Kotakonda Sivakumar M.Sc. B.Ed. M.Phil Junior Lecturer
6/105 Girija Street
Kadapa Dt AP
Plant king dom
Based on Eichler system of classification plants are classified into two sub king doms . They are 1. Cryptogamae. 2. Phaerogamae.
Cryptogamae:- They are flowerless, seedless and spore bearing plants. These sub kingdom is divided into three Divisions they are
1. Thallophytes 2. Bryophytes 3. Pteridophytes
Thallophytes:-Plant body is not differentiated into root,stem and leaves and such plant body is called thallus. They are divided into two sub divisions. They are
1. Algae 2. Fungi.
Algae:-chlorophyllous , autotrophic thallophytes,aquatic plants.Ex. Spirogyra.
Fungi:- Non green, heterotrophic thallophytes. Ex.Rhizopus.
Heterotrophic means they live as parasites( live on others for food and shelter), saprophytes( live on dead organic matter), symbionts , ( live in associations). mycorizae(association between fungal member and vascular plants).
Lichens means symbiotic association of algal member (provide food), fungal member (provide shelter) .
Bryophytes:- They are chlorophyllous, autotrophic, embryophytic and atracheophytic cryptogams. They found on damp and shady places . They are called amphibians of plant kingdom. Ex. Funaria.
Pteridophytes:- They are chlorophyllous, autotrophic, embryophytic and tracheophytic cryptogams. They are non flowering vascular plants. Hence they are called vascular cryptogams. Eg. Pteris.
Phanerogamae( flowering plants ):- Seed producing tracheophytes . It has only one division Spermatophyta.
Spermatophyta:- Seed plants with or without fruits. It classified into two sub-divisions namely 1.Gymnosperms 2. Angiosperms
Gymnosperms:- With out ovary and fruit. Seeds are naked without fruit wall.
Eg. Cycas , Pinus, Gnetum.
Angiosperms:- They are flowering plants. Seeds are in fruits. They divided into two classes. 1. Dicotyledons 2. Monocotyledons.
Dicotyledons:- They have two cotyledons in their seeds. Eg. Bean , Pea
Monocotyledons:- They have one cotyledons in their seeds. Eg. Maize. Sorghum.
Sub king dom Cryptogamae phaerogamae
Divisions Thallophyta Bryophyta Pteridophyta Spermatophyta
Eg. Funaria Eg. pteris
Sub-division Algae Fungi Gymnosperms Angiosperms
Eg. Spirogyra Eg. Rhizopus Eg. Cycas
Classes Dicotyledons Monocotyledons
Eg. Bean Eg. Maize.
Reproduction in Angiosperms
Life cycle of Angiosperms: In life cycle of angiosperms , there are two phases Sporophytic Phase and Gametophytic Phase.
Sporophytic Phase: It is dominant phase. Diploid phase. It is developed from zygote. This phase produce flowers.
Gametophytic Phase: It is haploid phase. It is developed from spore . Spores are formed by reduction division of spore mother cells .
Spore mother cells are two types, they are 1. Microspore mother cells. 2. Megaspore mother cells. Microspore mother cells are formed in microsporangium of anther. Microspore mother cells undergo reduction division and produce four microspores. Megaspore mother cells are formed in nucellus of ovule. Megaspore mother cells undergo reduction division and produce megaspore.
Haploid microspore and megaspore produce male and female gametophytes which give rise male gamete and egg cell. Male gamete and egg cell unite and form diploid zygote and it undergo divisions and produce embryo. Embryo give rise seed .Seed germinate and produce seedling and develop into sporophytic plant.
Microsporogenesis; Development of male gametophyte.
Structure of Anther: Anther consists of two lobes (Dithecous).Each lobe consists of two microsporangia or pollen chambers. Monothecous anther consists of only one lobe with two micro sporangia.
Structure of anther wall: It consists of epidermis, endothecium, middle layers and tapetum.
Epidermis: One celled thick protective layer. Epidermal cells present between two pollen sacs are called stomium . Stomium is useful during dehiscence of pollen sacs.
Endothecium: It is present below the epidermis. Cells with fibrous theckenings which are hygroscopic. They help in dehiscence.
Middle layers: Beneath endothecium,1-5 thin walled cells arranged
Tapetum; It is present below the middle layers . It encircles the sporogenous tissue. The tapetal cells large , thin walled, abundant cytoplasm and prominent nuclei.Tapetum is nutritive tissue useful for developing sporogenous tissue.
