Why would the tulip need to undergo meiosis?

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:

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.

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.

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.