Reproduction In Plants Flashcards
1
Q
What is meant by the term reproduction?
A
It is the production of a new generation of individuals of the same species, during which there is transmission of genetic material from parents to their offspring to ensure continued survival of a species.
OR: Formation of separately existing individuals of the same species by existing organisms
2
Q
What are the types of reproduction?
A
Sexual and Asexual
3
Q
Distinguish between sexual and asexual reproduction
A
Asexual reproduction is the production of offspring (new individuals) from a single organism without the formation of gametes.
The offspring (referred to as clone) are genetically identical to each other and to their parent, except when and if mutation occurs.
While: Sexual reproduction is the production of offspring by the fusion of haploid male and female gametes (fertilization) to form a diploid zygote, which develops into the mature organism.
4
Q
What is cloning?
A
This is the process by which an organism reproduces asexually to give individuals in a population genetically identical to each other and to their parent.
5
Q
Give similarities between sexual and asexual reproduction.
A
- In both mitosis is involved
- Both produce offspring
- In both there is transmission of genetic material from parents to their offspring
6
Q
State the various forms of asexual reproduction.
A
- Fission
- Sporulation
- Fragmentation
- Budding
- Vegetative propagation
7
Q
What is a spore?
A
A haploid reproductive cell, usually unicellular, capable of developing into an adult without fusion with another cell.
8
Q
Give an account of the five major ways by which asexual reproduction occurs.
A
a) Fission:
It is the division of the cell by mitosis into two or more equal sized daughter cells identical to the parent cell. Binary fission (‘splitting into two’) occurs in bacteria, amoeba, paramecium while multiple fission (schizogony) - ‘splitting into many’ occurs in plasmodium (a malaria parasite) immediately after infection as the parasite (merozoites) enters the liver and again as they invade the red blood cells.
b) Sporulation (spore formation):
Is the formation of small unicellular bodies called spores by cell divisions in organisms, which detach from the parent plant and grow into new organisms under favourable conditions. It occurs in fungi e.g. rhizopus, bacteria, mosses, ferns, Liverworts, algae and amoeba.
c) Fragmentation:
This is the breaking of a single organism into two or more genetically identical pieces, each of which is capable of regenerating into a new individual. It occurs in sponges, spirogyra, cnidarians, flatworms.
d) Budding:
This involves the parent cell forming an out growth (a small bulge), which increases in size and finally drops off (detaches) to become an independent organism. It occurs in flat worms e.g. tapeworms, yeast, hydra, obelia, bryophyllum.
e) Vegetative propagation:
This is asexual reproduction in which a part of flowering plant other than the flower detaches itself to develop into a new individual plant.
Parts of the plant such as root, stem, or leaf specialized to give rise to new individuals are called propagules and may also act as organs of perennation.
9
Q
What are perennating organs?
A
These are parts of flowering plants specialised for vegetative propagation as well as food storage, enabling plants bearing them to survive adverse (unfavourable) environmental conditions e.g. cold or dry periods.
The food manufactured by photosynthesis in aerial green leaves is translocated and subsequently stored as starch in rhizomes, corms, stem and root tubers, or as glucose in onion bulb.
10
Q
Explain the role played by perennating organs in plants.
A
⁃ During unfavourable conditions, the plant remains dormant
⁃ When conditions normalise, starch is hydrolysed to sugar and translocated to the young buds, enabling early growth, sprouting, and photosynthesis when there is little competition for nutrients from other species.
11
Q
Distinguish between organs of vegetative propagation and organs of perennation in plants.
A
Organs of vegetative progation are the parts of the flowering plant other than the flower, such as root, stem, or leaf specialized for developing into new individuals when detached.
While
Organs of perennation are plant parts specialised for storing the food used to develop into new individuals, enabling plants bearing them to survive adverse environmental conditions like drought.
12
Q
Give examples of organs of vegetative propagation.
