Q.8. Give the habit and habitat of any colonial algae. Describe the method of reproduction also. (1990, 91, 2000)
Related Questions -
Q. Describe the sexual reproduction in volvox. (2012)
Q. Describe the sexual reproduction in volvox with the help of diagrams only. (2006)
Ans. Systematic position: -
Division - Algae
Class - Chlorophyceae
Order - Volvocales
Family - Sphaerellaceae
Genus - Volvox
Habit and Habital: - Volvox is a green flagellate colonial algae. It is world-wide in distribution. Volvox about with 20 species found in world. It is occur in temporary and permanent fresh water of ponds, pools and ditches. It is found in the water as green balls of pin head size about 0.5 mm. The spring and rainy seasons are the periods of its active vegatative growth.
Structure: - Volvox is a colonial green alga. The plant body is called coenobium.
Colony formation in Volvox
Each coenobium is a hallow sphere or ellisoid with a sharply marked delicate mucilage bounding layer. The interior part of coenobium is composed of diffluent mucilage. The number of cell per coenobium varies e.g. 500 - 1000 in V. aureus, 1500 - 20,000 in V. globator and upto 60,000 in rouseletti. Each cell posses cup shaped chloroplasts. In contractile vacudes, a single pyrenoid and a stigma are present. The cell is biflagellate.
Reproduction: - It reproduces both sexually and asexually.
Asexual reproduction: - In volvox asexual reproduction takes place by means of special reproductive cells developed on the posterior end of the colony. These reproductive cells are called gonidia. They may be 5 to50 in different species coenibium. These gonidia divides and produces new colonies. The gonidium divides vertcally which is followed by second longitudinal division at right angle to the first. Thus 4 cells are formed. There cells divides longitudinally and arranged in an uncurved plate facing unwards.
The plakea stage is followed by another division and produce 16 cells. These are arranged in a hollow spherically. The division continue and a small opening phialopore, is also formed. Such divisions are followed by several cell generations and a new colony of cells is developed. This young coenobium inverts and the phialopore changes its position during inversion and reaches the other side. This inversion of the young coenobium takes place by simple method. Later on the portion of the coenbium oppsite the phialopore reaches on the outer side of the sphere. Now anterior ends of the cells face outwards. Flagella are very shortly. Such daughter colonies are free in the hollow of the parent colony of volvox. These daughter colonies move here and there into the hollow sphere, and then this daughter colony free and move in the water as a independent colonies.
Sexual reproduction: - Oogamous type of reprduction take place in volvox in which sex organs are Antheridium and Oogonium. They may be formed on the same plant (monoecious) i.e. V. globator or formed in different plant. (dioecious) i.e. V. aureus. The sexual cells are bigger than the colonies. Sex organs are called gametangium. The male gametangium is called anthridium while female oogonium.
Antheridium: - Antheridium is developed from antheridial intial cell. The proto -
plasm of Antheridium undergoes repeated longitudinal cell divisions similar to asexual gonidial cell into daughter colony. A large number 64-128 but it may be 16 to 512 indifferent species. These cells also undergo the prosses of invertion similar to daughter colonies. In group of a flate plate antherozoide are arranged and exhibit a similar inversion. Each male cells developed two flagelly and represents a spermatoziods or male gametes. These spermatoziods are arranged in hollow sphere. The spermatoziods are biflagellate, unicelluar, uninucleate, long, narow, conical or fusiform structures. The colours of antherozoid is yellowish-green chloroplast.
Oogonium: - The oogonial cell is larger than the vegetative cell it is large and flask-shaped and also projecting into the interior of the coenobium. Now it is called oogonium. Oogonia look like the gonidis but it may easily be distinguished from gonidia, that they do not divide like the latter. The entire protoplasm of oogonium converted into a single spherical egg oospheric. A beak-like structure present in the cell by which antherozoid enters into the oogonium. The egg or oosphere contain a large central placed nucleous and a parietal chloroplast with pyronoids. It is copiously stored with reserve substances often absorbed from the neighbouring cells through protoplasmic strands.
Fertilization: - At the time of fertilization a grooup of antherozoid (colony) reach near the female cell. The antherozoids interact till they reads the egg. The antherozoids are remains free only one of them antherozoid fuses with the egg. By the fusion of male antherozoid and egg the oospore formed. The oospore secretes a three layered smooth or spring wall.
Fig. - Part of female coenobium of Volvox A.with an Oogonium B. fertililization, C. with zygote
It appears haematochrome which gives to it organe red colour and may be called zygospore or oospore.
Oospore and its germination: - After fertilization the oospore is formed. It is three walled outer wall is smooth walled. Middle layer is mesospore and inner is endospore. The oospore contains reserve food material and other inclusions. There is always reduction division during germination, contants of oospore develop into a large swarmer. This swarmer liberates and swims in the water. Swarmer protoplasm divides again, and a hollow oosphere of cells is developed. Inversion takes place of this hollow oosphere. Now newly developed coenobium liberated in the water.
Q.9. Describe the life cycle of volvox only with line diagram. (2006, 08)
Ans.
Fig. - Graphical representation of life cycle of Volvox
Q.10. Write the names of important species of volvox. (2008)
Ans. Species of Volvox: -
Volvox is a simple coenobium green algae. This is pinhead-like in shape and found in fresh water reservoirs like small ditches, ponds and lakes. It is more common during reiny season. This is represented by species like Volvox globator, V. merelli, V. pralificus, V. aureus, V. companesis, V. carteri, V. africans and V. poonaensis etc.