|Wayne’s Word||Index||Noteworthy Plants||Trivia||Lemnaceae||Biology 101||Botany||Search|
Fertilization (syngamy) is the fusion of two haploid gametes (the sperm and the egg) to form a diploid (2n) zygote. This is how the chromosome number in a life cycle changes from haploid (n) to diploid (2n). The biflagellate sperm in the above illustration is characteristic of a moss. Human sperm have a single flagellum.
Mitosis is the division of a haploid (n) or diploid (2n) cell into two duplicate daughter cells. In a strict definition, mitosis (karyokinesis) refers to the division of a nucleus into two duplicate nuclei, each with identical sets of chromosomes. Cytoplasmic division or cytokinesis involves a cleavage furrow in animal cells and a cell plate in plant cells. An example of cell division in haploid cells is the male honey bee (drone bee) which develops from a haploid unfertilized egg.
See Mitosis In Exercise #2
Life Cycles Crossword Puzzle
Meiosis is a special kind of cell division in which the chromosome number is reduced in half. This is how the chromosome number in a life cycle changes from diploid (2n) to haploid (n). In humans, the only cells that undergo meiosis are egg mother cells (oöcytes) in the ovaries and sperm mother cells (spermatocytes) in the testes. Egg formation and sperm formation are referred to as oögenesis and spermatogenesis. In flowering plants, meiosis occurs in megaspore mother cells (megasporocytes) within the ovules of ovaries, and in microspore mother cells (microsporocytes) within the anthers of stamens. In the first division (blue cells in above illustration), the homologous chromosome doublets separate from each other so they are no longer in pairs. In the second and final division the chromatids of each doubled chromosome separate from each other forming the haploid gametes. During the first and second divisions of meiosis nondisjunction can occur, as shown in the following illustration.
Details Of 1st & 2nd Divisions Of Meiosis
Meiosis, Seedless Fruits & Parthenogenesis
In normal spermatogenesis, X-bearing and Y-bearing sperm are produced. If an X-bearing sperm unites with an X-bearing egg, the resulting zygote is female (XX). If a Y-bearing sperm unites with an X-bearing egg, the resulting zygote is male (XY). Sometimes the X and Y chromosomes do not separate properly during the first division (Anaphase I) or the second division (Anaphase II) during spermatogenesis, a phenomenon known as nondisjunction. Nondisjunction may result in sperm that carry an extra X or an extra Y chromosome, such as XX-bearing sperm, XY-bearing sperm and YY-bearing sperm. If these sperm unite with an X-bearing egg, the result could be XXX (triple-X syndrome), XXY (Klinefelter’s syndrome) or XYY (XYY-syndrome). The XXX and XXY chromosome anomalies can also result from an XX-bearing egg.
In the following diagram, normal spermatogenesis is compared with spermatogenesis with nondisjunction at meiosis I (anaphase I) and nondisjunction at meiosis II (anaphase II). If the doubled X and Y chromosomes move to the same cell at meiosis I, the resulting gametes will each contain single X and Y chromosomes. If meiosis I proceeds normally and nondisjunction occurs at meiosis II when the chromatids separate, it is possible to get gametes containing two single X chromosomes and gametes containing two single Y chromosomes:
Is Diploid (2n) & Everything Below The Red Line Is Haploid (n)
Generalized Life Cycle Pattern For Animals & Plants. A plant is defined here as a multicellular organism bearing roots, stems and leaves (Kingdom Plantae). The plant kingdom also includes nonvascular mosses & liverworts without true roots, stems & leaves.
In the human life cycle (and the life cycles of most multicellular animals), the only cells that are haploid are the sperm and egg.
From the zygote to the diploid mother cells inside the sex organs, all the cells are diploid with two sets of chromosomes. In addition, most multicellular animals are dioecious species with separate male and female individuals in the diploid population. Some plant species are also dioecious, including willows, cottonwoods, date palms, figs and marijuana.
