Study Notes on Euglena Viridis (With Digram)

In this article we will discuss about Euglena Viridis:- 1. Habit and Habitat of Euglena Viridis 2. Structure of Euglena Viridis 3. Locomotion 4. Nutrition 5. Respiration 6. Excretion 7. Osmoregulation 8. Reproduction 9. Encystment 10. Sensitivity.

Contents:

1. Habit and Habitat of Euglena Viridis
2. Structure of Euglena Viridis
3. Locomotion of Euglena Viridis
4. Nutrition in Euglena Viridis
5. Respiration in Euglena Viridis
6. Excretion in Euglena Viridis
7. Osmoregulation in Euglena Viridis
8. Reproduction in Euglena Viridis
9. Encystment in Euglena Viridis
10. Sensitivity of Euglena Viridis

1. Habit and Habitat of Euglena Viridis:

Euglena Viridis is found abundantly on the surface of fresh-water ponds. Sometimes the popu­lation of Euglena viridis becomes so dense that water appears to be green at the surface due to the green colour of Euglena.

In the laboratory, Euglena is cultured by introducing a few collected Euglena in cul­ture medium prepared by boiling cow or horse dung in distilled water.

2. Structure of Euglena Viridis:

Euglena viridis is spindle-shaped in appearance. The anterior end is blunt while the posterior end is pointed. The average length of the body is about 40-50 micra by 14-20 micra.

The outer limiting surface or pellicle is firm, elastic and gives the animal more or less a fixed shape. The pellicle is marked by deli­cate and spiral striations which can be seen with difficulty. Beneath the pellicle there are a few elastic fibrils arranged obliquely and longitudinally.

The pellicle is closely followed by a plasma membrane on the inner side. Within the plasma membrane there lies the general mass of cytoplasm differentiated into outer ectoplasm and inner endoplasm. The ectoplasm is thin, non-granular and more ‘sol’ in nature while the endoplasam is granular, vacuolated and more ‘gel’ in nature (Fig. 10.2).

The nucleus is large, spherical and almost centrally situated. It lies in a clear area among the chloroplasts. Suspended in the cytoplasm there are a number of radiating chloroplasts containing chlorophyll (Fig. 10.3C). The chloroplasts are elongated or ovoid in appear­ance. A peculiar type of animal starch, called paramylum, remains scattered in the cyto­plasm in the form of grains.

Sometimes the paramylum bodies show such an increase in number that they almost mask the chloroplasts. When such an Euglena is kept in darkness for several days the paramylum bodies decrease in number. Euglena, like green plants, can synthesise carbohydrate food by photosynthesis.

One to many contractile vacuoles are situated at the anterior end and in close proximation to the reservoir into which the products of contractile vacuoles are voided.

The anterior end bears a narrow depres­sion—the gullet or cytopharynx which leads to a flask-shaped and non-contractile reservoir. In the inner side of the pellicle at the gullet region there occurs a pair of ridges which acts as sphincter muscle.

Near the base of the gullet there is a large pigment spot or stigma (Fig. 10.3A). The stigma is bright red in colour and it is composed of small granules of carotenoid pigments embedded in colourless stroma.

A single flagellum, equal in length to the body, emerges out through the gullet. The flagellum bifurcates into two in the middle of the reservoir and the two roots go to the two compact basal granules or blepharoplasts situated in the cytoplasm just beneath the base of the reservoir. Some are inclined to think that there are two flagella—one short and one long.

Each originates separately from the two blepharoplasts and the shorter one soon after its origin unites with the longer one. The long flagellum is thick. The flagellum is made up of two parts—an elastic axial filament—the axoneme, made up of several fibrils and a contractile cytoplasmic sheath surrounding the axoneme (Fig. 10.3B).

The root of the flagellum close to the stigma bears a lens-like thickening or photoreceptor. Re­cent studies have shown that the stigma acts as a shield to the photoreceptor. When an Euglena rotates on its long axis, the presence of the stigma allows the light to strike the photoreceptor from the sides only. Euglena Viridis tries to orient itself in such a way that the photoreceptor be exposed from time to time.

3. Locomotion of Euglena Viridis:

Locomotion in Euglena viridis is affected in the following ways (Fig. 10.4).

(a) Locomotion with the help of flage­llum:

The actual mechanism involved in flagellar is not satisfactorily known and there are varieties of flagellar movements. To explain the forward move­ment it has been advanced that the flagellum makes a series of lateral movements and as a result, a pressure is exerted on water at right angles to its surface.

This pressure is resolved into two forces, one acting parallel and the other at right angles to the body axis. The parallel force causes the body to rotate while the force acting at right angles drives the animal forward.

Another observation states that Euglena viridis moves forward by the undulating motion of flagellum. A series of undulating waves pass along the flagellum from base to tip at the rate of twelve per second that push the animal forward. The flagellar action exerts forces on the surrounding medium that drives the water away from a stationary animal.

