BIO 5 GENERAL BIOLOGY
1. Basically there are two types of land plants: bryophytes and vascular plants. Mosses are bryophytes, and vascular plants include those plants that are most familiar to us: trees, shrubs, lawn grasses, garden plants. All land plants, both bryophytes and vascular plants, have solved the problems of life on land. The two groups, though, have dealt with these problems in different ways. Underscoring these differences is the fact that different portions of the life cycle are dominant in the two groups. This means that, for bryophytes, it is the gametophyte (1N) generation that confronts the environment sporophyte (2N) depends entirely on the gametophyte. In vascular plants, the sporophyte has developed adaptations for dealing with life on land, but the gametophyte either avoids these problems or is dependent on the sporophyte. This portion of the text supplement will provide information on the most primitive and oldest group of vascular plants, the ferns and plants that have life cycles similar to ferns, the fern allies.
2. Most of us are familiar with ferns and readily recognize them. The portion of the life cycle that we recognize as being ferns is the sporophyte (2N). In most ferns, the above ground portion of the plant consists of leaves which are called fronds (Fern_labeled_MC.jpg). Note that the fern in this image has fronds that are highly lobed but are not fully divided into leaflets. Most ferns have pinnately compound leaves (Fronds_MC.jpg ). Some fronds are 15-20 feet in length. Like the leaves of all vascular plants, fern leaves have veins that contain vascular tissue, xylem and phloem, mesophyll where photosynthesis occurs, and an upper and lower epidermis that is covered with a cuticle. The lower epidermis has stomata surrounded by guard cells. (See leaf structure in the Roots, Stems, and Leaves text supplement.) In most ferns, the stem grows underground. Underground stems are called rhizomes. Like all stems, fern stems have a pith surrounded by a ring of vascular bundles each with xylem and phloem, a cortex, and an epidermis that is covered by a cuticle. Fern roots arise directly from the rhizome. These are adventitious roots. Like all roots, fern roots have a central region of xylem surrounded by phloem, an endodermis, cortex, and epidermis that is not covered with cuticle. Because fern leaves are produced by the stem apical meristem and because ferns generally have rhizomes, fern leaves are coiled when they are immature. They uncoil only when they have grown to the surface of the soil. Because the immature leaves are coiled, they are not damaged as they push through the soil to reach ground level. The coiled, immature leaves are called fiddleheads. The image ( Osmunda_cinnamomea_KS.jpg) shows fiddleheads that are in the process of uncoiling. Though unusual, some ferns have above ground stems. We call these tree ferns ( Habit_MC.jpg).
3. Mature fern fronds often have light green, brown, or tan-colored patches on the lower surface ( Cyrtomium_sori_MC.jpg). These are often mistaken for insect infestations or other plant diseases. These are, however, perfectly normal structures found on ferns, they are called sori (plural for sorus). Each sorus is a cluster of sporangia (plural for sporangium). Often, the sporangia are covered by a productive layer. The image (Sorus_MC.jpg ) shows a sorus in cross section. Several sporangia are visible; each contains spores. The spores are 1N and are produced by meiosis. Also note in the image the protective layer that protects the sporangia while the spores are maturing. Additionally, note the leaf cross section (remember the sorus is located on the bottom surface of the leaf). The upper and lower epidermises, mesophyll, and veins are visible.
4. When the spores are mature, the protective layer that covers the sporangia dries up and folds back or falls off of the leaf. The sporangia are then fully exposed to the air. Each sporangium has a specialized layer of cells called the annulus ( Sporangium_xs_annulus_MC.jpg). The annulus is visible in the image in the sporangium that is on the left. The annulus is consists of cells that have very thick cell walls and it occurs as a band of cells. The function of the annulus is to throw the spores out of the sporangium and into the air. Note that the spores have very thick cell walls. Each spore is a 1N cell. Spores are carried by the wind away from the sporophyte that produced them.
5. If a spore lands in an environment that has enough water and the temperature is moderate, it germinates. This means that the spore wall breaks open and that the cell undergoes many mitotic divisions (mitosis). The result is a multicellular 1N gametophyte plant (Young_gametophyte_MC.jpg ). The image shows two gametophytes. Note that these plants are green and photosynthetic. They are anchored to the ground by rhizoids (unicellular hairs). The plants are heart-shaped. Each gametophyte produces two types of gamete-producing structures, antheridia (plural for antheridium) and archegonia (plural for archegonium). The image (Composite_view_MC.jpg ) shows an archegonium, a multicellular structure that produces an egg cell, and antheridia, each one a multicellular structure that produces several sperm cells. Note that the antheridia are produced on the portion of the gametophyte that is near the point of the heart-shaped gametophyte plant and that archegonia are produced near the lobes of the "heart".
