In this article we will discuss about Hydrodictyon. After reading this article we will learn about:- 1. Systaematic Position of Hydrodictyon 2. Occurrence of Hydrodictyon 3. Thallus Structure 4. Cell Structure 5. Reproduction.
Systaematic Position of Hydrodictyon:
Occurrence of Hydrodictyon:
Hydroclictyon, a non motile coenobium is macroscopic and beautiful alga. Due to its net like plant body, it is commonly known as ‘water net’. It is represented by 5 species. Only two species of Hydrodictyon i.e., H. reticulatum and H. indicum are reported from India. H. reticulatum is cosmopolitan in distribution.
The species are commonly found between spring and rainy season in slow running water or still water of ponds, pools and lakes. It generally floats on the surface of the water but may also lie on the bottom. Very often due to profuse growth, the nets assume big size and cover the entire pond.
Thallus Structure of Hydrodictyon:
A mature coenobium consists of a hollow cylindrical network which is closed at both the ends (Fig. 1). It is flat and saucer shaped and its maximum size is generally 20-30 cm. Rarely it may reach up to a length of 60 cm. The mature net of coenobium is made up of a few hundred to several thousand cells.
These cells are joined at the end and form pentagonal or hexagonal structures. These structures are called meshes. Bach mesh interspace is generally bounded by 5-6 or rarely three cells. At each angle of the net or mesh meet three cells (Fig. 2 A, B).
Cell Structure of Hydrodictyon:
Each cell is long, cylindrical or ovoid in shape. Its internal structure can be differentiated into two parts: cell wall and protoplasm. Cell wall is two layered and is made up of cellulose. It encloses protoplasm. When young, the cells are uninucleate, but at maturity they become multinucleate (coenocytic).
Cells contain reticulate chloroplast with many pyrenoids (Fig. 2C). All the typical structures of green algae like ribosomes, mitochondria, dictyosomes are also present. As the cell matures, a central vacuole appears and the protoplasm becomes peripheral.
Reproduction in Hydrodictyon:
It is of three types: Vegetative, asexual and sexual.
It takes place by fragmentation. Coenobium breaks up into small pieces called fragments. Which have capability to grow into new colonies. It may be due to water currents and movement of aquatic animals.
It takes place by the formation of auto colonies or daughter colonies (Fig. 3 A-G). These colonies are formed by the biflagellate, uninucleate zoospores. Under favourable conditions each coenocytic cell behaves as zoosporangium. Its nuclei undergo mitotic divisions to form a large number of nuclei (7000-20000).
Protoplasm gets segmented into as many segments as there are nuclei. Each segment gets surrounded by small amount of cytoplasm, a limiting membrane and develops two whiplash type equal flagella and represents biflagellate zoospore (Fig. 3 A-C). In Hydrodictyon a peculiar phenomenon is observed. The zoospores thus formed are never liberated outside the parent cell.
They remain motile within the restricted region i.e., within the cell. After swimming inside the cell, they ultimately withdraw their flagella and get themselves arranged into characteristic hexagonal or pentagonal fashion to form a new net (Fig. 3 D, E). This new net is called auto colony or daughter colony (Fig. 3 F, G).
The auto colonies are liberated by disintegration of the parent cell wall. The number of the cells in the daughter colony is fixed. Further growth of the coenobium is entirely due to increase in the cell size and not the number of the cells.
It is isogamous. Any vegetative cell of the coenobium can function as gametangium. The biflagellate gametes are produced by the cleavage of the protoplasm of the gametangia like that of zoospores (Fig. 4A, B). They are produced in large number and are smaller in size than the zoospores. They are liberated individually through a hole in the parent cell wall and swim freely in water.
The gametes are uninucleate and biflagellate. Hydrodictyon is monoecious. The gametes from the same or different coenobia after liberation fuse to form quadriflagellate zygotes (Fig. 4C).
Soon they lose their flagella and settle down. The immobilised zygote enlarges in size, becomes spherical and develops thick wall to form zygospore. First it is green but it becomes red because of the development of a red pigment haematochrome.
Germination of zygospore:
Zygospore is capable to tide over the low winter temperature. At the onset of the spring season, its diploid nucleus undergoes zygotic meiosis to form four, haploid uninucleate, biflagellate gonozoospores meiospores (Fig. 4 D-F). The zygospore wall bursts and the meiospores are liberated in the surrounding water. After swimming for some time these meiospores come to rest.
They retract their flagella, enlarge and form the thick walled angular cells called polyhedrons or polyeders (Fig. 4 G, H). This stage is known as polyhedron stage. The single nucleus of the polyhedron divides and re-divides several times and ultimately forms the second generation of zoospores (Fig. 41). These zoospores are also uninucleate and are anteriorly biflagellate.
The wall of the polyhedron cracks down and the zoospores emerge into a thin vesicle (Fig. 5J). These zoospores do not escape outside in the water but actively swim within the vesicle for some time.
They withdraw their flagella and arrange themselves in the form of a net of Hydrodictyon. It is a daughter or juvenile colony. It is released in water by the dissolution of the vesicle. Its cells grow in size and produce new coenobium where the cell number typical of the species is stored.
If a gamete fails to fuse, it develops into azygospore parthenogenetically. It gives rise to zoospores which, in turn, produce polyhedrons.