The eggs are produced in tiny, typically somewhat flask-like structures called archegonia. Each archegonium holds one egg in a swollen section called the venter and the sperm enter through the channel in the narrower, tubular section or neck.
On the side of the venter opposite the neck is the foot which anchors the archegonium to the gametophyte. In the early stages of archegonial development that channel does not exist, the area being filled with cells.
At maturity the cells in the centre of the neck disintegrate to create the channel. The channel is filled with mucilage that results from the breaking down of the cells that initially occupied the channel. A fertilized egg in an archegonium develops into the sporophyte. The sporophyte consists of a spore-containing capsule which, depending on the species, may be stalked or stalkless. Each spore contains a mix of genes from the two parents and on successful germination will give rise to a new gametophyte.
The following diagrams show some moss archegonia and antheridia. The archegonia are on the left and have been coloured green You can see the swollen venters near the archegonial bases. At the top of the neck each archegonium has a somewhat funnel-shaped mouth.
The antheridia are on the right, have been coloured blue and the middle antheridium is releasing a sperm mass, coloured brownish-orange. Archegonia and antheridia grow intermixed with hair-like to club-like paraphyses, left uncoloured in the diagrams. The previous paragraph mentioned that the antheridia and archegonia are tiny. Size varies, depending on species, but typically these gamete-producing organs are well under a millimetre in length.
Bryophyte antheridia are fairly uniform in structure and the same is true for the archegonia. The antheridia vary in size and shape from globose to somewhat cylindrical depending on species, but the diagram above captures the essence of any antheridium - a short, narrow stalk supporting a swollen, sperm-producing organ. Similarly, the archegonia vary in size, and relative lengths of the neck, the venter and the length of the supporting foot - but the diagram above shows the essential features of all archegonia.
Individual antheridia and archegonia are microscopic but at times you can see where they are formed. In this photo of the moss Rosulabryum billardieri each yellow ball is a cluster of antheridia. The same is the case in this photo of a thallose liverwort in the genus Fossombronia.
This photo shows male plants of the hornwort Phaeoceros inflatus and antheridia are produced in the many "blisters" visible on the thallus. Groups of archegonia are found under the white "blisters" shown in this photo of the thallose liverwort Lunularia cruciata.
Though there is much uniformity at the structural level, there is variety in the formation and arrangement of the archegonia and antheridia. The rest of this page will give an overview of the sexual reproduction cycle.
Once an antheridium has matured and contains viable sperm, the sperm need to get to the eggs in archegonia. The first step for the sperm to get out of the antheridia and the second is to then travel to the archegonia and fertilize the eggs within.
Water is essential for both steps. In some bryophytes a mature antheridium will hold free sperm, but more commonly that's not the case, Rather, each sperm is still held within the cell in which it formed. In such a case, when an antheridium opens, those sperm-containing cells are released and it is only at some stage after release from the antheridium that the single sperm, within each such cell, is liberated.
Such liberation may take place shortly after the opening of the antheridium or as long as 15 minutes later, depending on species. In the following, the expression "sperm mass" will mean either a mass of free sperm or a mass of sperm-containing cells, when it's not essential to distinguish the two. When a mature antheridium is moistened the cells at the apex absorb water, swell and finally burst or open in some way. The sperm mass inside a mature antheridium is under pressure. So, once an antheridium has opened, the sperm mass is forced out.
In some bryophytes the force is enough to shoot the sperm mass into the air, allowing dispersal over a relatively wide area. However, in most cases the sperm mass simply oozes into the area around the antheridium and further dispersal is by some other means.
While the entire sperm mass may sometimes be released during the forceful extrusion, release is more often a two-stage process. Typically a large percentage of the spore mass is quickly forced out by the built-up internal pressure, but a proportion remains within the antheridium and exits more slowly, over many minutes. Seeds and seed plants are discussed in the next chapter, below are considered aspects of the sexual cycle, in particular features of the less commonly seen gametophytes that are produced by seedless vascular plants.
Most ferns have a small, photosynthetic gametophyte that usually is less than 1 cm across and one cell thick, i. It is generally attached to a substrate via rhizoids filaments of non-photosynthetic cells. As was the case in the mosses, the fern gametophytes produce structures where the egg and the sperm are produced as a result of cells dividing in a particular pattern to produce archegonia Fig.
It is important to note that gametes are not produced by meiosis because all the cells of the gametophyte are haploid already. Fern gametophytes are generally have flask shaped female structures archegonia located in the notch between the lobes and globular male structures antheridia located on the on the lobes. While most fern gametophytes are hermaphroditic, some are unisexual and for some their sexual expression depends on environmental conditions. All the cells of the gametophyte are haploid but it produces a cell, the egg, with special developmental abilities.
