Sem II - Paper II Important Questions

       Important questions for  Botany II semester

Gymnosperms

Short Answer type Questions:

General characters of Gymnosperms

Pinus:  1.Mycorrhiza 2. Transfusion Tissue 3.Needle – Xerophytic Characters 4.Male cone 5.Female cone 6.Ovuliferous scales 7. Bract Scales  8. Sunken Stomata

Gnetum:  1.Collar or cupule    2. Male cone 3.Female Cone 4. Pavement tissue

5. TetrasporicEmbryosac6. Polyembryony

.Paleobotany

1. Impressions 2. Moulds 3. Casts 4. Compressions 5. Petrifactions

5. Paleozoic era 6. Mesozoic era 7.Coenozoic era

Essay Questions:

1. Root Anatomy of Pinus

2. Primary stem anatomy of Pinus

3. Internal structure of Pinus leaf or Needle

4. Pinus Ovule Structure

5. Root anatomy of Gnetum

6. Internal Structure of Gnetum leaf

7. Primary Stem anatomy

8. Structure of Gnetum Ovule

9. What are fossils? Different types of fossils and significance of fossils

10. Geological time scale

    ( Taxonomy of Angiosperms)

Short answer questions

1.Types of classifications

2. Chemotaxonomy

3. Binomial nomenclature

4. ICBN

5. Typification

6. Herbarium techniques

7. Economic importance of Rutaceae

8. Pollination in Fabaceae

9. Floral characters of Cucurbitaceae

10. Economic importance of Apiaceae

11. Pollination in Asclepiadaceae (clip mechanism )

12. Economic importance of Lamiaceae

13. Floral characters of Orchidaceae

14. Spikelet of Poaceae

Section – B

Essay questions

1. Compare the Bentham and Hookers system of classification with Englar & Prantls classification. Mention merits and demerits.

2. Write the salient features of  APG system of classification.

3. Write an account  on Numerical taxonomy.

4. Describe the floral characters and economic importance of Fabaceae.

5. Describe the floral characters and economic importance of Apiaceae.

6. Describe the floral characters, economic importance  and pollination mechanism of Asclepiadaceae.

7. Describe the floral characters, economic importance  and pollination mechanism of Lamiaceae.

8. Describe the floral characters, economic importance  and advanced characters  of Orchidaceae.

9. Describe the taxonomic features and economic importance of Poaceae.

10. Describe the floral characters, economic importance  and pollination mechanism of Asclepiadaceae.

Ecology

Short Answer type Question

1. Biotic and Abiotic factors 2. Niche 3. Autecology and Synecology  4. Climax or Climax community  5. Autogenic and allogenic succession  6. Food Chain 7. Food Web 8. Trophic levels

Essay Questions

1. Write on abiotic components of Environment.

2. What is ecologic succession? Explant Xerosere or Hydrosere

3. Explain the Process of Succession

4. What is ecosystem. Explain the energy flow through a pond ecosystem you have studied.

5. Write a brief essay on Ecological pyramid

6. Define Ecological Adaptations? Explain the adaptations of Hydrophytes 

7. or Xerophytes 

8. or Mangroves .

Polysiphonia

Polysiphonia

                                                                                                            Class: Rhodophyceae

                                                                                                            Sub-class: Florideae

                                                                                                            Order:       Ceramiales

                                                                                                            Family :Rhodomelaceae

                                                                                                            Genus  :Polysiphonia

Distribution and Habitat:

            It is a common red algae with about 200 species on sea coasts. It is abundant in Atlantic and Pacific oceans. Most of them are found in littoral zone in tidal marshes, brackish estuaries and tide pools frequently growing as epiphytes on rock weeds.

            The genus is represented in India by about 16 species, which occur in the southern and western coasts. P. platycarpa, P. ferulaceae, P. urceolata, P. variegata are common India species.

            P. variegate inhabits polluted water near estuaries and frequently found on roots of mangroves. P. urceolata is an epiphyte on Laminaria. P. fastigata is found on the fronds of a brown sea weed Ascophyllumnodurum and occasionally on Fucus.

