sexual reproduction in flowering plants Flashcards
ornithophily
pollination by birds
entomophily
pollination by insects
chiropterophily
pollination by bats
Stamen
The pollen-producing part of a flower, usually with a slender filament supporting the anther.
Anther
The part of the stamen where pollen is produced
Pistil
The ovule producing part of a flower.
ovary
The ovary often supports a long style, topped by a stigma.
anther wall
exothecium/epidermis , endothecium & tapetum
carpel
the female reproductive organ of a flower, consisting of an ovary, a stigma, and usually a style. It may occur singly or as one of a group
corolla
corolla or petals merely refers to the second whorl of a flower, which is interior to the calyx.
calyx
the whorl of sepals of a flower collectively forming the outer floral envelope or layer of the perianth enclosing and supporting the developing bud; usually green.
perianth
The perianth is the non-reproductive part of the flower, and structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla (petals)
In what type of plants are the petals small or absent and green?
In wind-pollinated plants
Why are the petals in wind-pollinated plants, small or absent and green?
Because they don’t rely on animals for pollination and therefore don’t need to attract them
structure of anther
bilobed, four-sided (tetragonal) , containing 4 microsporangia, 2 in each lobe
microsporangia
they develop into pollen sacs that are packed with pollen grains
structure of microsporangium (4) and their functions
the first 3 layers help in the protection and dehiscence of anther to release pollen.
*tapetum nourishes the developing pollen grains
tapetal cells have dense cytoplasm and generally more than 1 nucleus, why?
tapetal cells undergo endomitosis in which the nucleus divides within the nuclear membrane but cytokinesis doesnt occur. This results in …. bi nucleate.
what is the ploidy of microspore tetrads? or what is microsporogenesis?
Microspore tetrad cells are microspores arranged in a group of four. Microspores are formed from microspore mother cell (MMC). MMC is a diploid cell which undergoes meiosis to form four haploid microspores which are arranged in a tetrad. This process is known as microsporogenesis. Each of these four microspores develops into a pollen grain. Hence, ploidy of microspore tetrad cells is haploid.
production of pollen grains?
microsporogenesis: MMC-meiotic divns -microspore tetrad. -as the anthers mature and dehydrate,the microspores dissociate from each other and develop into pollen grains
layers of pollen grain? what are they made up of?
the hard outer layer,exine- sporopollenin thin and continuous layer,intine - cellulose and pectin
what is the function of germ pores?
germ pore helps in the formation of the pollen tube and the release of the male gametes during fertilisation.
when the pollen grain is mature,it contains 2 cells __ and ___? describe
VEGETATIVE CELL bigger -has abundant food reserve -large irregularly shaped nucleus GENERATIVE CELL -small -floats in cytoplasm of veg.cell -spindle shaped with dense cytoplasm and a nucleus. In over 60% of angiosperms,pollen grain is shed during this 2 celled stage.but in rest,gen. cell divides mitotically- 2 male gametes before pollen is shed (3celled)
What causes pollen allergy?
parthenium or carrot grass that came into India as a contaminant with imported wheat,has become ubiquitous in occurence and cause pollen allergy.
describe pollen viability ?
period for which pollen grains remain viable -highly variable
-to an extent,depends on prevailing temp&humidity
+some cereals rice,wheat..-lose via. within 30mins of their release
+some members of Solanaceae,rosaceae..- remain via.for month
cryopreservation-
crop breeding programs-pollen banks -pollen grains of large no.of species are stored in liquid nitrogen (-196 C)
define: monocarpellary,multicarpellary syncarpous, apocarpous
monocarpellary : single pistil
multi. .: more than 1
syncarpous: pistils are fused together eg; papaver
apocarpous: pistils are free eg:michellia
Structure of ovule
+The ovule -small structure attached to the placenta by means of a stalk- funicle.
+The body of the ovule fuses with funicle in the region called hilum - the junction between ovule and funicle.
+Each ovule has one or two protective envelopes called integuments.
