Reproduction in plants No individual organism can live forever, but reproduction makes sure that organisms do not become extinct. Organisms reproduce sexually or asexually and some organisms, such as angiosperms can reproduce both ways.
Learning Intentions And Success Criteria 1. The need for reproduction 2. Asexual reproduction: advantages & disadvantages 3. Sexual reproduction: advantages & disadvantages 4. Structure & function of the flower 5. Pollination: wind vs. animal 6. Fertilization and formation of the embryo 7. Seeds: ecological & economic importance
A distinction is made between two types of reproduction
Asexual reproduction In this type of reproduction only one parent is involved and all offspring have the same genetic composition as the parent (clone) No gametes (sex cells) are involved, the process occurs by mitosis
Advantages of asexual reproduction Only 1 parent is needed In favourable conditions, a large number of offspring are produced rapidly and simply = can crowd out the competition Energy expenditure is low, because no gametes are produced The offspring will easily adapt in the same environmental conditions as the parent because the offspring and parent are identical Desirable genetic characteristics are not lost in the offspring Don t have to rely on forms of dispersal e.g. wind
Disadvantages of asexual reproduction Since there is no genetic variation in the offspring, adaptation to changed environmental conditions is difficult All the offspring share the same weak characteristics. If the stable environment changes, the consequences could be fatal and drastically reduce their chances of survival e.g. disease susceptibility Overcrowding
Sexual reproduction Gametes (sex cells) are produced by meiosis Fertilisation takes place during which the male gamete (sperm) and a female gamete (egg cell) fuse to form a zygote, which will develop into a new individual through mitosis
Advantages of sexual reproduction The offspring show greater genetic variation. They are more adaptable and have a greater chance of survival in changing environments It forms the basis of evolution because it can result in the formation of new species Since the genetic material of the parents is reshuffled, the chances of inheriting a disease or weak characteristic from a parent decreases Can cross specific parents to obtain desirable characteristics in offspring
Disadvantages of sexual reproduction The process takes longer than asexual reproduction. Gamete formation takes time Fewer offspring are produced, decreasing the chances of survival Energy expenditure is higher than in asexual reproduction. Need special reproductive organs (flowers) Negative mutations are on recessive genes and may be expressed in the phenotype of the offspring Plants need agents to disperse pollen and seeds Offspring are often born vulnerable to predators and disease. Need parents for protection and food
Flowers as reproduction organs The perianth in dicotyledonous flowers is differentiated into two parts (calyx and corolla) Typical dicot flower consists of four whorls 1. Calyx 2. Corolla 3. Androecium (male) 4. Gynoecium (female)
Calyx The outermost whorl consists of five small, green sepals The sepals surround the other whorls and protect the flower in the bud stage
Corolla (petals) This whorl consist of five striking, brightly-coloured petals that are fused together to form a trumpet-shaped corolla The petals attract insects and birds to the flower for pollination
Androecium (stamens) This whorl composes the male part of the flowers, some flowers e.g Petunia have five stamens that are attached to the inside of the petals Each stamen consist of a long filament ending in a lobed anther. The anther contains pollen sacs (microsporangia) in which the pollen is formed Pollen grains are produced by meiosis from microspore mother cells (2n) in the microsporangia. Two haploid male gametes are found in each microspore.
Gynoecium (pistil) This innermost whorl is the female part of the flower It consist of a single stigma, a thin style and an ovary The surface of the stigma is sticky so that the pollen grains can stick to it. The ovary contains two locules with a large number of ovules Pollen grains are transferred to the stigma. A pollen tube grows down the style to the ovule and the male gametes rea released into the embryo sac. The fertilised ovule develops into a seed.
Pollination is the transfer of ripe pollen from an anther to a receptive stigma Two types of pollination can be distinguished 1. Self pollination: the transfer of ripe pollen from an anther to a receptive stigma of the same flower or other flowers on the same plant. 2. Cross pollination: is the transfer of ripe pollen from the anther of one flower to the receptive stigma of a flower on another plant.
Cross-pollination favourable Greater genetic variation Dioecious plants have only male or only female flowers and are there for dependent on cross pollination Protandry: pollen is ready for release before stigma matures Protogyny: stigma ripens before anther matures and pollen can be released Incompatibility between pollen and the stigma (pollen tube cannot develop)
Adaptation of flowers for pollination It occurs by means of various pollinators Wind Insects Birds Small mammals Water A pollinator is the agent that transfers ripe pollen from an anther to a receptive stigma so that fertilisation can occur.
Wind Insect Bird Petals Not attractive: usually green or small flowers on the tip. Small leaving stamens and stigma exposed Attractive: usually coloured. Large to protect stamens and pistil inside and allow insects to land Scent Unscented Scented, sometimes smelly to attract insects attractive: coloured large And tubular shape, strong Unscented Nectar No nectar produced Nectar produced as a reward for insects Stamens Long flexible, filaments and large mobile anthers exposed to wind. Filaments dangle outside flower and pollen is shaken out Stiff filaments and anthers rub against visiting insects Pollen Large quantities. Small, dry, light. Small quantities, rougher sometimes sticky. Insect brushes against anther as it drinks nectar and pollen sticks to it Stigmas examples Large, feathery, hang outside flower exposed to wind to catch passing pollen. Large surface area Some grasses, restios, trees (oaks, yellow woods) Small, unexposed, sticky with stiff style for pollencovered insects to rub against Cluster Disa, vygies Nectar produced Stiff filaments and anthers rub against visiting insects Small quantities, better chance of reaching stigmas Small, unexposed, sticky for bird to rub against Rat s tail, proteas strelitzia, salvia spp.
Seeds Produced by spermatophytes Each carpel (female organ) contains one or more ovules Seeds are formed from fertilised ovules Contains embryo plant with cotyledons (store food and protect embryo) Tough seed coat for protection (testa)
Significance of seeds Seeds provide a protective covering and a stored supply of nutrients for young plants Gymnosperm seeds are considered more primitive, because they are naked Angiosperm seeds are considered to be more advanced because they 1. Are enclosed by an ovary or fruit 2. Can remain dormant and survive unsuitable conditions 3. Are completely independent of the parent plant 4. Are waterproof preventing desiccation 5. Store food and are therefore ready for first growth. 6. Adapted for dispersal (less competition) 7. Sexual reproduction = genetic variation
Seeds as a food source Seeds contain stored carbohydrates, fats and oils and proteins Samp which contains pieces of mealie seeds and bean seeds are a significant part of stable diet of many South African people. The combination of mealie and bean seeds provide all the essential amino acids needed by the human body to produce proteins Seeds cultivated for food: coffee, cocoa, peas, soya beans, mealies
Seed banks Facilities that store seeds to maintain biodiversity and safeguard food security Seed banks store the seeds of plants that are either under threat or are going to be useful in the future Seeds are dried, packaged and stored in the seedbank vaults of many of the large herbaria and botanical gardens around the world Seed banks are established in case seeds elsewhere are destroyed by natural disasters, outbreaks of disease or war. These seed may include food crops that are becoming rare, or other rare species in order to protect biodiversity. Potatoes (Irish potato blight)