Thallus Structure:

            Polysiphonia has a filamentous thallus which is generally brownish red to purplish red in colour. The filaments branch and re branch several times giving the plant body a beautiful, delicate, feathery appearance.The thallus is attached to the substratum in water by a holdfast.

The plant body is heterotrichous consisting of an erect or projecting system and a filamentous prostrate system.

Basal Prostrate System:

            This creeps over the substratum. It is anchored to the substratum by means of thick-walled, elongated, unicellular rhizoids arising from the  peripheral cells facing the substratum. The distal ends of rhizoids expand to form flat irregularly lobed attachment pads or discs. The creeping filaments function as a means of perennation.

Upright or Vertical Filaments:

            These arise from the creeping filaments. They may attain a height of 25-30 cm. These filaments remain free-floating in water. The branching is lateral and branches are of two kinds, long and short.

            The short branches are of limited growth and are known as trichoblasts. The trichoblasts are colourless, hair-like and forked. They are borne on the long, erect branches of unliminted growth. The trichoblasts usually bear male and female reproductive structures. Trichoblasts are usually deciduous and are shed annually in the perennial species before winter.

            The long branches arise from the basal cells of short branches. They show unlimited growth due to the activity of apical cell and do not bear reproductive structures.

            The main filament and long branches, each consists of a system of parallel filaments. These are called the siphons. There is one central filament termed the axial or central siphon. The central siphon is surrounded by peripheral filaments called the pericentral siphons. The number of pericentral siphons varies from 4-20 (P. elongate – 4, P. spiralis – 5, P. variegate – 6).

            Such a thallus having central siphon surrounded by many pericentral siphons is called as Polysiphonous. This algal genus thus derives its name from the polysiphonous nature of its thallus. The cells of the central and pericentral siphons are inter connected through pit connections, a feature characteristic of the red algae.

            The long branches are polysiphonous or multiaxial. The short branches or trichoblasts are made up of only central siphons, and the pericentral siphons are absent, so they are monosiphonous.

            Because of nearly the same length of the cells in the central and pericentral siphons and the regular manner in which the cells are arranged, the main axis appears to be differentiated into nodes and internodes.

Cell Structure:

            The central and pericentral siphons are made up of elongated, cylindrical cells consisting of the cell wall enclosing the protoplast. The cell wall is thick and is differentiated into two layers. The outer layer is made up of pectic materials and the inner of cellulose.

            The centre of the cell is occupied by a vacuole bounded by tonoplast. The cytoplasm is restricted to the periphery of the cell. The peripheral cytoplasm encloses a single nucleus and a number of discoid chromatophores. Chromatophores are devoid of pyrenoids.

           

The photosynthetic pigments located in chromatophores are chlorophyll – a, Chlorophyll – d, α,β carotene, biliproteins – r phycoerythrin and r- phycocyanin and a few xanthophylls.

The reserve food materials are floridean starch and floridoside.

Growth:

            The thallus grows by means of a dome shaped apical cell. It is situated at the  extreme tip of the naked part of the central siphon. The apical cell by transverse division forms a series of segments parallel to its posterior face. These segments elongate to form the axial siphon.

            Some of the sub-terminal axial cells divide periclinically and form a definite number of pericentral cell around the axial row of cells. Both central and pericentral cells elongate into their respective siphon with pit connections.

Reproduction:

            In life cycle of Polysiphonia, three types of plants are found. They are :Gametopyte, the Carposporophyte and the tetrasporophyte.

Gametophyte:

            It is a free living haploid plant. It is concerned with sexual reproduction and bears sex organs.

Carposporopohyte:

            It is diploid plant and develops from the zygote. It remains attached to the female gametophyte plant, on which it is parasitic. It reproduces asexually by producing diploid spores called the carpospores.

Tetrasporophyte:

            The diploid carpospores germinate to give rise to the tetrasporophyte plant. It is an independent diploid plant. It reproduces asexually by producing the haploid tetraspores.

            The haploid gametophytes and diploid sporophyte plants are similar in their morphological structure, but differ in producing the reproductive organs.

