Chapter 22: Plant Growth, Reproduction, and Response

Carpel

Female structure of a flower

Endosperm

Food supply for the developing plant embryo.

Production of Female Gametophytes

One female gametophyte can form in each ovule of a flower's ovary. One cell in the ovule divides by meiosis to produce four female spores. In most flowering plants, three of these spores die. The nucleus of the last spore grows, dividing by mitosis three times, resulting in one spore with eight nuclei. Membranes grow between the nuclei to form seven cells. Together, these seven cells make up the female gametophyte, which is sometimes called an embryo sac. One large, central cell has two haploid nuclei, called polar nuclei. One of the other cells develops into an egg

Alternation of Generations

Plant life cycles alternate between a sporophyte phase, which produces spores, and a gametophyte stage, which produces gametes Type of life cycle that alternates between diploid and haploid phases

Double fertilization

The process in which one sperm fertilizes an egg and the other forms a triploid cell.

Seedless Vascular Plants: Ferns

The sporophyte is the dominant phase for all vascular plants, including seedless vascular plants such as ferns. This means that the plants you recognize as ferns are sporophytes. If you look at the underside of a fern leaf, called a frond, you might see sori. Sori are clusters of sporangia, which are spore-producing sacs. sori look like brown dots on the fern frond. Spores are released from the sporangia when they are mature. If a spore lands in a favorable spot for growing, it can develop into a gametophyte. A fern gametophyte is often called a prothallus. prothallus is a plant body about the size of your little fingernail. It anchors itself to the soil with tiny threadlike structures called rhizoids. The prothallus contains special reproductive structures that produce sperm and eggs. When free-standing water is present, male structures release sperm. Sperm then swim toward an egg. When a sperm fertilizes an egg, a zygote forms on the prothallus. Remember that the zygote is the beginning of the sporophyte generation. The zygote grows above the prothallus, which eventually rots away. The mature sporophyte is the familiar fern plant. Newly forming fronds are called fiddleheads, and they slowly uncurl as they grow. Eventually, the sporophyte will produce spores on the underside of each frond, and the cycle will begin again.

Seed Plants: Conifers

The sporophyte is the familiar form for all seed plants. Unlike most seedless plants, seed plants produce two types of spores that develop into male and female gametophytes. Another difference between most seedless plants and seed plants is that the gametophytes of seed plants are microscopic. A pine tree is a typical conifer sporophyte. If you look closely at a branch of a pine tree, you may notice two different types of cones. This is because cone-bearing plants have male and female cones. Female cones are usually larger and more scaly than male cones. They live and grow for several years. Each scale of a female pine cone has two ovules that produce spores. One spore in each ovule can develop into a microscopic female gametophyte, and the rest will die. Male spores are produced inside of male cones, which only live for a few weeks. Male spores develop into pollen grains, which are the very tiny male gametophytes of seed plants. male cones release clouds of pollen in the spring. When a pollen grain lands on a female cone, it sticks. Pollination occurs in a cone-bearing plant when a pollen grain reaches the small opening of an ovule. After pollination, eggs are produced inside the ovule and a pollen tube begins to grow from the pollen grain toward an egg. In pine species, it takes a year for the pollen tube to reach the egg, which is just millimeters away Two sperm also develop inside the pollen grain during this time. Eventually, these sperm travel down the pollen tube toward the egg. The sperm of seed plants do not have flagella, since they do not need to swim through water to reach an egg. One sperm may fertilize an egg, forming a zygote, which will develop into an embryo. Meanwhile, the ovule develops into a protective pine seed. Each scale of a female pine cone can be home to two developing pine seeds. Once the seeds are mature, the scales open up and release them. The life cycle then begins again with a new sporophyte—a pine tree seedling.

Dormancy

When a seed is dormant, the embryo has stopped growing. For some plant species, proper temperature, moisture, oxygen, and light levels are enough to end dormancy allows the next generation of plants to grow under favor- able conditions. Inside the seed coat, an embryo can withstand extremes that would kill a young seedling. Gardeners contend with seed dormancy all the time. When soil is turned over before planting a garden, fresh air and sunlight can cause the buried seeds of unexpected plants to come out of dormancy

Production of Male Gametophytes

anthers produce pollen grains, which are the male gametophytes of seed plants. Cells within the anthers divide by meiosis to produce four male spores. Each spore divides again, by mitosis, producing two haploid cells. These two cells, together with a thick wall that protects them, form a single pollen grain. Wind-pollinated plants have light, fine pollen grains that can be carried far by the Pollen from wind-pollinated plants, such as ragweed, is the source of some outdoor allergies

Moss sporophytes

are stalklike structures that grow up from the gametophyte ooks like a brown stem topped with a tiny cup called a capsule The capsule at the tip of the moss sporophyte contains spore-producing sacs called sporangia. When the spores are mature, the capsule opens and releases them. Spores allow seedless plants to disperse to new areas. If a spore lands in a favorable spot for growing, it can grow into a gametophyte. A moss gametophyte produces gametes in special reproductive structures. Each male structure produces hundreds of sperm with whiplike flagella, and each female structure produces a single egg. When water is present, sperm swim toward an egg. Once a sperm fertilizes an egg, the sporophyte phase begins once again

Nonvascular plants - moss

are the only plants in which the gametophyte phase is dominant. In other words, the green, carpetlike plants that you might recog- nize as moss are gametophytes

Flowering plants can be pollinated

by wind or animals When a pollen grain reaches the stigma of the same plant species, that flower has been pollinated. Pollination is a necessary step of sexual reproduction in flowering plants. Wind-pollinated species usually have small or incon- spicuous flowers and produce large amounts of pollen. A lot of energy is required to produce so much pollen. Many flowering plants are pollinated when insects, birds, or other animals visit flowers to collect pollen or nectar as a food source. In the process of feeding, an animal is dusted with pollen grains, As the animal searches for food in another flower, pollen from the first flower may brush against the stigma of the other flower. Because animal pollinators transfer pollen in this reliable way, pollination by an animal is more efficient than wind pollination. Animal pollinators are important factors in the success and diversity of flowering plants

Animals, wind, and water

can spread seeds.

Gametophyte

gamete-producing plant A spore divides by mitosis and grows into a mature gametophyte or gamete-producing plant. Specialized parts of a mature gametophyte produce gametes—sperm and eggs—through mitosis.

fruit

is the mature ovary of a flowering plant

Stamen

male structure of a flower

Petals

modified leaves (often bright colors). Help attract animal pollinators.

Sepals

modified leaves that protect the developing flower

Sporophyte

spore-producing plant A mature sporophyte has specialized cells that divide by meiosis to produce haploid spores.

zygote

the diploid phase of a plant life cycle begins with a fertilized egg, called a zygote. A zygote divides by mitosis and grows into a mature sporophyte

Germination

the embryo breaks out of the seed coat and begins to grow into a seedling begins when the embryo starts to take up water. Water causes the seed to swell and crack the seed coat. As the embryo grows, the embryonic root, called a radicle, breaks through the cracks. Water also activates enzymes inside the seed. These enzymes help to break down material in the endosperm into sugars, which are moved to the growing embryo. As the embryo continues to grow, a young shoot called the plumule eventually breaks through the surface of the soil When leaves emerge from the shoot, they begin to make food through photosynthesis. Once photosynthesis begins, the young plant is called a seedling

Ovary

where female gametophytes are produced. Found at the base of a flower.

Fertilization takes place

within the flower