogamous type.
C. Affinities of Vaucheria with Oomycetes (Fungi):
(i) The development of the sex organs in Vaucheria show striking resemblance to some members of Oomycetes.
(ii) Coenocytic nature of thallus is like that of Saprolegniaceae.
These features along with heterokontic flagella in antherozoids suggest that lower fungi could have been derived from Vaucheria like ancestors.