The waves proceed along the flagellum in a spiral manner and cause the body of the Euglena to rotate once in a second. Thus, in its locomotion it traces a spiral path about a straight line and moves forward. The rate of movement is 0.5 mm per second.

Rowing:

In normal locomotion, Euglena viridis can also move by rowing. While rowing the beat of the flagellum consists of an effective stroke and a recovery stroke. During effec­tive stroke the flagellum is held rigid and is slightly arched in the direction of the stroke. The effective stroke helps to push the water backwards and the body draws forwards.

During recovery stroke the flagellum is strongly curved and the flagellum is brought to its normal position and faces minimum resistance during recovery stroke (Fig. 10.4B).

(b) Euglenoid movement:

Euglena some­times shows a very peculiar motion in which waves of contraction pass along the body from anterior to posterior end and the ani­mal creeps forward. The contractions are brought about by the stretching of protoplasm on the pellicle or by the localised fibrils, called myonemes, in the ectoplasm.

4. Nutrition in Euglena Viridis:

The modes of nutrition in Euglena viridis are holophytic and saprozoic. Like a true plant it assimilates carbon and builds up carbohy­drates from carbon dioxide and water. The holophytic type of nutrition occurs in the presence of sun-light and the green pigment chlorophyll plays an important role in the process. Nitrogen and other minerals which remain dissolved in pond water is absorbed by the cell surface.

Excess of carbohydrates manufactured is stored as paramylum. Euglena remains an autoroph so long as it is in light and is provided with essential inorganic compounds. The whole autotrophic process in Euglena is dependent upon external sources of vitamin B₁₂ which is synthesized by bacteria and some micro­organisms.

At times when pond water becomes polluted with dead and decaying organic matter Euglena gives up the holophytic mode of nutrition and switches over to a saprozoic mode. Dead and decaying matters dissolved in pond water are digested extracellularly and then they are absorbed through the general body surface.

Some workers have reported that small organisms are forced to enter the reservoir by the movement of flagellum and they are engulfed. Such occurrence of holozoic mode of nutrition in Euglena is open to doubt.

5. Respiration in Euglena Viridis:

The respiration in Euglena viridis is aerobic. It absorbs dissolved oxygen from the surrounding medium by diffusion. In the process of photosynthesis, during day-time, a good amount of oxygen is liberated. There is every reason to believe that this oxygen is used in metabolic activities.

6. Excretion in Euglena Viridis:

The carbon dioxide accumulated in the process of respiration during day-time is used up in photosynthesis. Unused CO₂ escapes by diffusion through body surface. Nitro­genous waste matter also escapes in the same fashion.

7. Osmoregulation in Euglena Viridis:

Elimination of excess water is done by the contractile vacuole and its tributaries. The radiating or associating smaller vacuoles collect surplus water from the endoplasm and liberate their contents into the main vacuole (Fig. 10.3A), which gradually increases in size and finally contracts to force the fluid into the reservoir.

From the reservoir the fluid escapes through the gullet. Along with this, water soluble wastes are thrown out of the body.

8. Reproduction in Euglena Viridis:

Usual mode of reproduction in Euglena Viridis is longitudinal binary fission (Fig. 10.5). The producing daughter cells are mirror image, because the division is symmetrogenic.

During fission locomotory activities are suspended and the flagellum is withdrawn in some cases. The blepharoplast is the first to divide and the two halves remain attached by a spindle-like structure or by a strand.

This is followed by eumitotic type of division of the nucleus. The cleavage furrow starts appearing from the reservoir and proceeds longitudinally to divide the animal into two. In the two daughter Euglenae regeneration of lost parts occurs immediately after division.

Each one develops a new flagellum. In some cases the flagellum of the mother is retained by one of the daughters and a new one develops in the other.

Sometimes many Euglenae come close together, lose their flagella and round up. They secrete sticky substances in which they lie embedded. This condition is called palmella stage which is often seen as green scum on ponds (Fig. 10.6A). Individual members of the palmella carry on metabolic activities and reproduce by fission. When favourable conditions come back the Euglenae separate, regenerate the flagella and start living normal and active life.

9. Encystment in Euglena Viridis:

Euglena Viridis encysts during the periods of draught and extreme cold. The animal be­comes inactive, withdraws flagellum and assumes a round shape (Fig. 10.6B). Gradu­ally, protective walls are secreted. The cysts are red in colour due to the presence of a pigment called haematochrome.

On the return of favourable condition the cyst wall breaks and the Euglena comes out. Nuclear division may occur in encysted Euglena.

10. Sensitivity of Euglena Viridis:

Euglena Viridis shows photosensitivity and their responses vary according to the intensity of light source. Normally, it swims parallel to the light rays and towards the source of light. The stigma, together with the thickening on the flagellum, constitutes a sort of ‘optic orgamelle’ for the animal. The animal can also respond to various concentrations of chemicals, oxygen and carbon dioxide.