6. The image (Antheridia_ls_MC.jpg ) shows a portion of a gametophyte that has be cut in cross section. Several antheridia are visible. Note that these are round structures. Each of these contains sperm cells. A group of archegonia ( Archegonium_side_view_MC.jpg) are shown in side view. Each archegonium is flask shaped. The dark cell at the base of the "flask" is the egg cell. The egg cell is more clearly visible when the gametophyte is cut in cross section ( Archegonium_MC.jpg). Two archegonia are shown in the image. The archegonium on the left clearly shows the egg cell. Since both antheridia and archegonia are produced by the gametophyte, these are 1N structures, and the gametes, sperm and eggs, are 1N as well. When the gametes are mature, the sperm cells are released from the antheridia. These cells swim in a film of water from the antheridium to an archegonium ( Sperm_and_achegonium _MC.jpg). The narrow neck of the flask-shaped archegonium is visible on the left. The sperm cells are coiled-appearing cells.
7. A sperm cell (1N) fuses with the egg cell (1N) and a single 2N cells, the zygote, is formed. The zygote is located in the archegonium (where the egg was) and when it begins to undergo mitosis, a 2N mass of cells, the embryo or immature sporophyte, is formed. The embryo continues to grow. Soon it outgrows the archegonium, and it begins to form a first root and first leaf. Until the first leaf begins to carry on photosynthesis and the first root begins to absorb water, the young sporophyte or embryo is dependent on the gametophyte for food and water ( Young_sporophyte_MC.jpg). In the image, the first root is the thin brown structure on the right. The first leaf is on the left. The heart-shaped structure between the two is the gametophyte which persists until the young sporophyte establishes itself. Once the sporophyte is capable of photosynthesis and absorbing water, the gametophyte dies. The young sporophyte then matures into the plant that we recognize as being a fern.
8. The fern allies consist of plants that have life cycles similar to that of ferns. Two types of fern allies are the club mosses (Division Lycophyta (Lycopodium and Selaginella)) and the horsetails (Division Sphenophyta Equisetum). The club mosses superficially resemble mosses ( Huperzia_population_MC.jpg). However, the green photosynthetic plants in the image are sporophyte (2N) plants composed of roots, stems, and leaves, all containing vascular tissue. (Recall that the independent, photosynthetic portion of moss plants are gametophytes with no vascular tissues). Club mosses are characterized by having all above ground parts of the plant covered with small leaves ( L_inudata_plant_RK.jpg). Some of these leaves bear sporangia, shown by the arrow in the image of Lycopodium, a club moss ( H_lucidulum_sporangiaMC.jpg). The leaves that bear sporangia are called sporophylls. In some Lycopodium species have the sporophylls organized into a compact structure called a strobilus (plural = strobili) (Strobilus_ls_MC.jpg). The dark-staining structures in the image are sporangia, each containing several hundred spores.
9. A second type of club moss (Division Lycophyta) is Selaginella. Like Lycopodium, Selaginella sporophyte stems are covered with small leaves (Strobili_MC.jpg ). Note that the tips of some branches have a more compact arrangement of leaves. These structures are strobili. Strobili are composed of sporophylls, leaves that bear sporangia. Selaginella is different from Lycopodium in that two sizes of spores, microspores and megaspores, are produced (Megaspore _microspores_MC.jpg). Microspores are produced in microsporangia and megaspores are produced in megasporangia. Seed plants, like Selaginella, produce microspores in microsporangia and megaspores in megasporangia.
10. Horsetails (Division Sphenophyta, Equisetum) are another group of fern allies. Some horsetails have unbranched sporophytes that bear strobili (Plant_large_scan_MC.jpg ). Others produce fertile stems that are unbranched, non-photosynthetic but strobilus-bearing stems (Fertile_stems_KS.jpg ) and branched, photosynthetic stems that have no strobili (Scanned_Plant_MC.jpg ). The photosynthetic stem is at the right. Note that the branches are arranged in whorls. Equisetum stems have no photosynthetic leaves. The leaves of these plants consist of a whorl of fused tan, black, or white leaves ( bio5_lab\ferns_and_allies\Strobili_RK.jpg). The specimen in the image has dark brown whorls of non-photosynthetic leaves. Also not that the green, photosynthetic stem has distinct ridges and grooves. The strobili of Equisetum consist of a large cluster of sporangia-bearing branches (Sporangiophores_MC.jpg ). Four of these branches have been removed and the sporangia are visible. Spores are produced within each sporangium by meiosis. The 1N spores are released from the sporangium and are carried by the wind. If spores land in suitable environments, the undergo mitosis and a 1N photosynthetic gametophyte plant is produced (Gametophytes_MC.jpg ). Each gametophyte has antheridia that produce sperm and archegonia that each produce an egg. The sperm swim to the archegonium in a film of water and fuse with the egg to produce a zygote. The zygote undergoes mitosis to produce a young sporophyte (visible on the image). Eventually, the sporophyte continues to grow and mature and eventually the mature horsetail sporophytes is produced.