Plants and animal reproduction

• Plant Reproduction


  The propagation of flowering plants by sexual and asexual reproduction
• Flowering plants


  • They are the most important group of plants in most terrestrial ecosystems and in agriculture
• Flower structure


  Flowers are typically composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes. They are called determinate shoots, meaning that they stop growing after the flower and fruit are formed.
• Floral organs


  • Sepals
  • Petals
  • Stamens
  • Carpels (pistils)
• Sepals


  Enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance than the other floral organs
• Petals


  More brightly colored than sepals and attract the flower to insects and other pollinators
• Stamen


  Consists of a stalk called the filament and a terminal structure called the anther; within the anther are chambers called pollen sacs, in which pollen is produced
• Carpel


  Has an ovary at its base and a long, slender neck called the style. At the top of the style is the sticky structure called the stigma that serves as a landing platform for pollen. Within the ovary are one or more ovules, with the number depending on the species.
• Pistil


  Sometimes used to refer to a single carpel or to group of fused carpels
• Types of flowers based on the presence of the whorls

  • Complete - a flower that is built with four parts which include the sepals, petals, pistils, and stamens
  • Incomplete - If any of the sepals, petals, pistils, and stamens, which are integral in forming a flower, is missing
• Types of flowers based on the presence of the reproductive whorls

  • Perfect/Bisexual - a flower in which both male and female reproductive structures are present
  • Imperfect/Unisexual - a flower that does not have both male and female structures
• Examples of imperfect/unisexual flowers

  • Staminate flower (male)
  • Carpellate/pistillate flower (female)
• Examples of plants with imperfect/unisexual flowers

  • Squashes
  • Cucumbers
  • Corn
  • Grasses
• Gametophyte development and pollination

  Anthers and ovules bear sporangia, structures where spores are produced by meiosis and gametophytes develop. Pollen grains, each consisting of a mature male gametophyte surrounded by a spore wall, are formed within pollen sacs (microsporangia) of anthers. An egg-producing female gametophyte, or embryo sac, forms within each ovule. In angiosperms, pollination is the transfer of pollen from an anther to a stigma. If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows and digests its way down into the ovary via the style and discharges sperm in the vicinity of the embryo sac, resulting in fertilization of the egg.
• Double fertilization


  After landing on a receptive stigma, a pollen grain absorbs moisture and germinates; that is, it produces a pollen tube that extends down between the cells of the style toward the ovary.
• From ovule to seed


  After double fertilization, each ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch to varying extents, depending on the species. This is why seeds are such major sugar sinks. Initially, these nutrients are stored in the endosperm, but later in seed development in many species, the storage function of the endosperm is more or less taken over by the swelling cotyledons of the embryo.
• From ovary to fruit


  While the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and, when mature, aids in their dispersal by wind or animals. Fertilization triggers hormonal changes that cause the ovary to begin its transformation into a fruit. If the flower has not been pollinated, fruit usually does not develop, and the entire flower withers and falls away. During fruit development, the ovary wall becomes the pericarp, the thickened wall of the fruit. As the ovary grows, the other parts of the flower wither and are shed.
• Animal reproduction


  Animals reproduce with two principal modes: asexual (without sex) and sexual (fusion of gametes) reproduction. Asexual reproduction is the creation of new individuals whose genes all come from one parent without the fusion of egg and sperm. Sexual reproduction is the creation of offspring by the fusion of haploid gametes to form a zygote (fertilized egg), which is diploid.
• Female gamete


  The unfertilized egg (also called an ovum), is a relatively large cell and not motile.
• Male gamete


  The sperm, is generally a much smaller, motile cell.
• Advantages of sexual reproduction

  It increases genetic variability among offspring by generating unique combinations of genes inherited from two parents. This may enhance the reproductive success of parents when environmental factors (including pathogens) change relatively rapidly.
• Advantages of asexual reproduction

  It enables animals living in isolation to produce offspring without locating mates. It can also create numerous offspring in a short amount of time, which is ideal for colonizing a habitat rapidly.
• Metagenesis


  Also known as 'transformation development', refers to an alternation of asexual and sexual generations.
• Example of metagenesis


  • The hydrozoan Obelia, a polyp generation gives rise by budding to a generation of medusas. The motile medusas produce gametes and reproduce sexually, giving rise to new generation of polyps.
• Parthenogenesis


  Also known as 'virgin development', is a form of reproduction in which an unfertilized egg develops into an adult animal. This occurs for several generations, after which males develop, produce sperm, and mate with females to fertilize their eggs.
• Example of parthenogenesis


  • Honeybees. The queen honeybee is inseminated by a male during the "nuptial flight". The sperm she receives are stored in a little pouch connected with her genital tract but closed off by a muscular valve. As the queen lays eggs, she can either open this valve, permitting the sperm to escape and fertilize the eggs, or keep the valve closed, so that the eggs develop without fertilization. Generally, fertilization occurs in the fall, and the fertilized eggs are quiescent during the winter. The fertilized eggs become females (queens and workers); the unfertilized eggs become males (drones).
• Hermaphroditism


  Means that a single organism produces both eggs and sperm. Although this form of reproduction is still classified as sexual, (since both eggs and sperms are involved), it is an exception to the important generalization that sexual reproduction involves two different individuals.
• Example of hermaphroditism


  • Earthworm. Most hermaphrodites do not reproduce by self-fertilization. Rather, as in earthworms, two animals copulate, and each inseminates the other.
• Mechanisms of reproduction


  • Plant reproduction: Sexual, Asexual

  Animal reproduction: Internal, External

  Asexual reproduction mechanisms: Metagenesis, Parthenogenesis, Hermaphroditism
• Pollination: Transfer of pollen to stigma, pollen tube growth, double fertilization.
• • Gametophyte development: Male gametophytes (pollen grains) form in anthers, female gametophyte (embryo sac) forms in ovules.