A
- rhizomes e.g. ginger, couch grass, canna lily and spear grass;
- corms e.g. coco-yam (mayuni), crocus and gladiolus;
- stem tubers e.g. Irish potato, yams;
- root tubers e.g. dahlia;
- bulbs e.g. onion, garlic, tulip,
- swollen taproots e.g. carrot, turnip;
- stolons e.g. blackberry; runners e.g. straw berry, oxalis
13
Q
Give examples of organs of perennation.
A
- rhizomes
- corms
- stem
- root tubers
- bulbs
- swollen tap roots.
14
Q
Specialized organs of vegetative propagation must have buds, which only occur on stems.
Explain why root tubers and swollen taproots are used as organs of vegetative propagation despite roots lacking buds.
A
Root tubers and swollen taproots must bear a small part of old stem if they are to act as organs of vegetative propagation.
The swollen root together with buds at the base of old stem form organs of vegetative propagation and perennation.
15
Q
Give methods of artificial propagation in plants.
A
- Cutting:
A piece of root e.g. of lemon and tamarind, or stem e.g. sugarcane and cassava, or a complete leaf is
dipped in rooting mixture composed of plant hormones, and allowed to grow in a rooting composite or soil. - Layering:
Involves pegging down of stem of runners e.g. strawberry into the soil to induce development of adventitious roots, after which the new daughter plants are detached from the parent plants by cutting. - Grafting and budding:
It is the insertion of a stem or bud of one plant, the scion onto another closely related plant, the stock, ensuring that vascular tissues are in contact. It propagates lemons, apples, roses, hibiscus, and oranges.
Grafting in plants is not hampered by rejection, as is the case in animals.
16
Q
What are the advantages of natural vegetative propagation?
A
- It is a rapid means of reproduction and spread
- Offspring are genetically identical, preserving good strains.
- Perrenating organs enable survival in adverse conditions
- Their dispersal and spread is independent of external agents hence the process is faster.
- Plants are less affected by environmental factors
- Females pass all of their genes to the offspring
17
Q
What are the disadvantages of natural vegetative propagation?
A
- Leads to overcrowding and competition for nutrients, unless separated artificially.
- New varieties cannot be produced, except by mutation resulting into reduced vigour & strength
- Diseases typical of a species are rapidly transmitted and can decimate a crop
18
Q
Outline the process of sporulation in amoeba.
A
- It starts when the nucleus of a single cell of amoeba divides repeatedly and each unit is enclosed in a bit of cytoplasm, but all are encased by one cell membrane
- Some residual cytoplasm may be left which is discarded.
- The new nuclei with cytoplasm are surrounded by spore case (cyst) to withstand unfavourable conditions
- The spores formed are released by bursting of the cell membrane.
19
Q
What is parthenogenesis?
A
The development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilisation
20
Q
What is the meaning of diploid parthenogenesis and give an example?
A
- This is the development of embryo from unfertilized diploid eggs that formed by mitosis instead of meiosis, resulting into diploid offspring, which are clones of the parent.
- It occurs in Aphids, during which large numbers of wingless females are formed without necessitating the presence of males, in flatworms, rotifers, crustaceans.
21
Q
What is the meaning of haploid parthenogenesis and give an example?
A
- This is the development of embryo from unfertilized haploid eggs that formed by meiosis and may develop directly into haploid offspring.
- It occurs in honeybees, wasps, ants, whiptail lizards.
22
Q
Describe sex determination in honey bees.
A
- In honeybees, the queen bee can either fertilise the eggs as she lays them or allows them to pass unfertilised.
- Fertilised eggs become diploid females (fertile queens or sterile workers), and unfertilised eggs develop to become fertile haploid males (drones).
23
Q
What is the major disadvantage of parthenogenesis?
A
- During sudden environmental changes, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions
24
Q
Define apomixis.
A
It is the formation of plant embryo from an unfertilised haploid egg cell or from a diploid embryo sac mother cell or from a diploid cell in the ovule without fertilisation.
It occurs in potatoes and citruses.
25
What is parthenocarpy?
