Sexual Reproduction in Plants

Basic Plant Structure Plants have three vegetative organs: roots, stems, and leaves.

Asexual Reproduction All plant growth occurs by cell division (mitosis) and cell elongation. Cell division occurs primarily in cells known as meristems. Meristems are regions of embryonic tissue capable of growing into new plant parts. Meristems are found in both roots and shoots.

Primary Growth of Stems Primary meristems make the shoot or root grow longer. This kind of growth is called primary growth. Secondary meristems make the stem or root grow larger in diameter. This kind of growth is called secondary growth. Not all kinds of plants are capable of secondary growth. Secondary growth gives rise to wood, and plants that are not capable of secondary growth do not develop wood.

Plant Reproduction Asexual Reproduction Sexual Reproduction Because plant growth is indeterminate, each meristem can potentially develop into a complete plant. This means that it is very easy to clone plants, and many plants can grow from cuttings or broken plant parts. This is asexual reproduction (also called vegetative reproduction). Sexual Reproduction Flowers are special reproductive structures found in the Flowering Plants (=Angiosperms) A flower is a specialized shoot, adapted for sexual reproduction. A fruit develops from a flower following fertilization.

Major Plant Groups We are going to examine several groups that show these trends: 1. bryophytes: non-vascular plants including liverworts and mosses. 2. seedless vascular plants such as ferns and horsetails 3. gymnosperms, which have seeds and a vascular system, such as the conifers 4. angiosperms, the flowering plants that dominate the world today.

Bryophytes The bryophytes include the mosses, liverworts, and hornworts Bryophytes have no internal vascular system. Bryophytes spend most of their lives as haploids: the body of the moss plant is haploid.

Bryophyte Life Cycle The haploid gametophyte plant bodies are either male or female. Each produces a different kind of gamete (eggs or sperm) . The sperm are motile: they swim through drops of water (rain or dew) to reach the eggs. The eggs are encased within the female gametophyte’s body.

Bryophyte Life Cycle After fertilization, the diploid sporophyte grows as a stalk out of the female gametophyte’s body. After the diploid sporophyte matures, the cells in it undergo meiosis, forming haploid spores. The haploid spores disperse in the wind, and go on to form new gametophyte plants.

Seedless Vascular Plants The seedless vascular plants include ferns and horsetails. A vascular system to distribute nutrients throughout the plant allows them to grow tall. Some ferns grow up to 80 feet tall, and some extinct horsetails were also tree-sized.

Fern Life Cycle Specialized structures on the underside of the leaves develop, and inside them meiosis occurs. The haploid meiotic products are released as spores, which are dispersed to new locations and germinate into gametophytes.

Fern Life Cycle The haploid gametophytes are quite small, a few millimeters in diameter. They contain structures that produce sperm and eggs. The sperm swim to the eggs and fertilize them The fertilized eggs are diploid, and they grow into the new fern.

Seeds and Pollen A major development in plant evolution was the development of pollen grains and seeds. Pollen grains are the male gametophyte packaged in a hard coat that allows it to reach the female without having to swim through water. This is a large advantage on dry land. Seeds are diploid sporophyte embryos, packaged to survive a period of dormancy and bad environmental conditions. Seeds develop from the fertilized egg.

Pollination Pollination is the transfer of pollen from a stamen to a pistil.  Pollination starts the production of seeds.  To be pollinated, pollen must be moved from a stamen to the stigma.  When pollen from a plant's stamen is transferred to that same plant's stigma, it is called self-pollination.

Gymnosperms Gymnosperm means “naked seed”: their seeds develop on the outside of the plant, instead of inside an ovary as in the flowering plants. The most important gymnosperms today are the conifers: pines, redwoods, cedars, etc. All are woody plants with needles or scales as leaves.

Gymnosperms In conifers (pine trees) the tree produces both male and female cones. The male cones give off sperm in the form of pollen. The pollen grain travels on the wind and can land on a female cone.

Gymnosperms The female cone has water under the scales. The sperm gets stuck in the water and when the water dries up the sperm is carried directly to the ovaries of the female cone.

Angiosperms Angiosperms are flowering plants. Most of the plants we see are angiosperms. Unlike the other plant groups, angiosperms are often fertilized with the aid of animals. Some angiosperms have wind-dispersed pollen. Flowers on these plants are usually small and inconspicuous. Other angiosperms are self pollinators. Their own sperm can be used for fertilization.

Parts of a Flower

Fertilizing a flower For a seed to form the pollen must land on a stigma and then absorb nutrients. As it absorbs nutrients it grows a tube down the stigma until it reaches the ovary. Once it punches through to the ovary two sperm cells (created by mitosis) make their way to the ovary down the tube.

Fertilizing a Flower One sperm will fertilize the mature egg cell while the other sperm will fertilize the polar body. The polar body grows and turns into tissue called endosperm which will be absorbed by the mature zygote as food.