Lets not be mistaken I have a very distinct love of the pteridophytes and this chapter may be bias toward them something I cannot avoid and will not attempt to avoid following this disclosure. For over a decade I grew these amazing plants and learnt so much about their hidden beauty away from the glamourous colours of the angiosperms which have attracted gardens for generations. Too often people consider them the difficult plants in the garden to grow and maintain when in fact they are as easy as the common garden angiosperms many of which can become dare I say it weeds. Some were so prolific in reproduction we had problems eliminating them from our rarer cultured trays. Many grew around the nursery like weeds in the average garden nursery. The reproduction for cultivation of these exquisite plants is found under “Cultivation and Propagation Tips”.
The reproductive strategy of most pteridodphytes is not particularly efficient so the need to produce large quantities of spore is necessary for their very existence. Although a mature plant may produce many millions of spores which may be dispersed hundreds of kilometers by favourable winds, a spore must land in a suitable microhabitat in order to germinate into a sexual female reproductive prothalis. The viability of spores is anything from a few days to a few months. Green spores that contain chlorophyll are short lived and must develop a prothalis very soon after dispersal. The environmental conditions surrounding the prothalis must be conducive to allow fertilization, and once fertilization is complete, conditions must be favourable for the development of the sporeling. As the sporeling grows out of the microhabitat of the prothalis, it must then grow into an very different environment that is capable of supporting a mature fern.
Sporangia (Clustered in sori)
The sporangia is the structure which produces spores. In ferns the sporangia usually congregate into larger structures. The underside of a fern frond has various patterned congregations of the sporangia unique to different species.
Meiosis in the typical big fern produces the haploid spores with half the number of chromosomes of the parent plant. The spores are the product of meiosis. In ferns the seeds known as spore are very tiny. The largest are less than half a millimetre in diameter while the smallest are usually about 5 microns. 50 placed end to end would measure 1 millimeter. This makes them ideal for wind dispersal over long distances.
Spore germination: If the spores settle on a surface with suitable moisture they will grow into a gametophyte and that is a wholly separate individual plant.
Gametophyte is known as the prothallus will produce the new sex cells, the eggs and the sperm. It is a green; single celled thickness in most ferns or a single thickness near the edges and only a few cells thick near the central tube, photosynthetic structure that is usually heart or kidney shaped. The prothallus is also small and may measure as little as 6mm in length or as large as 15mm in length by 3mm to 8mm in width. They are rather difficult to find in nature if you are not familiar with their habitats however when you find one you often find hundreds or even thousands crowded together all competing to survive. It doesn’t have roots, stems or leaves but it does have filiform rhizoids that anchor it to the soil, rock or in the case of epiphytes to the limbs or branches of trees. They assist in the absorption of moisture and stabilize the prothallus during fertilization. The underside of the prothalli are where all the action occurs. The gametes are produced from the male and female sex organs.
Antheridia and archegonia may be borne monoeciously on the same prothalis or dioeciously in the case of heterosporus species, on separate male and female prothallia. Most pteridophytes produce spores of only one size (homosporous) however a significant number of species produce spores of two distinct sizes and are known as heterosporous pteridophytes. The minute microspores produce the male antheridia while the relatively very large megaspores produce the female archegonium.
Antheridium: The antheridium is the male sex organ and is usually located between the rhizoids. This is a small spherical structure that produces the flagellated sperm. Flagellated refers to the sperm having long flagella like tails a characteristic of fern sperm. The sperm or antherozoid as they are known in ferns are often armed with hair like or whip like cilia on their flagellae and are able to swim through water. Only a continuous film of water is needed with the sperm not fully developing or being released, prior to the film being present. On a given prothallus in many species the antheridia often mature before the archegonia to reduce the risk of self fertilization.
Archegonium: The archegonium is the female sex organ which is usually found closer to the cleft of the prothalis. The archegonium is a flask to funnel shaped structure with a long narrow funnel that produces a single egg.
Fertilisation: At this stage weather conditions play a vital role if the sperm are to reach the archegonium and the long swim down the neck to the waiting egg. The sperm need moisture and it can only be supplied by fog, mist light rain or humidity forming a film over the prothalis and down the tube neck. It is this moisture which the sperm rely on at the right time to commence their arduous swim. Too much will wash them away, too little will dehydrate them and make a discontinuous path for them to find the egg. Moist to wet habitats prior to the actual fertilization is imperative. The antherozoids are chemically attracted to the developing egg at the bottom of the flask. If a sperm manages to reach the egg and fertilization is successfully completed a diploid cell is final the stage before the cell grows to a point where it becomes a small fern.
Developing sporophyte: The young fern plant starts to grow from the prothallis which supplies the nutrition for the young fern in its endeavours to survive the next hostile move to the environment outside the prothallis’s tube and into adulthood.
Mature sporophyte: The mature fern consists of 3 major parts; the rhizome, the fronds and the sporangia. The mature fern is the sporophyte that produces spores which are released from sporangia.
1. The sporangia are the structures which produces and contain the spores.
2. A cluster of sporangia are known as Sori.
3. Fern spore are very small and may be as small as 2 microns.
4. Gametophyte or the prothallus as it is more commonly known grows independently of the sporophyte parent.
5. Antheridia and archegonia are born on the same prothalli and are known as monoecious Pteridophyte or homosporous species.
6. Antheridia and archegonia born on seperate prothallis are known as dioecious Pteridophyta or heterosporus species.
7. The antherozoids are chemically attracted to the single egg.
8. The egg develops at the base of an inverted funnel like tube on the underside of the prothallis.
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