This is fruit development without fertilisation, usually induced by auxins e.g. in apples
26
Give the major advantages of parthenogenesis.
- It avoids the problem in some animals of bringing together males and females at the right moment for successful fertilization.
- Produces a large number of organisms in a short time. E.g. in whiptail lizards all the parthenogenetic offspring are females, which all produce eggs, yet only half of the bisexual population are egg-laying females.
- It eliminates in each generation all lethal genes that thrive in homozygous state
27
What are the advantages of sexual reproduction?
- Brings about genetic variability in a population by recombining parental characteristics, enabling a species to adapt to the changing environmental conditions
- During the life cycle of the organism, resistant stages develop enabling survival of adverse conditions
- The formation of spores, seeds or larvae, which may be used to disperse offspring and so, reduce intraspecific competition.
- It leads to increased numbers of a population
28
What are the disadvantages of sexual reproduction?
- May result into lethal combination of genes
- In some animals, it is difficult to bringing together males and females at the right moment for successful fertilization.
- It relies so much on external agents hence reducing chances of occurrence
- It is a slower method of reproduction.
- There is wastage in production of males, many of which fail to reproduce & thus consume resources that could be applied in the production of females
- Females only pass half of the genes to the offspring because the genome is halved at meiosis
29
Outline the ways through which variation can arise in a population.
i) During fertilization when male and female gametes with different genotypes fuse to form a zygote.
ii) Mutations during which there is alteration in gene structure and sequence.
iii) During meiosis by crossover during prophase I and also during random segregation of chromosomes on the metaphase plate
iv) By the effects of the environment where the organisms live
30
What are the main characteristics or features of sexual reproduction?
- It involves production of gametes by two separate parents, a process called gametogenesis. Isogametes are identical though dissimilar genetically, Anisogametes (heterogametes) differ slightly in size. Some species exhibit oogamy, the gametes greatly differ in size and activity
- It involves fertilisation (syngamy), the union of male and female gametes
- Bacteria reproduce by conjugation.
31
Define the term life cycle.
- It is the progressive sequence of changes an organism goes through from fertilization till death.
32
What is meant by alternation of generations?
- It is the occurrence within the life cycle of an organism two or more distinct forms (generations), a haploid gametophyte and diploid sporophyte generations, which differ from each other in appearance, method of reproduction and genetic constitution.
33
Describe the alternation of generations in a moss. e.g. Funaria – a bryophyte
- A moss e.g. Funaria consists of two distinct forms in its life cycle, the haploid gametophyte, which is the dominant and sexual stage, and the diploid sporophyte, which is the asexual and less conspicuous stage.
- A gametophyte may bear both sex organs, the antheridia (sperm producing) and archegonia (egg producing) or they may be borne on separate gametophyte plants.
- The antheridia produce sperms by mitosis
- The archegonial produce eggs by mitosis
- On maturing, the antheridia shed sperms, that are aided by rain-splash to reach the open neck of archegonia
- They are attracted by chemicals e.g. sucrose enables them to reach the archegonia.
- The haploid sperms fuse with the haploid eggs to form diploid zygotes
- The zygotes develop into diploid sporophytes, which remain attached and surviving on the gametophytes.
- At maturity the sporophyte produces haploid spores by meiosis within a spore capsule, which splits open when dry and the spores are dispersed by wind.
- On landing on moist soil; each spore germinates into a green filamentous protonema which produces buds that grow into new haploid gametophyte.
34
Describe the alternation of generations in a common fern- a Pteridophyte / Filicinophyte:
- A common fern (Dryopteris) consists of two distinct forms in its life cycle, the diploid sporophyte, which is the dominant and asexual stage, and the haploid gametophyte, which is the sexual and less conspicuous stage.
- Diploid spore mother cells inside sporangia divide by meiosis to produce haploid spores
- When mature, the protective covering shrinks and the exposed sporangium wall begins to dry out.
- The walls rupture and the spores are discharged from the sporangium.