Moss Life Cycle. Mosses belong to the Division Bryophyta characterized by nonvascular plants with embryos that develop within multicellular female sex organs called archegonia. The dominant (conspicuous) part of the life cycle is the haploid, leafy gametophyte. The diploid sporophyte consists of a sporangium-bearing stalk that grows directly out of the gametophyte. Spore mother cells within the sporangium undergo meiosis, producing numerous haploid spores that fall to the ground like tiny particles of dust. Since the sporophyte is without chlorophyll, it is completely dependent on the autotrophic (photosynthetic) gametophyte for its water, minerals and carbohydrate nutrition. Consequently, the sporophyte of the moss is heterotrophic and parasitic on the gametophyte. Most moss gametophytes are dioecious, with separate male and female individuals in the population. The gametophytes are produced by “male” and “female” spores. Mosses have a primitive method of fertilization that involves a motile, biflagellate sperm that swims through water to reach the egg on female plants.
Some lichens superficially resemble mosses from a distance, particularly fruticose (branched) lichens growing on the branches and trunks of trees. Lichens are essentially fungi containing symbiotic algal cells. The photosynthetic algae provide carbohydrate nutrition for the fungus, while the fungus provides a protective place for the algal cells to thrive in an otherwise hostile environment. Because the relationship or “marriage” is beneficial to both partners, this particular example of symbiosis is classified as mutualism.
See The Classification Of Plants
Go To The Plant Division Bryophta
See Photos Of Mosses & Liverworts
Go To The Article About Rock Lichens
See Lichens: Nature’s Perfect Marriage
Fern Life Cycle. Ferns belong to the Division Pterophyta characterized by vascular plants with leaves (fronds) arising from subterranean, creeping rhizomes. In tree ferns, the leaves are produced on a definite woody trunk. The dominant (conspicuous) part of the life cycle is the diploid, leaf-bearing sporophyte. On the underside of the leaves are rows of brown sori. Each sorus is composed of a cluster of sporangia, and is often covered by a thin outer layer called the indusium. Some ferns such as Polypodium and Cyrtomium do not have the indusium. Ferns are classified by the arrangement of the sori and shape of the indusium. The sori and indusium superficially resemble an infestation of scale insects, and some people actually spray their ferns! Spore mother cells within the sporangium undergo meiosis, producing numerous haploid spores. The sporangia split open at maturity, releasing millions of spores that fall to the ground like tiny particles of dust. The splitting open of a sporangium is caused by a thick-walled, outer belt of cells called the annulus. As the cells of the annulus dry out, the annulus contracts and rips open the sporangial wall, thus dispersing the spores.
Each spore germinates and grows into a heart-shaped gametohyte (prothallus) which is smaller than your little finger nail. This haploid gametophyte bears male and female sex organs (antheridia and archegonia). With respect to populations of gametophytes, ferns are typically monoecious with both male and female sex organs on the same gametophytes. Unlike the unisexual gametophytes of a moss, a fern gametophyte is bisexual. Like mosses, ferns have a primitive method of fertilization that involves a multiciliate sperm that swims through water to reach the egg. The gametophytes and sporophytes of ferns are photosynthetic and autotrophic.
Photo Of A Haploid Fern Gametophyte
Go To The Plant Division Pterophyta
Fernlike Cycads With Giant Sperm
Photos Of Ferns & Fern Relatives
Photos Of Hawaiian Tree Ferns
A Scale Insect Called A Lerp
Flowering Plant Cycle. Flowering plants (angiosperms) belong to the vascular plant division Anthophyta. Like ferns, the diploid sporophyte consists of a herbaceous or woody plant with roots, stems and leaves. Unlike ferns, flowering plants produce reproductive organs called flowers and seed-bearing fruits. The term angiosperm is derived from angio (vessel) and sperm (seed), referring to the seed-bearing vessels (containers) called fruits. Flowers may be unisexual or bisexual, depending on whether they contain only one type of sex organ (the male stamen or female pistil), or whether they contain both stamens and pistil in the same flower. Species with separate male and female flowers on the same plant are termed monoecious, including oaks (Quercus), alder (Alnus) and walnut (Juglans). Species with separate male and female flowers on separate individuals are termed dioecious, including willows (Salix), cottonwoods (Populus), date palms (Phoenix), some figs (Ficus) and marijuana (Cannabis). The following illustration shows a typical bisexual flower:
In the above illustration of a bisexual flower, the “female” pistil is composed of the stigma, style and ovary. A simple pistil is composed of one carpel, while a compound pistil is composed of several carpels fused together. Carpels are actually modified leaves which can be readily observed when certain fruits dry and split open. For example the fruit or seed capsule of cotton is composed of five carpels, while yucca capsules contain three carpels. Some botanists prefer to use the term gynoecium instead of pistil. This term also applies to flowers with multiple pistils, each composed of separate and distinct carpels. The “male” stamen is composed of a pollen-bearing anther and a filament (stalk). Some flowers, such as species of eucalyptus and cactus, have literally hundreds of stamens. Unisexual flowers are either staminate (with one or more stamens) or pistillate (with one or more pistils). The variation in size, color, number and arrangement of floral parts in blossoms of different plant families is absolutely staggering.