  

Gametophyte:

            The free-living Polysiphonia plant is a haploid gametophyte. It undergoes sexual reproduction which is an advanced oogamous type. The gametophyte plant is heterothallic or dioecious i.e., male sex organs and female sex organs are borne on different plants called the male gametophyte and female gametophyte resepectively. The male and female plants are morphologically similar.

Male gametophyte:

The Male sex organs, spermatangia or antheridia develop on fertile trichoblasts present on tip of male gametophytic plant. The male trichoblast  when only 2-3 celled, divides dichotomously. In most of the species one branch remain sterile and the other bears spermatangia. In some species both branches become fertile.

The cells of fertile uniaxial trichoblast  divide periclinally to form  pericentral cells. The  pericentral cells form the  spermatangial mother cells on outer side. Each spermatangial mother cell cuts off 2-4 sporangia on outer side. The complete structure makes cone shaped cluster of spermatangia.

The mature spermatangium is a globular or oblong, unicellular structure. Its cell wall is differentiated into three layers – inner refractive , middle gelatinous and outer thick layer. The uninucleate protoplast of spermatangium forms a male gamete or spermatium. The spermatium is non-motile and is released through an apical pore.

Female gametophyte:

The female sex organ is called carpogonium. The carpogonium develops on trichoblast on female gametophyte plant.  The trichoblast initial arises from a cell, 2-4 cells behind the apical cell. It develops into 5-7 celled female trichoblast.

The 3 lower most cells of the female trichoblast divide vertically to form an ensheathing layer of pericentral cells. One of the pericentral cell on the adaxial surface (facing the axis side) functions as supporting cell.

The supporting cell cuts of a small initial cell at its free end known as procarp initial. The initial divides to form a small, curved four-celled branch called the carpogonial filament or procarp.

The terminal cell of the carpogonial filament functions as carpogonial mother cell. The carpogonial mother cell gets modified into the carpogonium. The carpogonium has a basal swollen portion having female nuclei or egg and an upper elongated, slender portion called the trichogyne. The trichogyne functions as a receptive organ

Meanwhile, the supporting cell cuts off two sterile cells, one towards its base called the basal sterile filament initial and one on the lateral side called the lateral sterile filament initial.

At this stage the carpogonium is ready for fertilisation.

Fertilisation:

The liberated spermatia are carried passively by the currents of sea water. As they reach the carpogonium, one of them adheres to trichogyne. At the poing of contact the common walls between them dissolves. The male nucleus then enters the trichogyne and passes down and reach the eagg. The male nucleus and the egg nucleus fuse and a diploid zygote is formed.

Post- Fertilisaion Changes:

After fertilisation many changes takes place within and around carpogonium. The two-celled lateral sterile filament divides and becomes 4-10 celled. Then basal sterile filament initial divides to form a 2-celled basal sterile filament. The sterile filaments are nutritive in function.

Meanwhile, the supporting cell divides transversely to form an auxillary cell at its upper end. It lies between the supporting cell and the carpogoniium. It has a haploid nucleus. Soon, the auxillary cell establishes a tubular connection with the carpogonium.

The diploid zygotic nucleus of the carpogonium dividies mitotically into two daughter nuclei. One of the daughter nuclei migrates into the auxillary cell through the tubular conncetion. Other daughter nuclei remain within the carpogonium. The haploid nuclei of the auxillary cell degenerate at this stage.

After, the migration of the diploid nucleus into the auxillary cell, the carpogonial branch gradually begins to degenerate.

The migrated nucleus present in the auxillary cell divides mitotically into two daughter nuclei. One of these remains in the auxillary cell and the other migrates into a small lateral outgrowth arising from the upper side of the auxillary cell. This small outgrowth on the auxillary cell is called as gonimoblast initial.

The gonimoblast initial cuts off a number of cells. Every time a new cell is cut, the zygote nucleus divides and its derivative enters the newly formed cells. Thus, gonimoblast initial grows into a number of short threads, the gonimoblast filament.

The terminal cell of the gonimoblast filament becomes swollen and it develops into a pear-shaped carposporangium. The diploid protoplast of the carposporangium develops into a single, diploid carpospores.