Microsporogenesis:In microsporangium microspore mother cells undergo meiosis and form four microspores called microsporogenesis. Four microspores in tetrad, later they separate and form microspores or pollen grains .Pollen grains are haploid, uninucleate, and unicellular.
Structure of pollen grain: Microspore or pollen grains are spherical or oval, with two layered spore wall. The outer layer is called exosporium or exine made up of sporopollenin and is thick walled with spiny ornamentations. The inner thin walled layer called endosporium or intine made up of pecto-cellulose. The thin places in outer exine are called germ pores. Pollen grain is centrally located haploid nucleus and dense cytoplasm.
Development of Male Gametophyte: pollen grain is the first cell of male gametophyte. Pollen grain divide periclinally into larger vegetative cell and small generative cell. At this two celled stage, pollen grains liberated and reach the stigma by pollination. Pollen grains germinate on stigma by absorbing stigmatic secretions and pollen grains swell and intine come out through germ pore as pollen tube. Into pollen tube vegetative nucleus and generative cell enter . Later generative cell come farward and then before fertilization vegetative nucleus degenerate. Generative cell divide mitotically and form two male gametes. Pollen tube reach ovule through style.
Monosiphonous: One pollen tube develops.
Polysiphonous: More than one pollen tube develops from each pollen grain Eg. Malvaceaae and Cucurbitaceae.
Ovule, Megasporogenesis and Development of embryo sac
Structure of ovule: Ovules are integumented megasporangia. Ovule consists of Body and funiculus.The body of the ovule contains a diploid nutritive tissue. Nucellus is protected by one or two or none membranes called integuments. The integuments and the nucellus unite at the base of the ovule This place is called chalaza. The integuments not cover the nucellus completely and leave an apical opening called micropyle .Anterior part is called micropyle and posterior part is called chalaza.Each ovule contains embryo sac or female gametophyte. Slender stalk of the ovary is called funiculus.
Based on number of integuments ovules are three types,
Ategmic ovules, nucellus is not covered by integuments. Eg. Loranthus.
Unitegmic ovule, nucellus is covered by one integuments. Eg. Asteraceae.
Bitegmic ovule, nucellus is covered by two integuments. Eg. Monocots.
Types of Ovules: Based on the position of micropyle in relation to the funiculus and other parts of the ovule, six types of ovules are identified.
Orthotropous ovule: It is straight ovule. In this type of ovule , the micropyle Chalaza and funiclus are arranged on the same vertical line
Anatropous ovule: It is inverted ovule and the curvature of the body of the ovule is 1800. Micropyle, chalaza are arranged in the same vertical line. Micropyle very close to the funiculus. Funiculus is attached to the body of the ovule is called hilum.Along the body of the ovule sort ridge is formed called raphe.Eg. Asteraceae . Gamopetalae.
Hemi-anatropous ovule: The body of the ovule is placed at 900 angle and arranged right angle to the fuiculus.Eg. Primula, Raunculus.
Campylotropous ovule:ovule body arranged right angle to the funiculus and micropyle comes to words funiculus. Embryo sac is straight or slightly curved. Eg. Fabaceae, Brassicaceae.
Amphitropous ovule: Ovule body is curved 1600 with horse shoe shaped embryo sac. Micropyle and funiculus close together. Eg. Alismaceae, Butamaceae.
Circinotropous ovule: The body of anatropous ovule rotates to 3600, so that the body becomes upright and appears like orthotropous ovule. The funiculus is coiled like watch spring around ovule. Eg. Opuntia.
Megasporogenesis and Development of Embryo sac (female gametophyte): 1. In the ovule, nucellus cell acts as archesporial cell. 2.Archesporial cell divide periclinally to form outer parietal cell and inner sporogenous cell
3. Sporogenous cell acts as megaspore mother cell.
4.Megaspore mother cell divide meiotically to form linear, megaspore terad. 5.The upper three megaspores are degenerate and lower one megaspore is functional. It is called functional megaspore.
6.Functional megaspore act as mother cell for the development of an embryosac.
7. In the functional megaspore, haploid nuceleus divide three times to form eight haploid nuclei.
8.Out of eight nuclei, four nuclei arranged at micropyle side and four nuclei arranged at chalaza side. In this stage functional megaspore is called embryosac.