+Integuments encircle the ovule except at the tip where a small opening called the micropyle is present.
+ Opposite the micropylar end is the chalaza. The chalaza represents the basal part of the ovule.
+A mass of cells; called nucellus; is enclosed within the integuments. The cells of nucellus have abundant reserve food material.
+Embryo sac or female gametophyte is located in the nucellus.
explain megasporogenesis.
process of formation of megaspores from the megaspore mother cell
-ovules generally differentiate a single megaspore mother cell in the micropylar region of the nucellus. MMC undergoes meiotic division - resulting in the production of 4 megaspores. -3 of the megaspores degenerate and only one is functional- develops into a female gametophyte, embryo sac. [monosporic development]
describe monosporic development ?
Nucleus undergo mitotic division2-nucleate embryo-sac is formed
Nuclei undergo mitotic division again4-nucleate embryo-sac is formed
Nuclei undergo yet another mitotic division8-nucleate embryo-sac is formed
Cell wall formation takes place leading to the organisation of embryo.
The nuclei formed inside get rearranged to form the final structure of Embryo sac as follows:Egg apparatus2 synergids, 1 egg cell located at micropylar end Antipodals3 cells at basal end of ovuleLocated at chalazal end Central cell2 polar nuclei fused together
embryo-sac is 7-celled, but 8-nucleate. why
in the central cell, 2 polar nuclei fuse to form a single cell.
Even though each pollen grain has two male gametes. Why are at least 10 pollen grains and not 5 pollen grains required to fertilise 10 ovules present in a particular carpel?
Double fertilisation -One of the male gametes fuse with the egg cell to form zygote while other male gamete fuses with polar nuclei and form primary endosperm nucleus.
define pollination
transfer of pollen grains from the anther to stigma
kinds of pollination
autogamy- same flower
geitonogamy- another flower of same plant common pansy(viola),oxalis,commelina.
-chasmogamous(exposed anthers and stigma)
-cleistogamous(doesn’t open at all)
xenogamy-diff plant of same species
What are the agents of pollination? their names?
abiotic- wind,water biotic-animals
wind: anemophily
water: hydrophily
insect: entamophily
bird: ormitophily
adaptations for wind polln:
- pollen grains : light and non sticky -often possess well exposed stamens(so pollen is easily dispersed in wind)
- large often feathery stigma (to easliy trap air borne pollen)
- flowers often have single ovule in each ovary
- numerous flowers packed into an inflorescence
- quite common in grasses.
adaptations of water polln
- limited to abt 30 genera,mostly monocotyledons.
- water is a regular mode of transport
- pollen grains in most species are long,ribbon-like and carried passively in water.
- pollen grain-protected from wetting by mucilagenous covering
examples of water polln (fresh $ marine)
freshwater- Vallisneria,hydrilla
marine-zostera
in vallisneria, flowers?
reach the water surface by a long stalk and pollen is released onto the water surface and it reaches the stigma by being carried by water currents
in some plants like sea-grasses, flowers?
remain submerged in water and pollen- released inside the water.
adaptations of animal/insect polln.
- flowers are large,colorful,fragrant and rich in nectar (floral rewards)
- pollen grains are gen.sticky
- if flower is small,theyre packed into inflorenscense
- flowers pollinated by flies and beetles secrete foul odours to attract these animals
- in some species,floral reward is to provide a safe space to lay eggs,eg:amorphophallus(tallest flower),moth and plant yucca (cant complete life cylces w/ one another) _moth-deposits egg in locule ,flower-gets pollinated _larvae of moth come out of eggs as seeds start developing.
why outbreeding devices? mention them
continued self polln. results in inbreeding depression.
4 devices:
-pollen and stigma receptivity- not synchronised.
