Plant Reproduction and Classification

Reproduction, Growth and Development in Plants

• Flowering plants/angiosperms have at least 300,000 species.
• Angiosperms reproduce sexually and asexually.
• The two types of plant reproduction are asexual and sexual.
• Asexual reproduction includes vegetative growth, parthenogenesis, and tissue-culture propagation.

Asexual Reproduction in Flowering Plants

• Asexual reproduction typically does not involve forming flowers, seeds, and fruits
• A single parent produces offspring which are genetically identical to each other and to the parent.
• Asexual reproduction proceeds by mitosis, without genetic recombination.
• Many methods involve modified vegetative organs, like stems (rhizomes, tubers, bulbs, corms, stolons), roots, and leaves.
• There are three types of asexual reproduction in flowering plants: Vegetative propagation, parthenogenesis and artificial propagation.

Vegetative Propagation

• A new plant develops from tissues or structures that drop from, or are separated from, the parent plant
• Roots, stems, and leaves, give rise to new differentiated tissues, which separate to become new individuals.
• Jade plants and African violets are examples.
• New plants arise at nodes along aboveground horizontal stems in strawberry plants.
• New plants arise at nodes of underground horizontal stems in Bermuda grass.
• New plants arise from axillary buds of short, thick, vertical underground stems in Gladiolus.
• New shoots arise from axillary buds; tubers are the enlarged tips of slender underground rhizomes in potatoes
• New bulbs arise from axillary buds on short underground stems in onions and lilies.

Artificial Propagation

• A new plant develops from cuttings or fragments of shoot systems.
• Tissue culture propagation involves a new plant being induced to arise from a parent plant cell.
• Tiny plant bits are grown in rotating flasks containing a liquid growth medium.
• Orchids, lilies, wheat, rice, corn, and tulips can be grown this way.

Parthenogenesis

• A reproductive strategy that involves the development of a female (rarely a male) gamete (sex cell) without fertilization.
• It naturally occurs in some plants and animals as well as being common in lower plants and invertebrate animals, but rare among higher vertebrates.
• Parthenogenesis in plants involves new individuals developing from an unfertilized haploid egg or adventitiously form tissue surrounding the embryo sac
• Parthenogenesis occurs with apomeiosis, or pseudogamous or autonomous endosperm formation, called apomixis.
• A familiar example is the reproduction of viable seeds with asexual embryos identical to the parent.
• An embryo develops without nuclear or cellular fusion.
• Embryos and seeds are formed without meiosis or fertilization.
• Examples include citrus fruits (orange, grapefruits, tangerines, rose) and in their hybrids.
• Parthenocarpy is the development of fruit without fertilization and parthenogenesis concerns the development of a new organism without fertilization of an ovum.
• Parthenocarpy produces seedless fruits, while pathogenesis, produces female clones that cannot reproduce sexually.
• Parthenogenesis is a type of apomixis, where the egg cells help in the development of an embryo
• Parthenogenesis produces a genetically identical clone of the mother cell.
• In parthenogenesis the embryos develop from an unfertilized egg, so the offspring are genetically identical, hence apomictic.
• In animals, parthenogenesis is reproduction without fertilization, and has the ovum develop into a new individual without fertilization of the sperm
• Parthenogenetic "species" result from the modification of ovum development, with changes in meiosis, that then lead to eggs produced with multiple sets of chromosomes
• Examples include rotifers, aphids, ants, wasps, and bees

Sexual Reproduction in Flowering Plants

• Sexual reproduction is the main reproductive mode of flowering plant life cycles
• Spores and gametes develop inside the specialized reproductive shoots called flowers.
• The fusion of reproductive cells (egg and sperm) involves meiosis.
• Fertilization, or the union of gametes, happens within the flower's ovary.
• Haploid gametes fuse to form a diploid zygote which then develops into an embryo and eventually a new plant.

Kinds of Flowers

• Perfect flowers have both male and female parts.
• Imperfect flowers are either male or female.

Flower Structure

• A typical dicotyledonous flower has four main whorls: calyx, corolla, androecium, and gynoecium.
• Nonfertile parts are the sepals and receptacle.
• Fertile parts are the male stamens and female carpels (ovary).
• The receptacle is the base of a flower for attachment of its parts.
• The calyx comprises individual units called the sepal.
• The corolla has individual units called a petal.
• The perianth signifies undifferentiated sepals and petals where there is no distinct calyx or corolla.
• The Androecium is the male part of the flower consisting of stamens
• The Gynoecium is the female part of the flower consisting of carpels

Gynoecium

• The gynoecium is the female part of a flower and is centrally located.
• Gynoecium consists of one/more carpels which are free or fused, containing 1 or more ovules inside the cavity.
• The ovary is the cavity with the ovules.
• Stigma is the sticky surface at the outer end of the carpel
• The style is the structure that connects the stigma and the ovary.
• The ovule contains the female gamete in the embryo-sac, and the embryosac is the female gametophyte in the life cycle.
• After fertilization the ovule develops to form the seed and the ovary develops to form the fruit.

