Plant Reproduction and Growth Concepts

Plant Reproduction and Growth

• Angiosperms (flowering plants) can reproduce asexually or sexually.
• Asexual reproduction is advantageous in stable environments, allowing new individuals to be cloned from parts of the parent plant.
• Vegetative reproduction is a type of asexual reproduction where new plants are produced from stems, roots, or leaves.
• Forms of vegetative reproduction include runners (e.g., strawberry plants), rhizomes (e.g., irises, potatoes), suckers (e.g., cherry, apple, raspberry plants), and adventitious plantlets (e.g., Kalanchoë daigremontiana).
• Sexual reproduction in plants involves alternation of generations.
• The diploid sporophyte generation produces a haploid gametophyte generation enclosed within the sporophyte.
• Male gametophytes are pollen grains that develop from microspores.
• The female gametophyte is the embryo sac, which develops from a megaspore.
• Angiosperm reproductive organs (flowers) are produced seasonally.
• Most flowers contain male parts (stamens) and female parts (carpels).
• Stamens have anthers at their tips.
• Carpels consist of an ovary, style, and stigma.
• Flowers with only male or female parts are imperfect.

Pollen Formation

• Pollen sacs within anthers contain microspore mother cells.
• Microspore mother cells undergo meiosis to form four haploid microspores.
• Microspores undergo mitosis to form pollen grains containing a generative cell and a tube cell nucleus.
• The tube cell nucleus forms the pollen tube.
• The generative cell divides to form two sperm cells.

Egg Formation

• The ovary, located at the base of the carpel, contains ovules.
• Each ovule has a diploid megaspore mother cell.
• Meiosis in the megaspore mother cell produces four haploid megaspores.
• Only one megaspore survives and undergoes repeated mitotic divisions to produce eight haploid nuclei.
• These nuclei form the embryo sac.

Pollination

• Pollination is the transfer of pollen from the anther to the stigma.
• If pollen from a flower pollinates the same flower's stigma, self-pollination occurs, which can lead to self-fertilization.
• Many angiosperms use animals (pollinators) to carry pollen grains from flower to flower, often rewarded with food like nectar.
• Flower color and form have evolved to attract specific pollinators.
• Effective pollination by animals requires a particular insect or animal to visit plant individuals of the same species.

Wind Pollination

• In some angiosperms and all gymnosperms, pollen is dispersed by wind and passively reaches the stigma.
• Plants dispersed by wind have small, green, and odorless flowers.
• Individuals of a given species need to grow where there's ample wind and relatively close to each other.

Fertilization

• Pollen grains adhere to the stigma, grow a pollen tube, and penetrate the style, reaching the ovule.
• The pollen tube releases two sperm cells.
• One sperm fertilizes the egg, and the other sperm fuses with polar nuclei to form endosperm (nutritive tissue).
• This process of using two sperm cells in fertilization is called double fertilization.

Seeds

• After fertilization, active cell division forms an embryo.
• The embryo becomes dormant after drying.
• The outer covering of the ovule develops into a seed coat, an impermeable layer that encloses the dormant embryo and food supply.
• Germination, resumption of metabolic activities, occurs when water and oxygen reach the embryo.
• Germination ensures that seeds germinate under favorable conditions.

Fruits

• During seed formation, the flower ovary develops into fruit.
• Fruits form in various ways and have diverse forms.
• Fleshy fruits are usually dispersed by birds and animals after excreting.
• Other fruits are dispersed by wind or by attaching themselves to the fur of mammals or the feathers of birds or water.

Germination

• A seed absorbs water when appropriate conditions are met.
• Aerobic respiration begins once the seed coat ruptures.
• The roots emerge first, followed by the stem and cotyledons (seed leaves) in eudicots, or the coleoptile in monocots.

Plant Hormones

• Plant development depends on the activities of meristematic tissues interacting with the environment via hormones.
• Differentiation in plants is largely reversible.
• Cells can express their hidden genetic information when provided with appropriate environmental cues.
• Plant hormones influence the expression of some plant genes.

Auxins

• Auxin is a plant hormone involved in phototropism.
• Phototropism is the growth of plants towards light.
• Experiments by Darwin and his son suggested a substance, later identified as auxin, was responsible for plant bending.
• Auxin causes the tissues on the shaded side of a seedling to grow more than those on the opposite side, bending the plant toward light.
• Synthetic auxins are used to control weeds.
• Synthetic auxins work by causing plants to grow to death, reducing ATP production.

Photoperiodism and Dormancy

• Photoperiodism is a mechanism that allows organisms to measure seasonal changes in day and night length.
• Plant flowering responses are categorized by their relation to day length: long-day, short-day. and day-neutral plants.
• Dormancy is a period when plant growth stops.
• Dormancy is often associated with winter in temperate zones due to low temperatures and water freezing.

Tropisms

• Tropisms are directional and irreversible growth responses to external stimuli, like phototropism, gravitropism and thigmotropism.