9. Out of four nuclei at micropyle side one nucleus act as upper polar nucleus. Out of four nuclei at chalaza side one nucleus act as lower polar nucleus, Two polar nuclei move towards centre of the embryosac and fused to form diploid secondary nucleus. Secondary nucleus enclosed in a central cell.
10.Three nuclei towards micropule side develops into egg apparatus.
11. Three nuclei towards chalaza side develops into antipodals.
12. Eight nucleated, seven called embryosac is described as female gametophyte.It is developed from a single megaspore, it is called monosporic type.
Structure of Embryo sac:The female gametophyte of angiosperms is called embryosac. The most common type of embryo sac in angiosperms is eight nucleated and seven celled . It is called polygonum type. It was first studied in Polygonum divaricatum by Strasburger . The embryo sac has three parts.
Egg apparatus:Three nuclei of the embryo sac towards micropyle develop into an egg apparatus on the anterior side. The middle cell of the egg apparatus is big and acts as an egg cell. The remaining two cells present on either side are called synergids.They contain finger like projections called filiform apparatus which may help in absorption and conduction of food materials from nucellus into the embryo sac. They also help in directing the movement of pollen tube towards the embryo sac by secreting some chemicals.
Antipodals: The three cells arranged on the posterior side of the embryo sac nearer to the chalaza are called antipodals. They are ephemeral and degenerate before or after fertilization. These cells are regarded as the vegetative cells of the embryo sac.
Central cell: It is the largest cell of the embryo sac which has centrally located vacuole and two polar nuclei. The two polar nuclei fuse to form a diploid nucleus. It is called secondary nucleus. This fusion occurs befor or after the entry of the pollen tube into the embryo sac.
Fertilization: The fusion of male and female gametes is called fertilization.
Process of fertilization: It occurs in following five steps.
1. Entry of the pollen tube into the ovule: It occur in one of the following three ways ,
a. Porogamy: Entry of pollen tube into the ovule through the micropyle is called porogamy. Eg. Ottelia
b. Chalazogamy: Pollen tube enter into the ovule through the chalaza . It is discovered by Treub in Casuarina.
c. Mesogamy: Pollen tube enter into the ovule through the integuments or funiculus.
2. Entry of pollen tube into the embryo sac: The pollen tube inters into the embryo sac only through micropyle either passing between egg cell and synergid or penetrating through synergid cell. Entry of pollen tube into embryo sac is directedy by filiform apparatus.
3. Dicharge of male gametes: After the entry of pollen tube into embryo sac, the male gametes released either by the rupture of pollen tube tip or formation of pore at the apex of the pollen tube. Pollen tube release male gametes and vegetative nucleus ,
4. Gametes fusion: One male gamete fuses with egg cell and forms diploid zygote(2n).
This fusion is called syngamy or fertilization. It was discovered by Strasburger.
5. Triple fusion and Double fertilization: The second male gamete fuses with secondary nucleus of the embryo sac and results the formation of triploid primary endosperm nucleus(PEN).It is called triple fusion It was first discovered by Nawaschin in Lilium and Fritillaria It is vegetative fertilization. Thus two fertilizations occur in angiospers. It is called double fertilization. Triple fusion results endospermic seeds.
Post fertilization changes: Changes afeter fertilization in the flower. Sepals,Petals, stamens, styles and stigmas fall off. In Solanaceae members calyx remain persistent and grow even after fertilization . In Asteraceae, persistent remain as pappus and help in fruit dispersal. Changes after fertilization in the ovule are as follows.
Parts of Ovule Changes occurring after fertilization
Funiculus— Stalk of the seed
Outer integument-- Testa
Micropyle-- Seed pore
Primary endosperm nucleus—Endosperm
Hilum— Scar of the seed.
Endosperm: In angiosperms the endosperm is formed from primary endosperm nucleus after fertilization. It is nutritive tissue and useful for the development of embryos.It is in triploid condition.In gymnosperms the female gametophyte is formed before fertilization and acts as endosperm. Hence haploid in condition.
Endosperm is completely used up during the development of seed.The ovule contains only seed without endosperm is called non endospermic seeds or ex-albuminous seeds.
Eg. Dolichos, Cicer.
In some plants endosperm not completely used up . A little amount of endosperm is with seed . Such seeds are called endospermic seed or albuminous seed. Eg. Cocos, Ricinus, Datura.
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