The Algae - Chlorophycae - Siphonales - Vaucheria

Phylum Thallophyta - The Algae - Chlorophycae - Siphonales - Vaucheria 
This Alga is largely terrestrial, occurring frequently on damp soils, as, for example, on the soil in flower pots, where it forms a green felt-like covering. It may also be found in fresh water, and it is under these conditions that asexual reproduction most frequently occurs.
Vaucheria consists of a non-septate, tubular coenocyte with lateral branches. It is dark green in colour and is very sensitive to changes of environment. The coenocyte is frequently attached to the substratum by
means of a branched holdfast or hapteron, which may be brown or white in colour and is devoid of chloroplasts. The remainder of the coenocyte contains a lining layer of protoplasm closely applied to the cell wall, and in it are embedded numerous nuclei and chloroplasts. The central part of the coenocyte consists of a vacuole containing cell sap. The chloroplasts are small and discoid, but in some species they become spherical when exposed to intense light.
In Vaucheria the reserve material is oil, which is stored in countless tiny droplets in the cytoplasm. Under normal circumstances no starch is formed, but under constant intense illumination it can be produced, which seems to indicate that the appropriate enzymes are present.
Injury to the thallus results in the formation of a septum sufficing to isolate the injured part, but otherwise, with the exception of those formed in relation to the sex organs, no septa occur.
Asexual Reproduction in Vaucheria
Asexual reproduction takes place by the formation of zoospores (Fig. 77), which are produced singly in club-shaped zoosporangia. The sporangia develop singly from the swollen ends of the branches by the accumulation of a large amount of cytoplasm with many nuclei and chloroplasts in the swollen tip, which, as a result, diminishes the size of the central vacuole, and thus these tips appear dark green in colour. Inside the zoosporangium a single large mass is formed which is termed the zoospore. It contains many nuclei which are arranged in a single layer near the surface and opposite each nucleus a pair of flagella is developed. The zoospore is liberated by the breakdown of the apical part of the wall of the sporangium to form a small pore through Which the large zoospore forces Its way. This zoospore IS generally mter­preted as a compound structure, resulting from the failure of the protoplast within the sporangium to divide into a number of uninucleate biflagellate zoospores. Zoospores are formed only by aquatic species or by terrestrial ones which have become flooded. They are usually developed during the night, or they may be induced by a sudden transference from light to darkness, or from running to quiet water. On liberation the zoospores are sluggish and only swim for a short time. On coming to rest the flagella are immedi­ately withdrawn and a thin membrane is secreted around the zoospore. Germination begins with the protrusion of two or more tubular outgrowths, one of which attaches itself to the substratum and forms the holdfast, whilst the other produces the filament.
Not all species of Vaucheria produce zoospores. Some develop what are termed aplanospores. These are simple non-flagellate structures which are produced in special sporangia called aplanosporangia. These aplano­sporangia develop as swellings either on the main or lateral branches of the thallus. When mature the aplanospores simply drop out of the aplano­sporangium through a perforation in the wall. This second method of asexual reproduction occurs most commonly among terrestrial species.
Sexual Reproduction in Vaucheria
In Vaucheria the sexual reproduction is oogamous and occurs most frequently in plants growing on damp soil or in quiet water, but is rarely found in plants growing in streams. The antheridia and oogonia are borne close to one another, either together on the same filament or on a common lateral branch or on adjoining lateral ones. Development of the oogonium begins (Fig. 78) with the formation of a swelling, which becomes filled with nuclei, oil and chloroplasts, though ultimately only a single central nucleus remains, which reaches a considerable size. The rest migrate back into the filament. The whole oogonium then becomes cut off from the rest of the coenocyte by a transverse wall or septum. Later a beak, or lateral swelling, is developed at one side of the oogonium, and in this region the wall becomes gelatinous and dissolves, leaving a pore. It is at this point that, subsequently, the antherozoids enter the oogonium. When mature the chloroplasts and oil take up a central position, leaving either a clear peripheral area of protoplasm or at least a clear area under the beak, the receptive spot, and the contents as a whole contract and become the oosphere.
The antheridia are formed at the ends of short lateral branches and develop simultaneously with the oogonia. Each antheridium is an elongated, strongly curved structure and is cut off from the thallus by a septum. The nuclei within the antheridium divide up and ultimately each becomes surrounded by a small mass of cytoplasm and constitutes an antherozoid. These antherozoids form a mass in the centre of the antheridium, whilst the periphery is filled with chloroplasts and residual cytoplasm. The antherozoids are liberated apically shortly before daybreak, as very minute, colourless, oval bodies with two laterally inserted flagella.
At the time of fertilization the oosphere exudes a droplet of gelatinous cytoplasm through the pore in the oogonium wall, and the antherozoids accumulate in numbers around the droplet. Several antherozoids may enter the oogonium, but only one enters the oosphere. The small male nucleus migrates to that of the oosphere, which is considerably larger, but does not immediately fuse \yith it. Whilst lying side by side the male nucleus increases in size, and when its volume equals that of the female nucleus the two fuse together.
After fertilization a membrane develops across the oogonial aperture, and subsequently a thick, several-layered envelope is formed around the zygote, thus producing an oospore. At the same time the oil globules unite to form one or more large central globules, and the zygote enters a resting period of several months before germination. It germinates directly to produce a new filament without the intervention of zoospores.

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