The Algae - Chlorophycae - Conjugales - Spirogyra

Phylum Thallophyta - The Algae - Chlorophycae - Conjugales - Spirogyra
Conjugales are Chlorophyceae devoid of flagellated reproductive boies. Sexual reproduction is by means of amoeboid gametes. Asexual reproduction is unknown. The vegetative cells are generally large, and possess large, elaborately shaped chloroplasts. The plants may be unicellular or the cells may be united together into filaments, which may fragment into separate unicellular parts.
Included in this group are the Desmids, which are among the most beautiful of the Green Algae. Most of them are unicellular, but the wall of the cell is divided into two halves, or semi-cells, each of which is often extremely complex in outline and is connected to the other by a narrow region called the isthmus.
Spirogyra 
Spirogyra is a very common bright-green Alga which may be found in large masses growing in ponds and slow-flowing rivers. The filaments are usually free, but are sometimes attached at one end to a substratum by a branched and colourless rhizoidal cell or holdfast. These masses are very slimy to the touch, owing to a mucous sheath surrounding the cells, and consist of enormous numbers of long, unbranched, green filaments.
There is no' morphological differentiation between the cells of the filament. A delicate sheath of cuticle surrounds the entire filament inside the mucous sheath, while each cell has its own polysaccharide wall, inside which is a peripheral layer of cytoplasm. The central part of the cell is occupied by a large vacuole containing cell sap. The nucleus is suspended in the centre of the vacuole by cytoplasmic strands. The most conspicuous structure in the cell is the chloroplast, which consists of one or more spiral bands lying in the peripheral layer of cytoplasm. It is the appearance of these bands that has given the plant its name. Embedded along the chloroplast are numerous pyrenoids around which is stored up the starch acquired during photosynthesis. In section the chloroplast is usually T -shaped, the inner flange being connected by fine filaments of cytoplasm to the nucleus.
The filaments grow by simple cell division, each cell dividing by a wall transverse to its length, and any cell of the filament being capable of division. Such a type of growth, where any cell of the filament can divide, is spoken of as intercalary growth, and may be contrasted with the apical growth of higher plants, where division is restricted chiefly to the cells of the growing point.
Cell division in Spirogyra begins before nuclear division, not after it as in higher plants, and the two processes are completed simultaneously. The new transverse wall starts as a ring on the inner surface of the longitudinal walls and grows inwards towards the centre. The cell does not necessarily divide into two equal parts, for the new wall lies exactly half-way between the nuclei, wherever that may happen to be in the cell. The position of the new wall is therefore fixed by the position of the nuclei and this position is determined while the mother nucleus is still in the metabolic condition. A similar condition exists in Cladophora.
Nuclear division in Spimgyra is also exceptional in that the nuclear membrane persists and divides \yith the nucleus, the spindle being formed inside the nucleus instead of in the cytoplasm.
Sexual Reproduction in Spyrogyra
Spirogyra grows in dense masses, and in this condition the filaments at the time of sexual reproduction come to lie side by side, probably as a result of surface tension changes. In this position sexual reproduction is possible by a process known as conjugation. Most frequently this process takes place in the spring, but it can be induced by the presence of a liberal supply of sugar, or by a deficiency of nitrogen. The filaments adhere closely to one another in pairs surrounded by a common investment of mucilage, and then, preliminary to conjugation, opposite cells in adjacent filaments develop short protuberances from their side walls, which are in contact with each other from the start, and as they grovv push the filaments apart. Each pair of protuberances when united forms a conjugation canal. The wall between them partially breaks down and a small conjugation pore is formed, through which the protoplasts of the two opposite cells come into contact. Conjugation by this method is called the ladder type from the appearance of the two parallel filaments linked by the conjugation tubes. At this stage the mucous sheath of the filaments disappears.
Meanwhile the contents of the conjugating cell which is about to function the male gamete contract away from the outer cell wall but without losing contact with the female. In this process many contractile vacuoles are formed, which pump water from the central cell vacuole into the space between the cell wall and the contracting cytoplasm. Subsequently the female gamete also contracts from the conjugation tube, leaving a space into which the male gamete passes, enters the other cell and fuses with the female gamete. It appears that the migration of the male gamete along the conjugation tube and the contraction of the female
are brought about by the pumping action of contractile vacuoles. In Mougeotia and certain callied genera the process is lightly different, both gametes Zygospore passing into the conjugation tube, where the zygote is formed. Such types are therefore isogamous.
The active gamete is looked upon as the male and the passive one as the female, although they do not show any morpho­logical differences. We have therefore here a case of physi­ological heterogamy. In some species of Spirogyra all the male gametes are formed in the cells
of one filament, and all the females in the other, with the result that after conjugation we find one filament entirely devoid of cell contents, while the other contains all the zygotes. In other species some cells function as males and others as females, with the result that the zygotes are distributed between both filaments. The former species are dioecious and the latter are monoecious.
There is also another type of conjugation called the chain type, in which adjacent cells of the same filament unite through tubes which are formed like side-links between cell and cell. In this case alternate cells behave as males and females respectively, and there is no sexual differentiation between the filaments as a whole. In some species this method is the rule.
Whatever the position of the formation of the zygote its subsequent development is the same. Immediately after the fusion of the gamete nuclei a zygospore is formed. A zygospore is a zygote enclosed in a thick wall. It is specially adapted for withstanding dry conditions, and is easily recognized by the thick yellow-brown wall which is usually produced. In Spirogyra the wall consists of three distinct layers, the characteristic chloroplast dis­appears and the cell becomes filled with brownish oil-drops.
The further development of the zygospore depends upon the decay of the parent filament. As this consists simply of the dead cell wall, its decom­position in water is fairly rapid, and in time the zygospores are set free and fall to the bottom of the pond. Activity only begins when the zygospore is lying in water: it will not germinate in a dried-up pond. Under suitable conditions the thick wall is burst open and the contents emerge as a green cell which divides transversly, the lower half forming a rhizoidal cell, while the upper half divides repeatedly to form a new filament. At the beginning of germination the nucleus of the zygospore divides into four by meiosis, but only one of the four monoploid nuclei survives, to become the nucleus of the first cell of the new plant.
There is no asexual method of reproduction in Spirogyra, and this and the entire absence of any truly motile gametes constitute the fundamental characteristics which distinguish this genus, as well as the whole of the Conjugales, from the rest of the Green Algae. It seems probable that the absence of motile gametes is a feature associated with the habit of the plant of living in dense masses where little or no difficulty is experienced in the association of cells potentially able to conjugate, as a result of which the gametes have, in the course of evolution, lost the power of motility.
Spirogyra has, however, a method of multiplying vegetatively by the fragmentation of the filaments into single cells, each of which becomes the starting point of new growth. In many species this process, which occurs mostly in early spring, is assisted by the action of a ring-like fold on the end walls of the cells, somewhat like that in Oedogonium, which expands laterally and thus ruptures the cuticular sheath of the filament.
Although the filaments of Spirogyra are multicellular they are not, biologically speaking, individuals, but rather colonies of comparatively independent cells. At sexual reproduction they behave like unicellular types in that the conjugating units are used up and disappear as such. This limits the reproductive capacity of the plant in comparison with higher forms in which a vegetative body persists and may reproduce many times.

Posted in Botany