Plants

• Plants are autotrophs: they get their energy from sunlight, and they synthesize their food from carbon dioxide.

• Plants are multicellular.• Plants live on land, or are

derived from land-dwelling plants. The algae and seaweeds that live in the sea are multicellular protists.

• Plants alternate between a haploid phase (the gametophyte) and a diploid phase (the sporophyte).

Evolutionary Trends• Plants are thought to have evolved from the green algae.• By moving onto the land, plants had to deal with 2 big issues:

gravity ( or lack of buoyancy) and dryness.• Major trends:

– 1. development of roots, shoots, vascular system. Roots needs to absorb nutrients, not just hold onto the surface. Shoots need to support photosynthetic system off the ground. Vascular system to transport materials between parts of the plant. Waxy cuticle on the leaves to prevent desiccation.

– 2. increasing the diploid phase of the life cycle, and decreasing the haploid phase. Diploid gives a backup copy of each gene, as a defense against random mutations. Allows a larger, more complex body.

– 3. Seed and pollen protection and dispersal. Development of very different male and female gametes, so only 1 type needs to be dispersed in the environment. The pollen (male gametes) needs to be protected from desiccation, and needs to find the female gametes successfully. Seeds also need to be protected from harsh conditions and to disperse to new locations.

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. They are short plants mostly growing in wet environments.

• Bryophytes have a waxy cuticle on their leaves to prevent desiccation.

• Bryophytes have no internal vascular system.

• Bryophytes spend most of their lives as haploids: the body of the moss plant is haploid.

• The only diploid structure is a stalk and spore capsule, which grow out of the haploid plant body.

• Peat moss is used to help soil hold water. It can also be used as fireplace fuel when it is dried. Peat bogs are very acidic, which allows plants like cranberries and blueberries to grow.

• Also, the acidic conditions preserve animal bodies—several humans who lived up to 5000 years ago have been dug out of peat bogs.

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.

• 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.

• Being seedless means that the diploid sporophyte grows out of the fertilized egg, attached to the gametophyte.

• The diploid sporophyte is much larger than the haploid gametophyte stage: most of what you see in these plants is the sporophyte.

• The sperm have flagella and swim to the eggs through drops of water (just like the bryophytes).

Fern Life Cycle• The main plant body in the diploid

sporophyte. 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.

• 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 sporophyte plant body.

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.

Gymnosperms• Gymnosperms were the first

plants to have pollen grains and seeds.

• 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.

• Conifers are our main source of wood and paper.

• Ginkos and cycads are other gymnosperms.

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: insects, birds, bats, that carry the pollen from one plant to another. The plants and their pollinators have co-evolved in a symbiotic relationship.

• Flowers produce the visual signals and the scents that pollinators use to find the plants. Flowers secrete nectar which is eaten by the pollinators. The pollen is carried from flower to flower on the body of the pollinator, as a consequence of its going into the flower in search of nectar.

• Some angiosperms have wind-dispersed pollen. Flowers on these plants are usually small and inconspicuous.

Angiosperm Gametophyte Stage

• The haploid gametophyte stage in angiosperms is very short and small.

• The male gametophyte is the pollen grain. Pollen is produced by the anthers. Each pollen grain contains 3 haploid cells, one to run the metabolism of the pollen grain and the other 2 (sperm cells) to fertilize the egg.

• The female gametophyte is the ovule. It is buried in an ovary, inside the plant body. It has 8 haploid cells. Three of them are used in fertilization.

Angiosperm Fertilization• The plant body is the diploid sporophyte. • Meiosis occurs in separate organs to produce male and female gametophytes. • The male gametophyte, the pollen, lands on the stigma of the flower. The pollen

grain then grows a tube down to the ovule, the female gametophyte.• The 2 sperm nuclei migrate down the pollen tube and enter the ovule.• One sperm fertilizes one of the ovule cells, producing a diploid embryo.• The other sperm fertilizes 2 ovule cells, producing the triploid endosperm.

Endosperm acts as a nutritive tissue for the developing seed, much like yolk does for animal eggs.

Seed Development• After fertilization, the egg and

endosperm grow and develop into a very small plant surrounded by protective layers.

• The seed then stops development and dries out: it becomes dormant. At this point it is released from the parent plant and dispersed to a new location.

• Seeds can remain dormant for a long time: the current record is about 2000 years for some lotus seeds.

• When the dormant seed finds proper conditions, it breaks out of dormancy, takes on water, and germinates.

• The embryonic plant grows its shoot downward, way from the light, and its shoot upward, towards the light.

• When light reaches the shoot, chlorophyll synthesis starts and the plant turns green, starting photosynthesis.

Angiosperm Life Cycle• Most of the angiosperm’s life is the

diploid sporophyte stage.• The male gametophyte is the

pollen grain; the female gametophyte is the ovule.

• Angiosperms have double fertilization: 2 sperm fertilize different cells in the ovule, producing the diploid embryo and the triploid endosperm.

• The embryo develops into a seed, a small immature plant, which goes into a dormant phase.

• The seed germinates, putting our a root and a shoot. The shoot turns green and starts photosynthesis when light hits it.