Systematic position
Class Chlorophyceae
Order Volvocales
Sub order Chlamydomonadineae
Family Sphaerellaceae
Genus Volvox
Occurrence
- It includes about 20 species.
- All are aquatic and free floating.
- Mostly occur in fresh water bodies
- It occurs as green rolling ball of pin head sized coenobia.
Vegetative structure
- Volvox is a colonial green alga and the colonies are oval to spherical in shape.
- Plant body is multicellular motile coenobium.
- Coenobia of volvox are spherical, oval and ellipsoidal inshape.
- Each cell is biflagellate and the alga rolls over the surface of water by joint action of flagella.
- The coenobium of volvox is an assemblage of similar and independent cells. Each cells perform it’s own function nutrition, respiratory, excretion.
- Each cells has it’s own gelatinous sheath. It is connected with the sheath of other cells by series of cytoplasmic strands called plasmodesmata.
Cell structure
- Each cell has two anteriorly inserted flagella of equal length and of whiplash type.
- These flagella project beyond the surface of colony into water.
- Two or more contractile vacuoles lie near the base of flagella.
LIFE CYCLE OF VOLVOX
Reproduction
Asexual Reproduction
- Occurs during growing season.
- At maturity few cells in posterior half are pushed back into hollow cavity.
- Cell withdraw their flagella, increase in size, become round shaped.
- These reproductive cells are called gonidia.
- The protoplasm of each gonidium divides by successive longitudnal division and forms daughter coenobium.
- Sometimes young coenobia are formed inside the young daughter coenobia called grand daughter colonies
Sexual reproduction
Oogamous type
- Male sex organ: anthridia (andogonia)
- Female sex organ: Oogonia(gynogonidia)
- Some species are monoecious and some are dieocious.
- Male gametangium gives rise to anthridium and female gametangium gives rise to oogonium.
- Antheridia (Androgonidia )and oogonia ( Gynogonidia)
- Each antherozoid is uninucleate, biflagellate, and posseses a small chloroplast.
- The mass of antherozoid is released at the time of maturity.
- The proplasm of female gametangium does not divide it changes into single, non flagellated green spherical egg or oosphere.
- The female gametangium called oogonium.
Fertilization
- The antherozoid are attracted chemotactially towards oogonium.
- Only one antherozoid fuses with egg from the side and the egg becomes zygote.
Zygote
Zygote divides repeatedly and forms a new colony by sequence of Plakea stage ( 8 celled stage)
- Zygote secretes it’s own cell wall. It is three layerered exospore, endospore, mesospore.
- The zygote remains embedded inside oogonium and tides over unfavourable conditions.
- Finallly the parent colony decays and zygote fall to the bottom of water body.
- Prior to germination the diploid nucleus of zygote divides by meiosis.
- Zygote protoplasm changes into zoospore.
- Zygote divides to form hollow sphere of cells produces a young coenobium.
Alternation of Generation
- Volvox has haplontic type of life cycle.
- Plant body is haploid and it produces haploid sex organ and gametes.
- Fusion of gametes results in the formation of Diploid zygote.
- Zygote is only diploid stage in life cycle
- It’s nucleus divides meiotically and results in the formation of zoospore.
- Haploid plant body alternates with the diploid zygote.