- If moisture is present, each spore germinates into the gametophyte, anchored to the soil by rhizoids.
- At the underside, the gametophyte/ prothallus bears antheridia and archegonia that produce haploid sperms and eggs by mitosis respectively.
- After rupture of antheridia, the ciliated sperms swim through water to fertilise eggs at the base of archegonia, and the diploid zygotes formed grow into young sporophytes, which remain supported on the prothallus till self-supporting.
35
What are the similarities between alternation of generations in a moss and a common fern?
- There is one dominant stage and the other stage is relatively inconspicuous.
- A moist or aquatic environment is required.
- Male gametes from the antheridia are brought into contact with eggs by some mechanisms.
- The gametophytes bear sperm producing antheridia and egg producing archegonia.
- Cases the spores are formed in specialized spore-bearing sporangia.
- There is a dispersal mechanism for scattering spores.
- Sporophytes are diploid and gametophytes are haploid.
- Spores are produced by meiosis and gametes by mitosis
- There is sexual and asexual reproduction
- Male gametes are motile while eggs are non-motile
36
State the differences between alternation of generations in a moss and a common fern.
Alternation of generations in a moss:
- Sporophyte is dependent upon the gametophyte nutritionally
- Each spore germinates first into protonema, which transforms into gametophyte
- Gametophyte is the dominant generation
- Both male and female reproductive organs may be borne on same or separate gametophyte plants
- Majorly depends on water for plant growth, transference of sperms, support and spore dispersal and spore germination
Alternation of generations in a Common fern:
- Sporophyte is a self supporting plant
- Each spore germinates directly into gametophyte
- Sporophyte is the dominant generation
- Both male and female reproductive organs are borne on the same gametophyte (prothallus)
- Dependence on water is less, mainly for swimming of sperms and germination of spores
37
What is the significance of alternation of generations to the life cycle of plants?
What is the significance of alternation of generations to the life cycle of plants?
- It enable exploitation of different habitats in the ecosystem by the different generations
- Promotes rapid multiplication of species since spores are enormously produced
- Enables plants to cope better with adverse environmental conditions for survival
- It reduces chances of extinction of a species since the different generations are interdependent
- Brings abut genetic variability by meiosis during spore formation
- Mitosis during gamete formation maintains the plant genome by producing haploid gametes
38
Define essential organs in plants.
These are those which directly take part in reproduction i.e. androecium and gynoecium
39
Define accessory or non-essential organs in plants.
These are those which assist but do not directly take part in reproduction i.e. corolla and calyx.
40
What are monoecious species?
These are species in which individual plants bear separate male and female flowers e.g. maize, oak, sycamore, coconut, date palms, castor oil, pumpkin
41
What are dioecious species?
These are species in which individual plants bear either only male or only female flowers, so that there are different sexes of the plant e.g., pawpaw and asparagus.
42
What are polygamous species?
These are species in which individual plants bear three types of flowers: bisexual, male and female e.g. mango
43
What is dichogamy?
The failure of the stamens and pistil to ripen at the same time in a hermaphrodite flower.
44
Define protandry.
It is the ripening of the stamens before the carpel in a hermaphrodite flower e.g. in sunflower, white deadnettle, salvia, and dandelion (mainly insect pollinated flowers).
45
What is protogyny?
This is ripening of carpels earlier than the stamens e.g. in arum lilies, and wild varieties of: wheat, barley, and oats. (Mainly wind pollinated flowers).
NB: protandrous flowers are more common than protogynous ones.
46
What is heterostyly?
It is a condition in which the style length is different in various flowers of an individual plant, so that pollen from a different flower only can bring about effective pollination. e.g. primrose, has two types of flowers that differ in length of style, with the pin-eyed flower having stamens situated below the stigma while in the thrum-eyed flower they are situated above.
47
Define Self-sterility/genetic self-incompatibility.
This is a condition, which results when the pollen and stigma recognize each other as being genetically related and pollen tube growth is either blocked or retarded. e.g. in clover and pears.