-anther and stigma are placed at diff positions(so no contact b/w them)
-self incompatibility (genetic mechanism)
-production of unisexual flowers
the function of filiform apparatus?
plays an imp. role in guiding the pollen tubes into the synergid.
how do plants ensure that polln. only takes place between compatible species?
pollination doesnt guarantee the right transfer of the right type of pollen. -pistil has the ability to recognise the pollen,whether it is of the right type or not (i.e,compatible or not). -if it is compatible,the pistil accepts the pollen and promotes post-polln. events for fertilisation. -if it is not compatible,the pistil rejects the pollen by preventing pollen germination on stigma or the pollen tube growth in the style. +The ability of the pistil to recognise pollen followed by its accceptance or rejection is the result of a continuous dialogue b/w pollen grain and the pistil. These dialogue-mediated-chemical components of the pollen interacting with those of the pistil.
explain pollen-pistil interaction?
- following compatible polln.,the pollen grain germinates on stigma- produce- a pollen tube through one of the germ pores.
- Contents of pollen grain move into the pollen tube.
- pollen tube grows and reaches ovary.
- after it reaches ovary,it enters the ovule through the micropyle- enters one of the synergids through the filiform apparatus. +All these events from pollen deposition on stigma until pollen tube enters ovule are together referred to as pollen-pistil interaction
why artificial hybridisation?
breeders are interested in crossing diff species and often genera to combine desirable characters to produce commercially ‘superior’ varieties.
if the female parent bears bisexual flowers, […] is called emasculation.
if the female parent bears bisexual flowers, removal of anthers from flower buds before the anther dehisces using a pair of forceps is called emasculation
covering emasculated flowers with a bag of suitable size, generally made up of butter paper,to prevent contamination of its stigma with unwanted pollen is called […]
bagging.
how is artificial hybridisation done?
2 imp things to take care of :
*to ensure that only the desired pollen grains are used for polln
*stigma has to be protected from being contaminated by unwanted pollen.
bisexual female parent:
-emasculation: anther removed from flower bud using forceps
-bagging: flower is then bagged- butter paper
-when the stigma of bagged flower attains receptivity,mature pollen grains collected from anther of male parent are dusted on stigma
-flowers are rebagged-fruits allowed to develop.
unisexual female parent:
no emasculation
-bagged before flower opens
-[rest same as above]
Why does endosperm development precede embryo development?
-primnary endosperm cell - divides repeatedly - triploid endosperm tissue.
Cells of this tissue - filled with reserve food materials - used for the developing embryo
stages of endosperm developement
1) PEN undergoes successive nucleur divisions - free nuclei. ——–> Free nucleur endosperm eg: water from tender coconut
2) Subsequently,cell wall formation takes place, endosperm becomes cellular ——-> Cellular endosperm eg:White kernel surrounding the coconut water
Endosperm may either be […] eg: pea,groundnut,beans… before seed maturation or […] eg:castor,coconut
Endosperm may either be completely consumed by developing embryo eg: pea,groundnut,beans… before seed maturation or it may persist in the mature seed and be used up during seed germination eg:castor,coconut
the early stages of embryo development […] is similar in both monocotyledons and dicotyledons the early stages of embryo
embryogeny is similar in both monocotyledons and dicotyledons
zygote divides to form […] and subsequently to the […].
zygote divides to form pro-embryo (2-celled)) and subsequently to the globular,heart-shaped and mature embryo
escribe the structure of a typical dicotyledonous embryo
A typical dicotyledonous embryo consists of an embryonal axis and 2 cotyledons.
The portion of the embryonal axis - above the level of cotyledon - epicotyl - terminates with the plumule or stem tip.
The cylindrical portion - below the level of cotyledons - hypocotyl - terminates at its lower end in the radicle or root tip –>covered with a root cap.
Describe the structure of the embryo of monocotyledons
only 1 cotyledon In the grass family,cotyledon - Scutellum - situated towards one side (lateral side)of the embryonal axis.
At its lower end,the embryonal axis has radical and root cap - enclosed in an undifferentiated sheath - coleorrhiza.
Portion of embryonal axis above the level of scutellum - epicotyl.
Epicotyl has - a shoot apex & a few leaf primordia enclosed in a hollow foliar structure - coleoptile.
