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PLANT KINGDOM
SEEDLESS PLANTS
I. Origin.
Earth Chronology:
4.6 billion years ago: the earth formed.
4-3.8 billion years ago: life originated.
3.8 billion years ago: prokaryote anaerobes, heterotrophs.
3.5 billion years ago: oldest known fossils: microfossils and stromatolites; photosynthesis.
2.5 billion years ago: photosynthesis established, oxygen accumulated.
1.5 billion years ago: first eukaryotes.
700 million years ago soft-bodies multicellular life.
540 million years ago hard-bodied multicellular life.
The colonization of land by plants probably occurred between 415 and 440 million years ago at the end of
the Silurian.
In a relatively short time of about 50 million years, plant diversified abundantly and colonized many land
areas.
Land plants probably are probably derived from a group of green algae called charophytes.
Land plants share with the green algae the following traits:
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Chlorophyll a and b, xanthophylls (yellow carotenoids) and carotenes (orange carotenoids).
Store carbohydrates in the form of starch.
Cell wall made mostly of cellulose.
Details of the formation of the cell plate.
DNA and RNA sequences support their close relation to the charophytes.
I. CHARACTERISTICS
1. Multicellular eukaryotes that are photosynthetic autotrophs.
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Chloroplasts
Chlorophyll a and b.
Accessory pigments: carotenes and xanthophylls
2. Cell wall made of cellulose, a glucose polymer.
3. Store food in plastids in the form of starch.
4. Alternation of generations.
5. Formation of cell plate during cytokinesis.
Evolutionary trend towards a larger sporophyte generation and a reduced gametophyte generation.
Bryophytes, Seedless Vascular Plants, Gymnosperms, Angiosperms.
II. ADAPTATIONS TO A TERRESTRIAL ENVIRONMENT.
1. Waxy cuticle to protect against desiccation.
2. Stomata for gas exchange and control of transpiration.
3. Multicellular gametangia made of a layer of sterile cells to protect gametes:
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Antheridia produce sperms.
Archegonia produce eggs.
4. After fertilization, the egg develops into a multicellular embryo within the archegonium.
5. Cell wall contains lignin, a polymer, to strengthen and support upright structures.
6. Transport system or vascular tissue:
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Phloem for the transport of dissolved carbohydrates.
Xylem for water and mineral transport.
Land plants can be grouped into four groups (non-taxonomic): bryophytes, seedless plants, gymnosperms
and flowering plants.
III. ALTERNATION OF GENERATIONS.
A life cycle characterized by a multicellular haploid gametophyte stage followed by a multicellular
diploid sporophyte stage.
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Gametophyte (n) produces haploid gametes (n).
Gametes fuse to form a diploid zygote, the new sporophyte (2n).
Embryo of the sporophyte (2n) develops in the archegonium of the gametophyte (n).
Sporophyte (2n) produces spores through meiosis.
Spores (n) are the first stage of the gametophyte generation (n).
BRYOPHYTES
About 15,000 species worldwide divided into three Divisions: mosses, liverworts and hornworts.
Their life cycle is similar but the three groups may not be closely related.
I. CHARACTERISTICS
1. Small plants found in moist environments, lack woody tissue and usually form mats spread over the
ground.
2. Gametophyte generation is dominant; sporophyte is parasitic on the gametophyte.
3. Bryophytes have cuticle, stomata and multicellular gametangia that allow them to survive on land.
4. Bryophytes need water to reproduce and most species lack vascular tissue (xylem and phloem).
5. Water transport is mostly through capillary action, diffusion and cytoplasmic streaming.
II. MOSSES
Gametophyte is a "leafy" plant.
Live in dense colonies forming mats on the ground, rocks, walls, tree trunks, etc.
Lack true roots, stems and leaves.
Rhizoids attach the gametophyte to the substrate.
Some species have water and sugar conducting cells but they lack true xylem and phloem.
The sperm must swim from the antheridium to the archegonium.
III. LIVERWORTS AND HORNWORTS
Liverwort gametophyte can be leafy or thalloid.
Liverworts can reproduce asexually by gemmae, small bundles of cells produced in cup-like structure.
Hornwort gametophytes are all thalloid. The sporophyte is horn-shaped and parasitic on the gametophyte.
Hornworts have a single large chloroplast in their cells.
IV. EVOLUTION.
The evolutionary origin of the bryophytes is obscure.
They do not seem to be in direct line with the vascular plants and might have evolved from a group o
green algae.
Alternatively, they may have evolved from early vascular plants by becoming simpler in their anatomy by
losing their vascular tissue.
FERNS AND FERN ALLIES.
Also known as pteridophytes, they first appeared about 400 million years ago.
I. CHARACTERISTICS
1. Ferns and fern allies have vascular tissue made of xylem and phloem; they posses true roots, stems
(rhizomes) and leaves (megaphylls).
2. The sporophyte is the dominant generation.
3. Gametophyte (prothallus) and sporophyte generations are photosynthetic and independent of one
another.
4. All species need water for reproduction; the sperm must swim from the antheridium to the
archegonium.
5.
Branching is dichotomous.
The Telome Theory attempts to explain the production of megaphylls in vascular plants: evolved from a
branch system.
In contrast, microphylls are considered to have evolved from small projections of the stem. They are
scale-like projection with a single vein.
II. FERNS
About 14,000 extant species. Many species are extinct.
Sporangia are often produced in clusters called sori (sing. sorus).
Ferns are valuable ornamentals. Azolla, an aquatic fern, is an important fertilizer in the cultivation of rice.
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associated to Anabaena, a nitrogen fixing cyanobacteria.
III. FERN ALLIES
Whiskfern sporophytes consist of a rhizome with rhizoids, and upright branches; they lack true leaves
and roots.
Horsetails have true roots, stems and leaves. The stems are hollow. Their epidermal cells are impregnated
with silica.
Horsetails were the dominant plants about 300 million years. They were among the main contributors to
the formation of coal, gas and petroleum (fossil fuels).
The sporophytes of clubmosses consist of true roots, stems and leaves (microphylls).
Some clubmosses (Selaginella) are heterosporous; Lycopodium is homosporous.
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Homospory: production of one kind of spores.
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Heterospory: production of two kinds of spores.
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Haploid megaspores develop into a female gametophyte.
Haploid microspores develop into a male gametophyte.
The sporangia of horsetails and clubmosses are arranged into an elongated conical structure called
strobilus (pl. strobili).
Seedless vascular plants arose about 420 million years ago in the mid-Silurian.
Coal formed from the remains of horsetails, ferns and other seedless plants that lived during the
Carboniferous, about 300 million years ago.
The sporangia of horsetails and clubmosses are arranged into an elongated conical structure called
strobilus (pl. strobili).