48
State the physiological adaptations of flowers to self-pollination.
- Stamens and carpels mature at the same time
- Hermaphrodite flowers may not open petals
- Hermaphrodite flowers may remain under ground
- Pollen is released onto the stigma by matured anthers before the petal opens.
- Pollen is compatible with the tissues of style, thus allowing for their germination
49
State the structural adaptations of flowers to self-pollination.
- Flowers are reduced and inconspicuous
- Stamens are situated above the stigma or the anther is close to the stigma
- stigma often coiled to touch ripe anthers
- Style and filaments coil on one another
- Flowers failing to open e.g. in commelina
50
State the physiological adaptations of flowers to cross-pollination.
- Stamens ripen before carpel (protandry e.g. in salvia, deadnettle and dandelion) or carpel ripens earlier (protogyny)
- Self incompatibility due to chemicals prevents germinating of pollen on the stigma of the same flower (self-sterility)
- It is genetically controlled by self-incompatibility genes e.g. in pears pollen only becomes functional if the stigma surface on which it is has a different genetic composition.
51
State the morphological adaptations of flowers to cross-pollination.
- Dioecious flowers have either stamens or pistil
- Stamens situated below the stigma .
- Hermaphrodite (bisexual) flower containing both sex organs.
- Production of nectar to attract insects.
- Hanging of stamens or whole flower downwards so that falling pollen drops clear of that plant.
52
Explain the mechanisms which limit inbreeding/promote outcrossing in plants.
- Dioeciousness, all flowers on the plant being either male or female e.g. pawpaw
- Monoeciousness, having separate male and female flowers on the same hermaphrodite plant.
- Heterostyly (differing style length), structure of the flower e.g. when a stigma is protected from coming into contact with its own pollen or stigma being taller than the anthers.
- Dichogamy, Stamens ripening before carpel (protandry) or carpel ripening earlier (protogyny).
- Self incompatibility due to chemicals prevents germinating of pollen on the stigma of the same flower
53
What are the consequences of self-pollination?
It promotes homozygosity i.e. transmission of genotype in the population, resulting into decreased fertility, reduced resistance to disease, thereby reducing evolutionary potential of the species in the long run.
54
What are the consequences of cross-pollination?
It promotes genetic diversity, resulting into hybrid vigour, causing increased resistance to diseases, high yield, and earlier maturity, thereby providing greater evolutionary potential.
55
Define Pollination.
This is the transfer of pollen grains from the anthers to the stigma of the flower.
56
What is Self-pollination?
Form of pollination in which pollen from the anthers is transferred to the stigma of the flower on the same plant.
57
What is Cross-pollination?
Form of pollination in which pollen from the anthers is transferred to the stigma of another flower on a different plant but of the same species.
58
What are the advantages of self-pollination?
- It is reliable especially where members of a species are rare and far apart
- It is the only means where pollinating agents are unreliable e.g. mountain tops lack insects, and during harsh climate.
- It is not wasteful of pollen grains
59
What are the advantages of cross-pollination?
It encourages genetic variation, increasing the hybrid vigor
60
What are the disadvantages of self-pollination?
It promotes homozygosity i.e. transmission of genotype in the population, resulting into decreased fertility and reduced resistance to disease
61
What are the disadvantages of cross-pollination?
It is wasteful of pollen grains since pollinating agents are not very efficient
62
Give an ecological importance of Monoeciousness in plants.
-It allows self-fertilization for continuity of species where plants of that species are few or separated by long distance
63
Give an ecological importance of Dioeciousness in plants.
- It encourages genetic variation, thereby increasing the hybrid vigour since it allows cross- pollination
64
Why is it that in dioecious plants, male plants are usually associated with dry soils while female plants are associated with moist soils?
-Pollen production does not require much water, while seed formation and fruit ripening require much water.
65
Why are nearly all dioecious plants wind-pollinated?
It may not be possible to have animal pollinators bring in pollen from the same plant species, which would reduce chances of fertilization.