With these development taking place, the supporting cell, the auxillary cell and cells of sterile filaments fuse resulting in a large, irregularly shaped structure called the placental cell. The placental cell provides nourishments to the growing carposporophyte.

Meanwhile, the pericentral cells of the female trichoblast adjacent to the supporting cell grow into outgrowths known as the enveloping threads.

The enveloping threads finally develop into an urn-shaped envelope or sheath around the developing frutification. This sheath is called the pericarp. It consists of two layers and has a wide aperature, the ostiole at its distal end.

The entire structure consisting of the placental cell, gonimoblast filaments bearing carposporangia and the surround pericarp is called the cystocarp. It is partly haploid and partly a diploid structure.

Carposporophyte or Cystocarp:

The post-fertilisation fructification respresenting the diploid phase and second individual in the life-cycle of Polysiphonia is called as carposporophyte. It remains attached to the female thallus of Polysiphonia and thus lives parasitic on it.

It is urn-shaped and protected by a two layered wall called the pericarp. The pericarp is a haploid structure. The cystocarp has an opening on its top called the Ostiole. The cystocarp contains a placental cell, gonimoblast filaments and carposporangia.

Each carposporangia produces a single, uninucleate, diploid carpospores. Carpospores are liberated through the ostiole and they are carried by the water currents.

Germination of Carpospore:

On coming in contact with substratum diploid carpospores secrets a wall around it and attaches itself to the substratum. It divides transversly and forms a smaller lower cell and a larger upper cell. Each of these again divide transversely forming a four-celled small filament.

The basal cell of the filament is called the rhizoidal cell. It is colourless, elongated and form the rhizoids. The end cell of the filament is dome shaped and functions as the apical cell. The lower axial cell divide vertically to cut off the pericentral cells. In this way a full-fledged tetrasporophyte is formed.

Tetrasporophyte:

It is an independent, diploid plant developed from the carpospores. It is morphologically similar to the haploid gametophyte. It consists of a central siphon encircled by the perincentral silphons. The thallus is laterally branched.

Asexual Reproduction:

The tetrasporophyte plant reproduces asexually by means of non-motile haploid spores called tetraspores. Tetraspores are produced within spherical, sac-like diploid structures called the tetrasporangia.

The tetrasporangia are developed from the pericentral cells. Only one of the pericentral cell in each transverse tier produces a tetrasporangium. The fertile branches bearing the tetrasporangia become swollen and twisted.

The fertile pericentral cell is smaller in size than the other cells in the same tier. This cell divides by a vertical wall into two cells, the outer and inner.

The outer cell again divide to form two cover cells. The inner cell functions as the sporangial mother cell. The sporangial mother cell divides by a transverse wall into a lower stalk cell and an upper tetrasporangium cell.

The tetrasporangium cell increases considerably in size. It has a diploid nucleus. The nucleus undergoes meiosis forming four haploid daughter nuclei.

This is followed by cleavage of the cytoplasm from the periphery towards the centre resulting in formation of four uninucleate meiospores. These four meiospores are arranged tetrahedrally in the tetrasporangium, hence called as tetraspores.

When tetraspores reach maturity, the sporangial wall ruptures and two cover cells move apart longitudinally. Thus, the tetraspores are liberated.

The liberated tetraspores germinates and give rise to the haploid gametophyte plants. Two of the tetraspores give rise to male and other two to the female plants.

Volvox

Chlorophyceae

The class chlorophyceae is represented by about 425 genera and6500 species they are commonly called as green algae because of the presence of the pigment chlorophyll a and b in their plastids

1. They have a wide distribution in aquatic (both fresh water and marine and terrestrial habits)

2. Cell wall compassed of two layers, inner layer made of cellulose and an outer layer consisting pectin

3. The conspicuous part of protoplast is chloroplast which varies in number and shape from genus to genus

Chloroplast may be cup shaped(clamydomonas) discoid(vauchert) stellare(zygnema) or reticulate (oedogonium)

4. The main pigment are chlorophyll a and chlorophyll b  α, β,γ, carotene, astaxanthin, leutein, neoxanthin, siphonein etc.

