Hydrodictyon
Q.11. Describe the life history of Hydrodictyon.
Related Questions -
Q. Describe the reproductive process of hydrodictyon.
Q. Briefly describe the cells structure of hydrodictyon. (2012)
Ans. Systematic position: -
Class - Chlorophyceae
Order - Chlorococcales
Family - Hydrodictyaceae
Genus - Hydrodictyon
Occurrence: - Hydrodictyon or ‘water net’ is a free floating fresh water algae. It grows during winter and spring season and sometimes almost completely covers the pond. The genus is represented by five species. Hydrodictyon indicum and H. reticulatum are common Indian species which occur in temporary or permanent water reservoirs.
Thallus structure: - Hydrodictyon is a macroscopic non-motile coenobium
algae forming free floating ‘water nets’ in fresh water reservoirs. The mature coenobium is cylindrical-shaped and hollow in the center, usually measuring 2-80 cm. in length. The coenobium consists of few hundred to several thousand cells. The long cylindrical and pencil-shaped or ovoid cells are joined at the ends forming a design of hexagons and pentagons.
All the cells of a colony have the same general structure. Each cell is surrounded by a firm cell wall of cellulose. There is a thin layer of cytoplasm in the inner of the cell wall which encloses large central vacuole. The young cell is uninucleate with a parietal chloroplast, but at maturity it becomes multinucleate and has a reticulate chloroplast. There are several pyenoids in a mature cell.
The number of cells in a coenobium is fixed when it is young. Further growth of the coenobium is entirely due to the increase in a cell size and not the number of cells.
Reproduction: -
Hydrodictyon reproduces mainly by asexual and sexual methods, but vegetative multiplication may also take place by fragmentation of the coenobium into small segments.
(i) Asexual reproduction: - Aseual reproduction takes place by means of small, uninucleate and biflagellate zoospores. All the cells of a coenobium are capable of forming zoospores which eventually develop into new coenodia. The multiplication is very rapid. Pyrenoids disappear in the cell and starch grains are accumulated in the chloroplast before the formation of zoospores. The multinucleate protoplast divides into 7,000-20,000 uninucleate segments. Each segment gets surrounded by a small amount of cytoplasm and a limiting membrane. It evetually develops into a biflagellate zoospore. Zoospores form a dense column in the center of the mother cell due to sudden contraction. The real swarming starts once the zoospores tear loose from the central column; then they swimrely and haphazardly inside the mother cell. The wall of the mother cell, however, does not rupture and the zoospores remain inside the mother cell. They buzz about repidly within the mother cell. After a period of rapid movement the zoospores loose their flagella and sattle down on the inner side of the cell wall. The arrange themselves into characteristic hexagonal or pentagonal fashion to form a new net.
Although swarmers move quite haphazardly without any definite direction, the position of cells at the end of the movement desides the configuration of the future net.
The young coenobium is liberated by softening of the wall of the mother cell. In a young coenobium the cells grow mainly along their long axis.
(ii) Sexual reproduction: -
Isogamous kind of sexual reproduction is known in Hydrodictyon where the plants are monoecious. Gametes are produced in a pretty large number (i.e. 30,000 to 1,000,000) from a cell. Each gamete is small biflagellate structure. The gametes are liberated from the parent cell and moves about freely. During the process, the two isogametes fuse and form zygote. Generally, gametes concerned come from different cells, occasionally from the same cell.
The zygote becomes spherical and secretes a thin wall. It remains motile for a short time in H. patenaeforme while in others it is non-motile. It may or may not undergo a resting period.
During germination, zygote enlarges in size and the nucleus undergoes a meiotic division which may be followed by a mitotic division. As a result four or eight uninucleate and biflagellate zoospores are formed.
They increased in size and secretes a thick wall and are called polyhedron or polyeder. It is a non-motile polyhedral cell and is the usual means of perennation. In favourable conditions, the content of polyhedron divides mitotically and produces several biflagellate zoospores. The zoospores are liberated in the vesicle formed by the inner wall of the polyhedron and swim for a short time. Soon they lose flagella and organise in a way as to form a new coenobium which is set free by the rupture of vesicle. It grows to normal size by mere elongation of cells.
Since the net of Hydrodictyon is formed by joining together of a large number of zoospores (each of which acts as an individual cell of the net), some investigators consider it as collection of a number of individual plants.