OR: It is therefore much easier for wind to bring in pollen from same plant species to increase chances of fertilization
66
Suggest two reasons why dioecious plants are rarer than monoecious plants despite the advantages of cross-pollination.
- Half of the individuals in dioecious plants do not produce seeds, whereas all individuals in monoecious plants bear seeds.
- A lot of pollen is wasted in dioecious plants because the male and female plants are not necessarily together.
67
Why is dioecism common in animals than in plants?
-There is less wastage of gametes in animals than in plants because males and females can move about.
68
Give differences between wind pollinated and insect pollinated flowers.
Wind pollinated flowers e.g. guinea grass, maize, rice:
- Inconspicuous flowers, small and reduced in size.
- Petals may be absent or dull coloured if present.
- No nectar and not scented
- Pollen produced in large quantity, light, smooth, small and may have wing-like extensions.
- Anthers are large and versatilily attached to filament.
- Stamens hang outside the flower to release pollen.
- stigmas are large, feathery or branched to trap pollen.
- Stigmas hang out side the flower.
- Flowers often unisexual, having male and female reproductive parts on separate flowers
Insect pollinated flowers e.g. crotalaria, sunflower, coffee, etc:
- Conspicuous flowers, large with brightly coloured petals, and often held in inflorescence if they are tiny and inconspicuous.
- Produce nectar and or scent to attract insects.
- Less in quantity, sticky, relatively heavy, and large pollen is produced.
- Anthers are small and basifixedly or dorsifixedly attached to filament.
- Stamens are confined within the flower.
- Stigma relatively small, glandular, and sticky.
- Stigma lies inside the corolla.
- Flowers bisexual, enclosing reproductive organs e.g. stamens and carpels.
69
Describe the significant events that lead to the production and release of male gametes in flowering plants.
- Repeated mitotic divisions occur within the sporogenous tissue, producing a mass of swollen cells, the microspore mother cells.
- Mitosis stops and each micro spore mother cell divides by meiosis to form a tetrad of haploid microspores, each of which gives rise to a pollen grain.
- Each pollen grain develops a hard resistant, sculptured outer wall, the exine and an inner cellulose wall, the intine.
- The single haploid pollen grain nucleus divides by mitosis form a pollen tube nucleus and the generative nucleus.
- When the pollen grains mature, the walls between each pair of pollen sacs breakdown and the anther lobe rupture longitudinally along the line of weakness to release pollen grains.
70
Describe the significant events that lead to the production and release of female gametes in flowering plants.
- Each ovary develops one or more ovules, which contain a mass of cells, the nucellus, which is completely surrounded by two integuments, except at a small pore called micropyle.
- One cell of the nucellus develops into a diploid embryo sac mother cell.
- The embryo sac mother cell divides meiotically to give four haploid megaspore cells, only one of which develops to form the embryo sac while the three degenerate.
- The embryo sac nucleus divides mitotically to form two nuclei, which migrate to opposite poles, from where each divides mitotically to form four haploid nuclei at each pole.
- One nucleus from either end of the sac move to the centre and fuse to form a diploid primary endosperm nucleus.
- The remaining six nuclei each gets enclosed in a thin cell wall, and one of the three nuclei near the micropyle becomes the female gamete (egg nucleus).
- The remaining two nuclei near the micropyle, the synergids disintegrate while the three nuclei at the opposite end to the micropyle become the antipodal cells.
- Thus, a mature embryo sac contains a total of 7 nuclei, comprised of: two synergids, three antipodal cells, and diploid primary endosperm nucleus.
71
State the main features of embryo sac development in flowering plants.
- Mitosis occurs three times
- There is formation of diploid nucleus by fusion of two male nuclei
- Produces six haploid cells
- Forms antipodal, synergid and egg cells.
72
In terms of development, what is the embryo sac the equivalent of in males?
The pollen grain of the male
73
Describe the events which occur in the flower from pollination to fertilization.
