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Biology

Chapter 17: 1-14 - Plants

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Lecture

Plants, Classification and the Alteration of Generations

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Plant Classification

The plant kingdom includes all photosynthetic multicellular organisms, and a few non-photosynthetic organisms which have plant-like structures. All plants have adaptations which help them survive on land, away from a constant water source. Plants have lignen, a polymer which stiffens the cell walls and allows the plant to grow tall in its quest for sunlight. Most plants have a cuticle or waxy covering which limits water loss. Plant leaves have holes called stomata that the plant can open or shut to control gas exchange for photosynthesis or water loss. Plant embryos develop inside an encased archegoinium (algae are not so well protected).

All plants alternate between a haploid-cell gametophyte generation, which produces egg and sperm gametes or haploid cells for sexual reproduction, and the diploid sporophyte generation which produces haploid spores.

Plants are grouped into four major types.

Mosses

Bryophytes (mosses, worts) are simple seedless plants which do not have vascular structures (inner tubes for transporting water and nutrients or sugars). Their sperm are motile, and must be able to move from the structures which produce them (antheridia) to the structures which produce the egg cells (archegonia). As a result, bryophytes are usuall small, and must grow near water sources to support sperm motility for reproduction.

"Nursery log" supporting moss and lichens.

Ferns

Ferns are seedless plants which have vascular structures, which means they can grow much larger than the mosses. All ferns have a dominant sporophyte generation; unless you look closely you won't find the tiny gametophyte generation forms, which are often less that half an inch in size. The sporophyte produces easily visible spores which grow into a prothallus (the gametophyte); the gametophyte produces antheridia and archgonia which in turn make sperm and eggs. The motile sperm swim to reach the eggs; after fertilization occurs, the sporophyte plant which we are used to associating with ferns begins to grow.

Whisk ferns are the simplest ferns, lacking true roots and leaves but having vascular stems.
Horsetails [shown below] have hollow stems and bear some resemblence to modern trees. They have true roots, stems (both below-ground rhyzomes and above ground ariel stems) and small leaves.

Club mosses have branched stems, roots, and small leaves.

True ferns [shown above] have fronds, roots, rhizomes and stems, all of which contain vascular tissues. Fern fronds can be simply branched or many-branched. Tropical ferns can grow many feet high and resemble palm trees, with a single, strong stem and many fronds at the top.

Gymnosperms

Gymnosperms are plants which produce seeds iinside a vessel or case. All gymnosperms have a dominant sporophyte generation which supports the gametophyte generation, rather than alternating with it. The fir tree that you see is the sporophyte generation; the gametophyte generation is the cone.

Angiosperms

Angiosperms are flowering plants which produce seeds inside a fruit. They are found in almost every habitat, so with about 235,000 known species, they are considered the most successful plant form. Because each seed is both protected and fed by its own fruit covering, angiosperm seeds have a higher rate of survival than conifer seeds. Angiosperms differ greatly in form, and include most of our food crops (rice, wheat, corn, barley), deciduous trees (oak, cherry, walnut), textile sources (cotton, flax), and other foods (fruits, vegetables, chocoloate, coffee) and many other products, such as rubber and oils for perfumes.

The sporphyte generation is dominant in angiosperms. The gametophyte is the flower, which contains four main parts:

The anther produces pollen, which is carried to the stigma of the carpel (on the same flower, on other flowers of the same plant, or on flowers of other plants of the same species). Each pollen grain has two sperm. When the pollen is deposited on the stigma, it produces a tube which reaches down through the style to the ovule and into an egg sac in the ovule. The egg is fertilized by one of the sperm, forming the zygote. The other sperm develops into the seed body, case, and fruit. This "double fertilization" ensures that the developing embryo is protected and has a supply of nutrients for its early development, until it has the root and leaf structures it needs to produce its own food.