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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.
Some evidence that suggests that plants evolved from the green algae is:
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.
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).
Spores germinate to produce a branching horizontal filament called a protonema. The familiar green gametophyte body and rhizoids arise from the protonema.
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.
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:
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
Phylum Pterophyta- Ferns, Whisk Ferns, Horsetails
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.
The gametophyte is small and heart shaped.
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.whorls of small leaves or branches. Sporangia are located in a strobilus at the tip of the stem.Below: Horsetails
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.