- When a ripe pollen grain lands on the right stigma, it absorbs water, the outer wall (exine) breaks open and the inner wall (intine) germinates to form a pollen tube.
- The tube nucleus is positioned at the tip of the growing pollen tube controlling growth while the 2 male nuclei (2 sperms) formed by mitosis from the generative nucleus follow closely behind.
- The pollen tube tip pierces the stigmatic surface and penetrates the style tissue via intercellular spaces.
- Chemical substances in the style and embryo sac enable the unidirectional rapid growth of the pollen tube towards the ovary.
- After reaching the ovary, the pollen tube grows towards the ovule and usually enters through the micropyle, chalaza, or integuments.
- The pollen tube then penetrates the embryo sac, bursts open and the tube nucleus degenerates.
- The first fertilization occurs when one sperm fuses with the egg to form a diploid zygote, while the second fertilization occurs when another sperm fuses with the diploid secondary nucleus to form a triploid primary endosperm nucleus.
- This double fertilization is unique to only flowering plants.
74
What are the results of fertilization in a flower?
Before fertilisation:
- Zygote
- Ovule
- Ovary
Integuments:
(a) outer
(b) inner Ovary wall
- Triploid primary endosperm nucleus
- Micropyle
- Nucellus
- Calyx
- Petals, stigma, style & stamens
After fertilisation the parts become:
- Embryo
- Seed; protects embryo
- Fruit; protects seeds and enables their dispersal
(a) Testa/seed coat
(b) Tegmen; both offer protection to the seeds
- Fruit wall/pericarp; offers protection to the fruit contents
- Endosperm; Acts as food store in some seeds e.g. cereals like maize, wheat
- Micropyle; For entry of water and oxygen at germination
- Disappears
- Wither away or may persist
- They dry and fall off
75
Explain what is meant by double fertilization in relation to flowers.
Double fertilization: is a unique process occurring only in flowering plants in which one male nucleus fuses with the functional egg nucleus to form a diploid zygote while another male nucleus fuses with the diploid secondary nucleus to form a triploid primary endosperm nucleus simultaneously.
76
What are the advantages of propagation by seed?
- Enable plants to be better adapted for land environment since water is less required for sexual reproduction
- Embryo is protected within the seed
- There is food reserve for embryo growth in the cotyledon or endosperm
- Seeds are easy to store
- Seeds are easy to disperse and transport
- Allows for mixing of genes which increases the hybrid vigor
- There is increased resistance to diseases
- Plants mature early
- There is tolerance to unfavourable conditions
77
What are the disadvantages of propagation by seed?
- Seeds are easily destroyed by pests
- Seeds have limited food reserves
- Requires selection of suitable seeds
- Needs technical knowing
- Initial inputs are expensive
- Dispersal may not be easy because of the large size of seeds usually
78
Explain how the science of palynology (pollen analysis) can provide information about past climate and human activities of a particular period of time.
- The highly sculptured pattern of the exine (outer wall) of pollen grain is specific to plant species or genus
- Since the exine has a waterproof lining resistant to decay, pollen grains last for a longtime in the sediments.
- The pollen from successive layers of sediments can be extracted and analysed by radiocarbon dating to establish the climatic history e.g. past temperature.
- Human interference with the natural vegetation is also reflected in the pollen record e.g. absence of pollen from trees in some areas would indicate forest clearance.
79
Give differences between sexual and asexual reproduction
Sexual reproduction:
- Involves fertilization to form diploid zygote
- Involves fusion of gametes
- Offspring show genetic variability
- Population numbers increase slowly -May involve one or two parents
- It is a less rapid process
- Offspring mature slower
- Occurs among all living organisms
- Male and female gametes produced by gametogenesis
Asexual reproduction:
- No zygote formation
- No fusion of gametes
- Offspring are usually identical to each other and to their parents; i.e. a clone
- Population numbers increase rapidly
- Always involves only one parent
- it occurs rapidly in favourable conditions
- Offspring mature faster
- Occurs mainly among plants and simpler animals
- No gamete formation
