Euglenophyta: General Features and Relationships

In this article we will discuss about the general features and phylogenetic relationships of euglenophyta.

General Features of Euglenophyta:

The Euglenophyta represent mostly unicellular naked motile individuals which usually occur in fresh-water. Some species form dendroid colony. The thallus possesses a definite nucleus and the grass-green colour is due to the presence of chlorophyll localized in chromatophores.

Few species lack chromatophores and are colourless. Each euglenoid cell bears several chromatophores which may be discoid, band-shaped, or stellate.

There are species whose cell may contain a red pigment—haematochrome. Cells contain pyrenoids which may be within or outside the chromatophores. The reserve food is paramylum—a carbohydrate related to starch, and fats. Besides assimilation of carbon with the help of chlorophyll the nutrition may also be holophytic, and holozoic.

Number of flagella is one, two or rarely three being attached to the anterior end of the cell, close to which are the gullet and red eye-spot.

Reproduction is by cell division along the long axis. Many species form thick- Walled resting cells—cysts, which germinate into new individuals. Sexual reproduc­tion is practically unknown except in one genus only.

This division includes only one class Euglenophyceae.

Phylogenetic Relationships of Euglenophyta:

The Euglenophyta exhibit both plant as well as animal characters. The presence of gullet, holozoic mode of nutrition and the absence of chlorophyll in some species resemble animals and are classified as Protozoa by zoologists.

On the other hand, they are classified by botanists under Algae, since they have simplest vegetative body and most of the forms possess chlorophyll with which they can carry on photosynthesis like green plants. The genus Colacium has true algal organization with immobile cells which have walls and Colacium forms colonies.

One may regard these euglenoids as animals which show some plant characters or may think of them as essentially plants, some of which have lost their chlorophyll and so have assumed as animal-like method of securing food.

They are possibly very primitive organisms, apparently closely related to the original stock from which probably both the plant and animal kingdoms have evolved. At least, some biologists hold this opinion.

Some avoid the difficulty of classifying these forms as plants or animals by placing them in a special group, the Protista, in which border-line organisms like, the bac­teria, the slime molds, and similar other flagellates are included.

Formerly, botanists placed the euglenoids along with all the other pigmented, flagellate unicellular organisms in a group known as the Flagellata, or the flagellates. There is a tendency to distribute them to the various taxa. They differ from the Cyano- phyta for having definite chromatophores.

They are not included under the Chloro- phyta as their food reserve is not starch. They are thus classified as a separate taxon Euglenophyta.

Genus Euglena:

This is a single-celled uniflagellate free-swimming organism occurring exceedingly in fresh-water small ponds, especially rich in organic matter. A number of species inhabits damp mud and salt-marshes. All the species of Euglena are always solitary and never form colonies. The highly differentiated naked body shows a more definite

trend in the direction of animal world than, the pigmented plants. The shape of the individual cell is spindle-shaped, sometimes spirally twisted and at times forms a colourless tip at the posterior end, with anteriorly placed flagellum and the posterior end is more or less pointed. The exterior position of the cytoplasm is differentiated into a soft more or less rigid periplast.

The cell contains a centrally placed nucleus which is recognizable without staining and numerous discoid to band-shaped green chloroplasts which carry on photosynthesis (Fig. 93A). Pyrenoids may or may not be present, when present may be naked or with a sheath of paramylum granules.

The chloroplasts in most cases are bright-green in colour with the exception where the cells may also produce a red pigment (haematochrome).

The products of assimilation are paramylum granules of characteristic shape originating in the cytoplasm. Euglena ingests solid foods. A few species lack chloroplasts, and these apparently are saprophytes absorbing organic matter by diffusion through cell surface.

At the anterior end of a cell there is a lask-shaped gullet with a narrow neck and an enlarged posterior, the reservoir, adjoining to which are one or more contractile vacuoles (Fig. 93B). In the vicinity of the gullet there is a red eye-spot which is sensitive to light.

The single flagellum bifurcates at its lower end within the reservoir and each fork terminates in a blepharoplast (Fig. 93G). In reproduction the nucleus divides mitotically into two daughter nuclei which is followed by longitudinal division of the cell producing two new indivi­duals (Fig. 93D to F).

Sometimes the daughter cells are retained within an envelope and pass through palmelloid condition by further dividing and redividing. Forma­tion of thick-walled cells to protect themselves against unfavourable condition is quite common in Euglena. Sexual reproduction is by conjugation of amoeboid isogametes.