5. Associated with the chloroplast are bodies known pyrenoids.

6. Generally cells are uninucleate, but multinucleate conducts is also not uncomman members of siphonales and cladophorales are coenocyctic.

7. Both flagellated and non-flagellate cells are present. Number of flagella may be two, four or more. Flagella are whiplash type.

8. Vegetative propagation takes places by fragmentation and cell division.

9. Asexual reproduction takes place through the formation of Zoospores, aplanospores, akinites and hypnospores.

10. Sexual reproduction may be isogamous, anisogamous or oogamous.

Fritsch (1935) recognized nine orders in Chlorophyceae

1) Volvocales 2) Chlorococcales 3) Ultotrichales 4) Cladophorales 5) Chaetophorales

6) Oedogoniales 7) Conjugales 8) Siphonales 9) Charales

Smith (1955) included the green algae in one division Chlorophyta and divided it into two classes – Chlorophyceae and Charophyceae.

The class Chlorophyceae was divided into 12 orders, and only a single order was included in Charophyceae.

Volvacales

The volvacales include chiefly the microscopic genera of the green algae in which the thallus is one-celled and generally motile throughout life. The unicell is furnished with 2 or 4 whiplash type flagella.

This is the only order of the green algae in which the vegetative cell is actively motile. In some genera, the motile cells are joined into groups to form an colony. A colony is aggregation of individuals mechanically held together generally in a gelatinous sheath. The individuals in the colony have little or no dependence upon others.

The order volvocales, include both unicellular and colonial forms which occur widely in fresh water plankton. They are absent from sea. Many of them prefer water rich in nitrogen and organic substances tne his occur in quiet pools.

The order comprises 6 families with about 60 genera and 500 species.

Volvox

Class – Chlorophyceae

Order – Volvacales

Sub-order – Chlamydomonadinae

Family –Sphaerellaceae

(Volvocaceae)

Genus – Volvox

Distribution and Habitat:

Volvox is green, flagellate, colonial, planktonic algae. The genus has 20 species, which occur both in temporary and freshwaters of ponds, pools and ditches. They appear like spherical balls of pin head size, just visible to the naked eye.

The colonies multiply so rapidly during rainy season that the water of small ponds may be coloured green by their presence.

The Volvox includes about 20 species, which are worldwide in distribution. The genus is represented in India by 6 species, of these V. glabatox, V. aureus, V. prolificus, V. africanus are more common.

Thallus Structure:

The volvox is coenobium which is largest and highly differentiated. The coenobia is of the size of a pin-head and is spherical or ovoid in outline, hollow and only one cell thick. The size varies from 0.5 to 2.8 mm in diameter.

Each colony consists of 500 to 60,000 cells. The number of cells in each colony is in multiple of two, and each species has a definite number of cells. A colony consisting of a definite number of cells arranged in a specific manner and forming an integrated whole is called a coenobium.

The numerous cells of the coenobium are embedded in the gelatinous or mucilaginous matrix and are arranged in a single layer in the periphery. The coenobium is thus a hollow sphere with the interior showing no cellular organization. The central part of the colony is filled with a water (e.g., V. glabator) or gelatinous substances (V. aureus).

Each cell in the coenobium is connected to its neighbor cell by broad protoplasmic strands which are formed during the cell division. These strands from a continuous spherical network. In some species e.g., V. monoae and V. tertius protoplasmic strands are absent.

Cell Structure:

Each cell of the colony is pyriform in shape with a broad posterior and narrow anterior end. Each cell of the colony has its own gelatinous sheath or mucilaginous envelope and thus is separated from its neighbouring cell. This shows that Volvox is not an individual but an association of a number of independent cells.

The cell has a thick cell wall differentiated into outer firm and inner mucilaginous layer. Each cell has two flagella at its anterior end. The flagella are acronematic type and project beyond the surface of the colony into the water. The flagella arise from the basal bodies. Due to the concerted movement of flagella, the colony rolls along rather rapidly in water behaving like a single organism.

