Plants (kingdom Plantae) are autotrophs; they make their own organic nutrients. The term "organic" refers to compounds that contain carbon. Organic nutrients such as sugars are made by photosynthesis.
Plants are adapted to living on land. For example, the above-ground parts of most plants are covered by a waxy layer called a cuticle to prevent water loss.
Aquatic plants are secondarily adapted to living in water.
Some evidence that suggests that plants evolved from the green algae is:
- they both use chlorophyll a, chlorophyll b, and carotenoid pigments during photosynthesis.
- the primary food reserve of both is starch.
- they both have cellulose cell walls.
Genetic and morphological evidence indicates that plants evolved from a group of green algae called charophyceans. Many charophyceans inhabit shallow freshwater environments. Natural selection may have favored individuals capable of surviving occasional drying in these environments and this gave rise to land plants.
These traits occur in plants but not charophyceans. Some evolved independently in other algae.
- Apical meristems
- Alternation of generations
- Spores with protective walls
- Spores produced in sporangia
- Gametes are produced in multicellular structures called gametangia; Antheridia produce sperm; Archegonia produce eggs
- Multicellular dependent embryos
- Many have a cuticle that waterproofs and offers some protection
The basic alternation of generations life cycle is illustrated below.
The diploid plant that produces spores is called a sporophyte. The haploid plant that produces gametes is called a gametophyte.
Some protists also have an alternation of generations life cycle but the structures that produce gametes in protists are usually single cells. Plants produce gametes in multicullar structures that have an outer protective layer. Sperm are produced in structures called antheridia (sing. antheridium), eggs are produced in archegonia (sing. archegonium),. As in protists and fungi, spores of plants are produced in sporangia (sing. sporangium).
A dependent sporophyte is a sporophyte that is small and grows attached to the gametophyte. It obtains nutrients from the gametophyte. An independent sporophyte grows separately from the gametophyte. Similarly, a dependent gametophyte is small and grows attached to the sporophyte while an independent gametophyte grows separately from the sporophyte.
The evolutionary trend in plants has been from plants with a dominant gametophyte and reduced, dependent sporophyte (ex. Mosses) to plants with a dominant, independent sporophyte and a reduced, dependent gametophyte (ex. Seed plants).
Evolutionary relationships among the plants are shown below.
We will study the following phyla of plants.
Liverworts (Phylum Hepatophyta)
Mosses (Phylum Bryophyta)
Hornworts (Phylum Anthocerophyta
Bryophytes (no vascular tissue)
Club mosses, Spike Mosses, Quillworts (Phylum Lycophyta)
Horsetails, Whisk Ferns, Ferns (Phylum Pterophyta)
|Seedless vascular plants|
Conifers (Phylum Coniferophyta)
Cycads (Phylum Cycadophyta)
Ginkgos (Phylum Ginkgophyta)
Gnetophytes (Phylum Gnetophyta)
Gymnosperms (vascular, naked seeds)
Flowering Plants (Phylum Anthophyta)
Angiosperms (vascular, protected seeds)
Mosses, liverworts, and hornworts lack complex vascular tissue for transporting water and nutrients. Some mosses have simple vascular tissue.
The lack of an extensive system of vascular tissue restricts their size. The largest are less than 20 cm (8 in.).
Rhizoids are root-like structures that absorb water and nutrients. They do not have true roots, stems, or leaves because they lack vascular tissue.
Bryophytes are generally restricted to moist areas because the sperm are flagellated and therefore require at least a film of water to swim to the egg.
The sporophyte of bryophytes is dependent on the gametophyte, that is, it derives its nutrition from the gametophyte. The gametophyte is independent.
Left: Moss growing on a rock
Figure X: The sporophytes are attached to the gametophytes.
Spores germinate to produce a branching horizontal filament called a protonema. The familiar green gametophyte body and rhizoids arise from the protonema.
Left: Mnium archegonial head X 40. Eggs are produced in archegonia.
Mosses also reproduce asexually by fragmentation.
Marchantia is an example.
Some stalks contain antheridia and others contain archegonia. Sporophytes are small and produce windblown spores.
The upper surface of the thallus (plant body) produces cup-shaped structures called gemma cups. Groups of cells (called gemma) within the cups are capable of breaking off and producing a new plant.
The gametophyte of hornworts resembles liverworts. The sporophytes are horn-like projections that rise from the gametophyte.
Bryophytes can colonize rocks and help initiate the soil-formation process.
Sphagnum (peat moss) is a moss that may accumulate due to low rates of decomposition. It is used for fuel in several parts of the world. It's spongy structure enables it to store water, thus making it useful for improving soil quality by adding peat moss to the soil.
Approximately 93% of plant species are vascular plants.
Vascular plants contain vascular tissue.
There are two kinds of vascular tissue:
Xylem conducts water and minerals up from the soil. The cell walls of xylem cells help support the plant.
Phloem conducts organic nutrients from one part of the plant to another.
True roots, stems, and leaves are found only in vascular plants because these structures must contain vascular tissue.
The sporophyte of vascular plants is dominant.
Seedless vascular plants include ferns, whisk ferns, club mosses, and horsetails.
The plants do not produce seeds so, like bryophytes, they are dispersed (spread) by windblown spores.
The gametophyte and sporophyte are independent.
They are vascular plants and therefore have true roots, stems, and leaves.
The sperm are flagellated and require water for reproduction. These plants are therefore limited to moist areas.
Many of the seedless vascular plants were once tree-sized. During the carboniferous period (near the end of the Paleozoic), these plants were so abundant that in some areas, their remains accumulated faster than they decomposed. These accumulations produced our fossil fuels.
The earliest known vascular plants had a pattern of branching that increased the number of sporangia.
Leaves of later plants probably evolved from webbing between the branches.
Phylum Lycophyta- Club Mosses, Spike Mosses, Quillworts
Lycophytes have microphylls- small, narrow leaves with a single vein. Most other vascular plants have megaphylls- large, branched vascular tissue.
Many are epiphytes- they grow on other plants but do not harm them.
The sporophytes contain both upright and horizontal stems that grow along the ground.
Club mosses have true roots, stems, and leaves.
Spores are produced in reproductive structures called strobili (sing. strobilus).
Phylum Pterophyta- Ferns, Whisk Ferns, Horsetails
Left: The sporophyte is the dominant generation.
In most ferns, the stem is a horizontal, underground structure called a rhizome. The leaves grow above-ground (see the photograph above).
A sorus (pl. sori) is a cluster of sporangia. Sori are located on the underside of the leaves.
Left: Sori can be seen on the underside of this fern leaf.
Left: Close-up of a sorus. Sporangia can be seen underneath the cover (indusium).
The gametophyte is small and heart shaped.
|Left: Fern gametophyte X 40 showing antheridia. Fern gametophytes are small, heart-shaped structures. The dark-stained cells near the center are antheridia.|
Whisk ferns are seedless vascular plants that closely resemble the first vascular plants.
They have stems and vascular tissue but do not have true roots or leaves. Rhizoids function to anchor the plant and absorb nutrients.
The above-ground stems are photosynthetic and contain whorls of small leaves or branches. Sporangia are located in a strobilus at the tip of the stem.
The strobilus at the tip of the plant contains sporangia.
These plants are sometimes called scouring rushes because the epidermal (outer) cells contain silica in their cell walls. They were used by Native Americans and early pioneers for scrubbing and polishing.