Microspore mother cells in the pollen sacs of the anthers undergo meiosis to produce haploid microspores. Each microspore completes meiosis I and meiosis II resulting in a tetrad of four microspores. These haploid microspores become pollen grains. The haploid nucleus inside each microspore divides into a tube nucleus and generative nucleus before it becomes a mature pollen grain. The pollen grains are released at this binucleate stage and are carried by wind, insects or water to the receptive female part of a flower called the stigma. Some pollen grains are shed at the 3-nucleate stage, after the generative nucleus has divided into two sperm nuclei.
Note: According to Peter Raven (Biology of Plants, 1992), the pollen grain of a lily (Lilium) is composed of two cells, with a spindle-shaped generative cell contained within the cytoplasm of a larger tube cell. According to Raven (1992), mature pollen grains of rosinweed (Silphium) contain two filamentous sperm cells which are suspended in the cytoplasm of the larger tube cell. The sperm have a only a small amount of cytoplasm and no flagella. Dr. Raven has some convincing photo images to show the cellular configuration of pollen grains.
During the process of pollination, pollen is transferred from the anther to the receptive stigma at the top of the pistil. In self pollination, pollen is transferred from the anther to the pistil of the same flower or between flowers on the same plant. In cross pollination, pollen is transferred from the anther of plant A to the stigma of a different plant B. Pollen is transferred by insects, wind and water. Insect-pollinated flowers are typically strongly-scented with showy petals and sweet nectar. Flowers are not always sweet-scented, especially in the case of carrion flowers which smell like rotting flesh and attract fly and beetle pollinators (see stinking flowers link below). Wind-pollinated flowers typically produce prodigious amounts of pollen and are responsible for the hay fever of allergy sufferers. Surf grass (Phyllospadix), the marine angiosperm that grows in the rocky intertidal zone of southern California, is a dioecious species with ribbonlike pollen grains carried by currents and crashing surf. The stamens and pistil of bisexual flowers often mature at different time intervals. This strategy favors cross pollination. Flowers in which the female stigma is receptive before the anthers release pollen are termed protogynous, including wolffia (the world’s smallest flowering plant) and figs. Flowers in which the anther releases pollen before the stigma is receptive are termed protandrous. In both type of flowers, cross pollination typically occurs between different plants in which the stigmas and anthers are receptive and shedding pollen at the same time.