Each cell has a single large, cup-shaped chloroplast situated at the posterior side of the cell. Chloroplast has 1 or 2 pyrenoids in its centre. A single nucleus is present in the cytoplasm which fills the concavity of the chloroplast. The nucleus is connected with the neuromotor apparatus consisting of bhlepharoplast, rhizoplast and centrome.

The reddish brown eye spot is located in the anterior region of the chloroplast. Volvox being ovoid or a sphere, the end which contains few or no reproductive cells is regarded as anterior end of the plant and opposite the posterior end. The cells at the anterior end have larger eye-spots than those at the posterior end.

In the young Volvox colonies the cells are similar in size and shape and are purely vegetative in function. As the colony grows older the cells get differentiated. A certain number, especially in the posterior half of the colony, the cells become considerably enlarged and function either as asexual or sexual reproductive cells. The reproductive cells are recongnisible by their large size, definite large nuclei, more denser granular protoplasmic contents, numerous pyrenoids.

Volvox is an assemblage of similar and independent cells. Each cell functions like an individual carrying out its own nutrition, respiration and excretion. There is no co-operation between the cells in these functions.

Movement of the colony:

Volvox is a motile alga. By concerted action of the flagella of the cells, the coenobium rotates above its axis, thus making a forward movement. The colony does not roll forward like a ball but rotates about its axis, with one end of the colony always leading in progression. Hence, Clamydomonas is also called as rolling alga.  This shows that the beat of flagella of all the cells in the colony is co-ordinated.

Reproduction:

In young Volvox colonies all the cells are similar and vegetative in function. A certain number, especially in the posterior half of the colony, become considerably enlarged and function either as asexual or sexual reproductive cells.

The reproductive cells are recognizable by their large size, definite larger nuclei and more denser granular protoplasmic contents. They divide and form new colonies.

Asexual Reproduction:

Asexual reproduction occur during favourable conditions in spring and early summer months.

Once the young coenobium attains maturity, a few cells (2-50) in the posterior half of the colony, are pushed back into the hollow cavity. These cells withdraw their flagella, increase ten or more times the size of the vegetative cells and become round in shape. Such reproductive cells are called parthenogonidia or gonidia. The cells have well – defined, central nucleus and dense granular cytoplasm with numerous pyrenoids within their chloroplast.

Development of Daughter Colony:

The first division of the gonidium is longitudinal with respect to the parent coenobium as a whole. The second longitudinal division is at right angles to the first. The four resulting cells divide lengthwise so that a curved plate-like eight-celled plakea stage is formed. The cells are arranged in such a manner that their concave surface faces towards outside.

Another longitudinal division in the plakea stage results in the formation of 16 celled stage. At the 16-celled stage the cells in the young daughter colonies become arranged in form of a hemisphere. Finally, a hollow sphere with a small aperture is formed. This aperture is called the phialophore. It is directed towards the exterior of the mother coenobium.

The cells continue to divide longitudinally, until a specific number of cells are formed specific to a particular species.

At the end of division stage all the cells are naked and in contact with one another. The anterior flagellar end of each cell is directed inwards or towards the center of the sphere.

Now, the young daughter colony turns inside out through the phialophore to bring flagella from its interior to its outer surface. This is called the inversion. Due to this inversion, the inside portion of the young daughter coenobium turns towards outer side. The phialophore is ultimately becomes closed.

Each cell of the daughter coenobium acquires a cell wall and separate form one another. The flagella elongate rapidly. The daughter colony is still retain within the parent or gonidial cell wall. The latter develops into a mucilaginous membrane surrounding the young volvox colony.

Several daughter colonies may be developed simultaneously in the parent colony. Finally, the young colonies escape either by the disintegration of the parent colony or through a pore at the position of the original gonidium. In the former case, the parent colony immediately perishes.

After releasing from the parent colony, increase in size of the colony is partly due to cell enlargement and mainly due to the swelling of the mucilaginous cell walls.

Sometimes, due to long time for disintegration of the parent colony, we may find colony containing daughter colony which in turn may contain granddaughter colony e.g., V. Africana.

