Reproduction In Plants Flashcards
What is meant by the term reproduction?
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
What are the types of reproduction?
Sexual and Asexual
Distinguish between sexual and asexual reproduction
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.
What is cloning?
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.
Give similarities between sexual and asexual reproduction.
- In both mitosis is involved
- Both produce offspring
- In both there is transmission of genetic material from parents to their offspring
State the various forms of asexual reproduction.
- Fission
- Sporulation
- Fragmentation
- Budding
- Vegetative propagation
What is a spore?
A haploid reproductive cell, usually unicellular, capable of developing into an adult without fusion with another cell.
Give an account of the five major ways by which asexual reproduction occurs.
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.
What are perennating organs?
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.
Explain the role played by perennating organs in plants.
⁃ 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.
Distinguish between organs of vegetative propagation and organs of perennation in plants.
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.
Give examples of organs of vegetative propagation.
- 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
Give examples of organs of perennation.
- rhizomes
- corms
- stem
- root tubers
- bulbs
- swollen tap roots.
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.
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.
Give methods of artificial propagation in plants.
- 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.
What are the advantages of natural vegetative propagation?
- 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
What are the disadvantages of natural vegetative propagation?
- 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
Outline the process of sporulation in amoeba.
- 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.
What is parthenogenesis?
The development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilisation
What is the meaning of diploid parthenogenesis and give an example?
- 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.
What is the meaning of haploid parthenogenesis and give an example?
- 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.
Describe sex determination in honey bees.
- 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).
What is the major disadvantage of parthenogenesis?
- During sudden environmental changes, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions
Define apomixis.
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.
What is parthenocarpy?
This is fruit development without fertilisation, usually induced by auxins e.g. in apples
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
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
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
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
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.
Define the term life cycle.
- It is the progressive sequence of changes an organism goes through from fertilization till death.
