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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

Life Cycle – 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.
Volvox : Oogonium Development, Fertilization & 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.
Volvox : Parent & Daughter Colonies

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.
Graphical Representation : Volvox Haplontic Life Cycle

Anjali Thakur

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