The antheridia release sperm which have flagella that allow them to swim to the archegonia , where the eggs are located, swim down a narrow canal and fuse with the egg cell at the base. The zygote develops into a sporophyte, producing stems and roots. The stems produce leaves which shade the gametophyte and it soon dies Fig.
The sporophyte continues to grow to produce the fern that we recognize. It has the same structure as most plants: a root-shoot axis with leaves produced by the shoot. Most of the ferns in this area have stems rhizomes that are below ground and relatively short. What we see are the leaves emerging above ground from this rhizome. At some point this diploid organism produces structures termed sporangia, inside of which are spore mother cells that undergo meiosis to produce a group of four a tetrad haploid spores that are released to the environment.
When these germinate, they grow into haploid gametophytes and the process is repeated. Other ferns have entire portions of their leaves that are obviously different and where spores are produced.
A few ferns in this area are dimorphic see sensitive fern , producing two types of leaves, some that are green and photosynthetic and which never produce spores and other leaves that are non-photosynthetic and produce abundant spores, while being nourished by the photosynthetic part of plant.
While this is the general pattern for ferns , there is some variation, one example of which is the water fern Marsilea , which has several interesting features see the information sheet on Marsilea.
As is the case in a number of ferns, spores are produced on a specialized leaf that is very different looking from normal photosynthetic leaves. Whereas the normal leaves are green and shaped like clover leaves, the spore bearing leaves are initially packaged into a seed-like structure, hard on the outside and capable of being dried out and revitalized germinated when re-wetted.
It produces spores in clusters and there are two types of spores, male spores called microspores and female spores called megaspores, each in separate sporangia.
The technical name for plants that produce two types of spores is heterosporous. In contrast, most ferns are homosporous , producing only one type of spore that generally produces hermaphroditic bisexual gametophytes described above ; a few homosporous forms produce unisexual gametophytes, both male and female , but both coming from identical looking spore s.
The two types of spores of Marsilea are readily distinguished by size. The megaspores are around 1 mm in length and germinate to produce egg-producing, female gametophytes. The spermatazoids are chemically attracted to the female gametophyte. The female gametophyte is substantially larger than the male gametophyte but it still is small and , like the male gametophyte, exhibits endosporic development, its development occurs within the spore case of the megaspore , with only the very short neck extending from it.
It produces a single archegonium with a single egg which the sperm swims to and fertilizes, forming a zygote. While the new sporophyte plant seemingly sprouts from the female spore, it actually is coming from a female gametophyte that is growing inside the spore case.
Another interesting fern is the Appalachian bristle fern, which is only known from the gametophyte form. Apparently, it has been reproducing asexually for millions of years! There are several other species of ferns known only as gametophytes. The basic pattern found in ferns, with a dominant sporophyte generation and a diminutive gametophyte generation, is found in the horsetails , a group of vascular plants that originated in the Paleozoic and produced a number of tree forms that were significant in producing extensive deposits that became coal and oil.
There only remains one genus of horsetails and there are less than 20 species worldwide. All are herbaceous with perennial rhizomes that send up vertical branches which have a very distinctive pattern of growth with photosynthetic stems , very small scale-like leaves, and whorled branches or no branches.
Spores are produced in a terminal cone-shaped structure, which is a cluster of sporangia. Spores are dispersed by the wind but their movement and release from the sporangium may be aided by structures called elators, strap-like appendages on the spore that move in response to the absorption and loss of water.
Antheridia and archegonia are usually both produced from the same gametophyte , although it may be unisexual for a period of time.
Sperm are multi — flagellated and need to swim to reach the egg. Fertilization results in a zygote that develops into a diploid sporophyte that soon overgrows the gametophyte that it emerges from, producing roots and both horizontal stems rhizomes and vertical stems. The group has species and is considerably more diverse than the horsetails but much less diverse than the ferns 12, species. The sporophytes of extant clubmosses, spike mosses and quillworts are all herbaceous perennials.
They generally spread extensively with above-ground and below-ground stems tropical members are usually epiphytes. Although ancient members of the group exhibited woody growth, none of the species alive today do.
Clubmosses Fig. These gametophytes live much longer than most gametophytes of vascular plants, some over 15 years. Spikemosses and quillworts are heterosporous and, like the aquatic fern Marsilea , the gametophyte s develop endosporically ; living off the material that was provisioned in the spore by the sporophyte plant. The male gametophyte is very short-lived and has little stored material the microspore is small , but t he female gametophyte is considerably bigger and lives for months on material present in the spore.
0コメント