Sexual Reproduction:

Sexual reproduction takes place at the end of the growing season. It is oogamous. Certain special cells in the posterior region of the colony enlarge, retract the flagella to become gametangia. The male gametangia are called antheridium and female gametangia are called oogonium.

Many species of Volvox are homothallic or monoecious, they produce male and female sex organs in the same colony, e.g., V. globator. Some species are heterothallic or dioecious, they produce male and female sex organs in two different colonies, e.g., V. aureus.

The monoecious species are protandrous i.e., antheridia mature before the oogonia.

Antheridia or Androgonidia:

The cell destined to form the antheridium enlarges, retracts its flagella and pushes back into the colony. It keeps connections with the adjacent vegetative cells by cytoplasmic strands.

The protoplast of an antheridium undergoes mitotic divisions and form 16-512 small, elongated rod like cells. These are male reproductive cells – sperms or antherozoids.

They are arranged in form of bowl-shaped plate. The sperm cells have their anterior ends directed towards the inside, where flagella begin to develop. As the bowls matures, they undergo inversion just like the asexual colony, so that, the flagella are on the exterior (convex) side of the bowl.

Each antherozoid is a biflagellated, elongated, conical or spindle shaped, pale yellow body. The antherozoids are liberated as a unit, swim around as a unit. They do not separate until they reach the egg.

Oogonium:

Also called as gynogonidia.  The cell predestined to form oogonium enlarges, looses its flagella, and become rounded or flask-shaped and projects into the anterior of the colony.

The entire content of the oogonium gets metamorphosed into a single, non-flagellated, green, spherical egg or oosphere. It has a large central nucleus, and a parietal chloroplast containing numerous pyrenoids. It has large amount of reserve food materials.

The oosphere often shows a beak-like protrusion which serves as the receptive spot, where the sperm enters during fertilization.

Fertilisation:

Before fertilization, the entire mass of antherozoids swims as an unit. Due to chaemotactic stimulation the packet of antherozoids break up in the vicinity of the egg. A single antherozoid swim through the gelatinous oogonial wall, enter the egg through the beak-like receptive spot and fuses with egg nucleus to form a diploid zygote.

Zygote:

Soon after fertilization, the zygote secretes a thick, three-layered, smooth or spiny wall around it and becomes an oospore. It accumulates haematochrome pigment due to which the protoplast becomes orange red.

The oospore undergoes a period of dormancy and is retained in the parent colony until the latter decay or disintegrates. Thus, oospore constitutes the perennating stage in the life history of Volvox. Eventually, the parent colony disintegrates and the oospere fall to the bottom of the pool, where it ripens and may remain viable for several years.

Germination of Oospore:

The oospore germinates during favourable conditions. Prior to germination, the zygote nucleus undergoes meiosis to form four haploid daughter nuclei. Out of 4 nuclei, three nuclei degenerate.

At the time of germination, the outer most layer of the oospere splits. The middle layer them splits and the delicate inner layer protrudes to form a vesicle. The haploid protoplast containing the functional daughter nucleus migrates into the vesicle.

The zygote protoplast develops as biflagellate zoospore or meiospores inside the vesicle. The zoospores rarely escapes from the vesicle. It divides to form a hollow sphere of cells. This sphere is of about 125 to 500 cells, undergoes the process of inversion and develops into a new colony or coenobium.

In V. aureus and V. minor the two thick outer layers of the zygote wall split. The haploid protoplast surrounded by the inner layer escapes. By repeated division it directly develops into a colony in the same manner as in asexual reproduction.

The young colony contains small number of cells. The maximum size constant for the species is attained after it has passed through several asexual cycle. With each cycle or generation, the daughter coenobium becomes longer n size and has increased number of cells until a fully developed colony is formed.

Life-Cycle:

The life cycle of Volvox is haplontic or haploid type. The plant body belongs to haploid generation. It reproduces asexually by daughter colonies. Sexually, it reproduces by haploid antherozoids or sperms and egg. The diploid zygote is formed by the fusion of a sperm and egg. The zygote undergoes meiosis and forms the colony of haploid daughter cells.

The haploid phase is dominate and diploid phase is represented only by the diploid zygote.