Pollination is followed by another event called fertilization. Upon landing on a suitable stigma, the binucleate pollen grain (containing a tube nucleus and generative nucleus) germinates and develops a pollen tube that grows down through the style and into the ovary where it penetrates an ovule through an opening in the ovule wall called the micropyle. The pollen tube growth is controlled by the tube nucleus which occupies the distal end of the pollen tube. At this time the generative nucleus divides into two sperm nuclei, so that a mature male gametophyte consists of a pollen grain and elongated pollen tube containing three haploid nuclei. [Note: If you consider the pollen grain to be composed of two cells, then the mature male gametophyte consists of three cells: An elongated, uninucleate tube cell or pollen tube containing two minute sperm cells.] Inside the ovule is a female gametophyte or embryo sac containing seven cells and eight nuclei. Each cell has a single haploid nucleus, except the endosperm mother cell which contains two haploid polar nuclei. Flowering plants exhibit the unique phenomenon known as double fertilization involving two sperm from the pollen tube. Upon reaching the embryo sac (inside the ovule), sperm #1 fuses with the egg to form a diploid zygote (n + n =2n). Sperm #2 fuses with the two polar nuclei (within the endosperm mother cell) to form the triploid endosperm (n + n + n = 3n). The zygote develops into an embryo and the endosperm develops into nutritive tissue surrounding the embryo. All of this is happening within the ovule which increases in size and becomes the mature seed. The outer two layers of the ovule (called the integument layers) become the seed coat. The ovary also enlarges and develops into a fruit. Ripened ovaries (called fruits) may be fleshy or dry, depending on the species. Fruits may be dispersed by the wind, seawater or hitchhiking on the fur of animals. The following links shows a summary of he major types of fruits, some record-breaking fruits, and fascinating methods of seed and fruit dispersal. The Mexican jumping bean is included here because it is actually a dry fruit that breaks apart into three sections. Some of these sections (carpels) contain a little round seed, but the sections that roll around by seemingly perpetual motion actually contain the robust larva of the jumping bean moth
Summary of alternation of diploid with haploid phases in botany life cycles.
The general evolutionary trend (left to right) is a gradual increase in the diploid sporophyte phase and a decrease in the gametophyte phase. In the filamentous green alga (Spirogyra) and black bread mold (Rhizopus), the only part of the life cycle that is diploid is the zygote or dormant zygote (zygospore), while the entire algal or fungal body (thallus) is haploid. In the moss, the diploid phase consists of a sporangium and stalk that grows out of the haploid female gametophyte. In the fern and flowering plant, the entire leaf-bearing plant is diploid. The haploid gametophye of a fern is reduced to a small, heart-shaped prothallus. In flowering plants, the haploid gametophyte is greatly reduced and consists of two microscopic structures: A seven-celled, eight-nucleate embryo sac containing the egg and endosperm mother cell, and a pollen grain plus pollen tube containing 3 nuclei, two of which are the sperm which penetrate the embryo sac during fertilization. Once inside the embryo sac, sperm #1 unites with the egg nucleus to form a zygote and sperm #2 unites with the two polar nuclei inside the endosperm mother cell to form the endosperm. The zygote becomes the embryo of a seed and the endosperm develops into the nutritive tissue surrounding the embryo. The entire ovule which contains the embryo sac becomes a seed. The outer wall of the ovule (composed of two layers called the integument) becomes the seed coat. Illustration from Biology 100 Laboratory Manual and Workbook by W. P. Armstrong, Burgess International Group, Inc., 1988.
Many species of plants reproduce asexually without gametes. They simply clone themselves by the formation of bulbs, corms, tubers, rhizomes, runners, turions, plantlets and “pups.” In the duckweed family (Lemnaceae) daughter plants are produced vegetatively in budding pouches. Each “mother plant” produces up to a dozen daughter plants during its lifetime of 1-2 (or more) months. The daughter plants repeat the budding history of their clonal parents, resulting in exponential growth. It has been estimated that the Indian Wolffia microscopica may reproduce asexually by budding every 30 hours under optimal growing conditions. At the end of 4 months this would result in about 1 nonillion plants (1 followed by 30 zeros) occupying a total volume roughly equivalent to the planet earth. Some of these methods are discussed under vegetative terminology at the following links:
Vegetative Terminology Part 1
When placed in fertile potting soil, the leaves of African violet (Saintpaulia ionantha) will readily produce new plantlets along the leaf margins. Although this member of the gesneria family (Gesneraceae) from coastal Tanzania has fuzzy leaves, it is not adapted to drought conditions. It grows well as a house plant or in well-drained, damp areas with partial sun. Other plants that bear plantlets along the leaf margins are “air plant” (Kalanchoe pinnata = Bryophyllum pinnatum) and “mother fern” (Asplenium bulbiferum). Sometimes the term viviparous is used for plants that bear live young.
See The Remarkable Mother Fern
Return To The Biology 100 Home Page
Return To WAYNE’S WORD Home Page
Return To NOTEWORTHY PLANTS Page
Go To Biology GEE WHIZ TRIVIA Page
Go To The