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Reproduction in lower and higher plants, POLLINATION
transfer of pollen…
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POLLINATION
- transfer of pollen grains from anther to stigma of the flower
Self- pollination
- occurs in single or 2 flowers of single plant
autogamy
- pollination in which bisexual flower is pollinated by its own pollen grains
- offsprings are genetically identical to parents
geitonogamy
- transfer of pollen grainsto a stigma of diff flower on the same plant
- doesnt bring genetic variation
Cross-pollination
- transfer of pollen grain from anther of one flower to stigma of other flower of diff plant of same species
xenogamy
- pollen grain of one flower is deposited on stigma of a flowerof diff plant of same species with the help of pollinating agents
- genetically varied offspring
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outbreeding devices
- self pollination causes inbreeding depression,so there r mechanisms in plants to discourage sp and promote cp
- for genetic diversity and evolution
unisexuality
- plants bear either male or female flowers,dioecism
- as flowers r unisexual,sp is not possible
dichogamy
- anther and filament mature at diff times in bisexual flowers to prevent sp
protandry
- androecium matures earlier than gynoecium
protogyny
- gynoecium matures earlier than androecium
preptotency
- pg of other flowers germinate faster than pg of the same flower
heterostyly
- in some plants,there r 2-3 types of flowers in which stigma and anther r placed at diff levels
- in heteromorphicflowers,pg produced from anther pollinate stigma produced at the same level
herkogamy
- mechanical device to prevent sp in bisexual flowers
- natural physical barrier is present betn sex organs to avoid contact of pg with stigma of same flower
self incompatibility
- genetic mechanism in which germination of pg on the stigma of the same flower is inhibited
APOMIXIS
- phenomena of formatio of embryo through assexual reproduction without formation of gamete and act of fertilization
- no meisosis,syngamy
- embryo develops in ovule and ovulr develops to seed
- when gametphyte organ or cell produces embryo like structue\re without fertilization-apogamy
- diploid sporophyte cell produces diploid gametophyte without meiosis-apospory(orange,mango)
recurrent apomixis
- embryo sac arises from archesporial cells or from other part of nucellus
- diplospory-unreduced embryo sac is derived from diploid megaspore mother cells
- in apospory,nucellar cells give rise to apomictic embryo sac
non-recurrent apomixis
- megaspore mother cell undergo meiosis to form haploid embryo sac,embryo either arises from the egg cell through parthenogenesis or from other haploid cells of gametophyte through apogamy
- plants are gentically sterile and do not reproduce sexually
adventive embryony
- embryos devlop from somatic nucellus or integuments along with zygotic embryo
- in citrus(orange,lemon,mango)
- gives rise to conition-polyembrony
STRUCTURE OF ANTHER
- immature stage=grp of parenchymatous tissues surrounded by single layer of epidermis
- 2 lobes(dithecous) and tetrasporanginate(since each lobe contains 2 pollen sacs)
T.S
- archesporial cells divide into=inner sporogenous cell(mmc) and outer primary parietal celll
- sporogenous cell form sporogenous tissue(mmc):each mmc divde meiotically to give tetrad(4) of haploid microspore(pollen grain)
- parietal cell undergoes division to form wall layer;(4 layers for mature anther)
1.epidermis-outermost protective layer of tabular cells
2.endothecium-sub-epidermal layer of radially elongated cells of fibrous thickening
3.middle layer-1-2 thin walled cell layers,may disintegrate in mature anther
4.tapetum-inner most nutritive layer,immediately encloses mmc
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STRUCTURE OF MICROSPORE
- non-motile,haploid,unicelllular with 1 nucleus
- 2 layered wall=sporoderm;
1.exine-outermost layer,thick,made of complex non-biodegradable substance(sporopollenin),resistant to chemicals,at some places it is thin to show thin areas-germ pores-meant for growth of emerging pollen tube
2.intine-inner wall,made of cellulose and pectin
DEVELOPMENT
- first mitotic division to produce bigger naked vegetative cells(rich in food,irregular shaped nucleus) and small thin walled generative cell (float in cytoplasmof vegetative cells)
- 2nd mitotic division of generative cell takes place in pollen grain or pollen tube,in this gives rise to non-motile male gametes
- pollen grains are release from angiosperm at this 2-celled stage
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PARTHENOCARPY
- fruit is developed without the process of fertilization
- occurs naturally in papaya,banana,pineapple
- placental tissue in unfertilised ovary produces auxin responsible for enlargement of ovary into fruit
- fruit resemble normal fruit but is seedless
POLYEMBRYONY
- development of more than 1 embryo in seed
- noticed by leeuwenhoek
- results in multiple seedlings
- additional embryos result from differentiation and development of various maternal and zygotic tissues associated with ovule of seed
in cleavage polyembrony-zygote proembryo divides into many parts or units and each unit develop into embryo
- polyembryony increases the chances of survival of new plants