Euglenophycae - Euglena Viridis - Its structure and Reproduction

Phylum Thallophyta - The Algae - Euglenophycae
The Euglenophyceae include both green and colourless organisms. Those which possess chlorophyll are able to assimilate carbon dioxide and live as plants. Those which are colourless may either live as saprophytes, or holozoically like animals by ingesting solid food. The Euglenophyceae were formerly included in the old group known as the Flagellata which it was impossible to classify collectively either as plants or animals; but according to recent methods of classification the Flagellata are split up into a number of separate groups, some of which are placed in the Thallophyta, among the Algae, while others form a separate group of the Protozoa among the animals. In the past it was not uncommon to consider the whole group of the Flagellata as members of the Protozoa, green and colourless alike, and moreover to extend the scope of the group to include many unicellular and colonial Chlorophyceae, a system which still survives in some quarters, though certainly erroneous.
In the Euglenophyceae there is no definite cell wall, merely a denser protoplasmic surface layer, with the result that the protoplast may exhibit contractile and amoeboid movements. One or more mobile appendages, the flagella, are present, and the motile condition is dominant in the life history. Pulsating vacuoles are present. The organisms may live independently or may unite to form colonies held together by mucilage or possessing stalked investments.
Sexual reproduction is unknown.Multiplication is by binary fission as the result of a longitudinal split of the protoplast during the motile phase. Many produce thick-walled resting spores. The Euglenophyceae contain the single order Euglenales. 'vVe shall consider one common green example of the group, Euglena viridis.
Euglena viridis 
This little organism is found very commonly in fresh-water ponds and ditches, where it may at times occur in vast numbers producing a green colouration of the whole of the water. There are, however, a number of allied species, some of which occur in brackish or even in sea water.
The organism consists of a single oval or fusiform protoplast which often terminates in a point at the posterior end and is enclosed in a spirally striated membrane. The upper or anterior end of the organism is rounded, and slightly to one side is an indentation into the protoplast, leading to a canal through which passes out a single flagellum about as long as the body of the organism. At the base of this canal lies a large vacuolar reservoir which has a thickening at its upper end which Reservoir may act as a sphincter for opening
and closing the canal. Near the reservoir are one or more pulsating vacuoles which may fuse with, or in any case discharge their contents into, the reservoir. Adjacent to the reservoir is a prominent red eye spot wich has been shown to be light-sensitive.
The nucleus  occupies a posterior position. It is surrounded by a colour less area of cytoplasm. The chlorophyll is contained in a series of about ten rod-like chloroplasts which radiate out from the centre of the cell. Each chloroplast has, at the end nearest the centre, a group of large solid granules of paramylon of very diverse shapes. These grains actually develop in the cytoplasm but come to lie attached to the inner ends of the chloroplasts. Paramylon is a polysaccharide allied to starch. It differs, however, in giving no colour with iodine, and it is not attacked by the enzyme diastase.
A few species of Euglena are devoid of chlorophyll. Some live as sapro­phytes, even in the intestines of animals, such as frogs and tadpoles, while a few have been described as capable of ingesting solid food material.
The organisms are all capable of active movement, swimming by the lash­ing of the flagellum and the corkscrew-like turning of the whole body. They also perform a characteristic, rhythmical and contractile motion, termed euglenoid movement. The flagella show an axial thread with a protoplasmic sheath, and there are indications that the axial thread itself consists of a spiral strand of still finer fibres.
Reproduction in Euglena Viridis
Reproduction is effected by the longitudinal division of the body into two.
This is called binary fission. In most instances the individual comes to rest, ecretes an envelope of mucilage, and then proceeds to divide, beginning at the front end of the protoplast. In some species the cells so formed may round themselves off and divide again and again, so that a large number of spherical cells are formmed all enclosed in the original membrane. Eventually this breaks and the individual separate and grow into normal eugleoid
cells. In dividing while at rest Euglena differs from the majority of the other Flagellates, and shows an approach to the true Algae.
Cysts with thick walls are frequently found; they are generally spherical and the walls are striated. Very often such cysts are red in colour due to a red pigment, haematochrome, in the cells. Such encystment may be only temporary, and the individual may retain its flagellum. If, however, the process is employed to tide the organism over a longer period of unfavourable conditions the enclosed cell may retract its flagellum.
Except in one doubtful case sexual reproduction is unknown in any species of Euglena.

Posted in Botany