Floral Formula

• Symmetry: Zygomorphic flowers can only be cut vertically into two equal parts in one particular plane.
• Symmetry: Actinomorphic flowers can be cut vertically into two equal parts in any radial plane.
• Sexual Parts: Having separate male and female parts on the same individual is classified as monoecious
• Sexual Parts: Having the same parts on separate plants is classified as dioecious
• From the Floral Formula (2) section: n = number of free sepals, (n) = number of fused sepals
• From the Floral Formula (2) section: n = number of free petals, (n) = number of fused petals
• From the Floral Formula (2) section: n = number of free stamens, (n) = number of fused stamens, α = numerous free stamens (α)= numerous fused stamens
• G/G with a line above is a superior ovary
• G/G with a line below is an inferior ovary
• -G- is an ovary at the same level of attachment with accessory parts
• Gn is n = number of free ovaries
• G(n) is (n) = number of fused ovaries
• From the Floral Formula (4) section attachments: Androecium attached to the corolla
• Calyx and corolla cannot be differentiated when the perianth (P) is present
• From number of whorls section: Kn + n= Two whorls of sepals
• From number of whorls section: Cn + Two whorls of petals
• Delonix regia, or Flame of the Forest Classification: Kingdom: Plantae Division: Angiospermophyta Class: Dicotyledoneae Order: Caesalpinoidales Family: Caesalpinaceae Genus: Delonix Species: regia Scientific name: Delonix regia.
• Delonix regia features include: Tree, compound pinnate leaves, red flowers (10 stamens, 5 petals, 5 sepals, bisexual, superior ovary), fruit is a legume.

Pollen Formation

• Each anther contains four pollen sacs.
• Cells inside pollen sacs undergo meiosis and cytoplasmic division to create haploid microspores.
• Meiosis I and II together followed by cytoplasmic division, results in four haploid (n) microspores.
• Microspores then undergo mitosis to form pollen grains.
• Mitosis results in a two-celled haploid pollen grain.
• One cell will give rise to a pollen tube, and the other develops into a sperm-producing cell.
• In pollen grain development, the young pollen sac with numerous microsporocytes, transforms into microsporocytes then goes through meiosis to become microspores.
• After mitosis the cell will form a pollen grain, with an immature male gametophyte that will form sperm cells/tube cells.

Eggs Formation

• An ovule forms in the ovary of a flower of the mature sporophyte.
• Cells divide by meiosis (I and II) followed by cytoplasmic division, resulting in four haploid (n) megaspores.
• Mitosis occurs without division of the cytoplasm, in remaining megaspore.
• After two rounds of mitosis without cytoplasmic division, the megaspore ends up with eight haploid nuclei, in seven cells.
• Six have a single nucleus, but one cell has two nuclei.

Embryo Sac

• In embryo sac development: The young ovule (megasporangium) becomes a megasporocyte then becomes a functional megaspore & 3 disintegrating megaspores after undergoing meiosis.
• After 3 mitosis divisions a embryo sac can develop, gametophyte with antipodal cells + polar nuclei in a central cell + synergids + egg.

Plants and Pollinators

• Pollen evolved by 390 MYA.
• Pollen is sperm packed inside a nutritious package that was first transferred by wind currents, and later by insects.
• Plants that attracted insect pollinators with flowers had a reproductive advantage

Pollination and Fertilization

• Pollination is the transfer of pollen grains to a receptive stigma
• This transfer can be by wind, water, or animals
• When a pollen grain lands on the stigma it germinates
• A pollen tube grows through the ovary tissue, carrying two sperm nuclei.
• A pollen tube reaches an ovule, penetrates the embryo sac, and deposits the sperm
• One sperm fertilizes the egg, while the other fuses with both nuclei of endosperm mother cell.
• Endosperm Formation only occurs in angiosperms where the sperm nucleus combines with two nuclei to create a triploid cell.
• This triploid cell becomes the nutritious tissue of the seed known as the endosperm.
• The seed is the mature ovule and the fruit is the mature ovary

Seed Formation

• Fertilization of the egg produces a diploid sporophyte zygote.
• The zygote undergoes mitotic divisions to become an embryo sporophyte, protected by a seed coat along with food reserves.

Structure of a Seed

• The protective seed coat is derived from integuments which enclose the ovule
• The embryo contains 1 or two cotyledons, namely a single one for monocots, and two for dicots
• Nutritious endosperm is food reserve

Nourishing the Embryo

• Dicot embryos absorb nutrients from endosperm and store them in the two cotyledons.
• Monocot digestive enzymes are stockpiled in the single cotyledon.
• Enzymes do not tap into the endosperm until the seed germinates.

Fruit Development

• Simple fruits are derived from an ovary of one flower.
• Aggregate fruit develops from many ovaries in one flower.
• Multiple fruits develop from ovaries of many flowers.
• Accessory fruit is where most tissues are not derived from the ovary.
• Aggregate fruits are formed from the many carpels of a single flower.
• Aggregate fruits make up many simple fruits attached to a fleshy receptacle
• Blackberries and raspberries are aggregate fruits
• Multiple Fruits are formed from individual ovaries that developed from many flowers that grew clumped together such as a pineapple
• Fruits are adapted to seed dispersal via wind, water, or animals

Glossary Terms

• Parthenogenesis: A process in which eggs develop without being fertilized such as the eggs of Daphina bees, wasps, and ants in which females produce offspring from unfertilized eggs.
• Stolon: Creeping horizontal plant stem that roots along its length from which new plants form.
• Apomixis: asexual seed production.
• Ovule: Structure that houses a female gametophyte, and that may become a seed.
• Double fertilization: When two fertilizations occur in the flowering plants life cycle, where one fertilization results in forming a zygote, and the other results in forming the endosperm.
• Gamete: a sex cell, sperm or egg
• Parthenocarpy: the natural or artificially induced production of fruit without fertilization of ovules, which makes the fruit seedless

Test your knowledge of plant reproduction, including fertilization, pollen and egg formation, and distinctions between sexual and asexual reproduction. Also includes questions on plant classification and the characteristics of specific plant families like Orchidaceae.