Seedless Vascular Plants: Ferns, Club Mosses, and Whisk Ferns
Division Pterophyta
The Division Pterophyta contains the ferns and is the division of seedless vascular plants that is probably most familiar to people. The ferns prefer moist and shady environments. For example, they are abundant in tropical and temperate rainforest floors. Ferns are much larger than the seedless vascular plants. During the Carboniferous period, some fern species loomed as tall as trees. Even today, there are a few very large fern species. Many fern species are cultivated as potted plants as well as indoor and outdoor landscapes.
Figure 15. Fern fronds growing in a landscaped area.
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Ferns contain roots, stems, and leaves. The sporophyte generation is dominant in ferns and all seedless vascular plants. The fern leaf is called a frond. The fronds are the photosynthetic body of the plant. The fronds also produce spores. As ferns grow, unfurled fronds called fiddleheads emerge from the soil. The blade of the frond protects the tender leaflets as they break through the soil. Many ferns produce underground stems called rhizomes. New ferns plants may erupt from these rhizomes forming a colony of fern clones, a method of asexual reproduction.
Figure 16. A fern fiddlehead or an unfurled frond. All of the leaflets are still coiled up and protected.
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Figure 17. This image shows where the name fiddlehead comes from, the shape of a violin "fiddle" head. The wooden violin head coils inward, just like that of an emerging fern frond.
Ferns typically produce their spores in structures called sori. The sori are found on the underneath side of the fronds. These sori appear as brown patches and are often disturbing to novice horticulturists who do not recognize them as a sign of a healthy fern. Each sorus may form numerous sporangia. The sporangia are shielded by a structure called the indusium. Within the sporangia, haploid spores are produced by meiotic divisions. As the sporangium matures, a ring of thick-walled cells called the annulus is formed. When the sporangium matures, this ring pulls backwards, ripping open the sporangium and releasing the spores. If the spores land in favorable conditions, they may grow by mitotic cell divisions to produce haploid gametophytes.
Figure 18. This image shows a close-up of the brown spots found on the bottom of fern fronds. The brown spots are where ferns produce their spores in these structures called sori.
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Figure 19. Micrograph of a cross-section of a Sorus.
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The fern gametophyte is very small and inconspicuous. Generally, they are barely visible to the unaided eye. The structure is heart-shaped and called a prothallus. The produces both antheridia and archegonia although they are generally not mature at the same time. The antheridia are found towards the tip of the structure, and the archegonia are found near the "V" of the heart shape. The antheridia produce haploid sperm by mitotic cell divisions. The sperm are flagellated and require water to swim to the egg on a prothallus with mature archegonia. When the sperm fertilizes an egg, a diploid zygote if formed. If conditions are favorable, this zygote will divide by mitosis to produce a new diploid sporophyte fern plant.
Figure 20. Fern prothallus with antheridia. This heart-shaped structure is the gametophyte and the antheridia can be seen as the dark spots towards the tip of the structure.
Figure 21. Fern prothallus with archegonia. This heart-shaped structure is the gametophyte and the archegonia can be seen as the few dark spots in the "V" of the heart shape.
Figure 22. Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte.
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