Reproduction and Life Cycle of Ferns and their Allies
J.R. Croft (firstname.lastname@example.org)
The life cycle and reproduction of the pteridophytes or ferns and their allies is characterized by a complete absence of conspicuous flowers and fruit, the production of minute fine spores, often in copious quantities, and by alternating generations of separate spore producing plants (sporophytes) and gamete producing plants (gametophytes).
In these respects they are similar to the structurally less complex bryophytes or mosses and liverworts. In the bryophytes the gametophyte is the dominant more conspicuous and long-lived generation; the sporophyte is almost completely dependent on the gametophyte for support and nutrition for its entire lifespan. In the pteridophytes the sporophyte generation dwarfs and overtakes the gametophyte almost immediately and is totally independent of it for support and nutrition; it is the most commonly observed component that is recognized as a fern or fern ally.
The general cycle involves the production of asexual spores on the sporophyte which are shed and germinate to grow into gametophytes which produce male organs with motile sperm and female organs with non-motile egg cells. Fertilization follows and the egg cell develops into a new sporophyte to complete and continue the cycle.
Figure: Life cycle of a typical fern; illustration by Murray Fagg ©
Some pteridophytes dispense with the sexual aspect of the development cycle entirely in a process know as apogamy whereby a haploid sporophyte develops directly from the prothallus. It is not common but has been reported in a wide range of genera such as Asplenium, Cheilanthes, Pellaea, Pteris, Trichomanes, Isoetes. Sporelings develop directly from the prothallus without fertilization.
Given that pteridophytes produce so many easily dispersed spores, why do not all ferns and their allies occur everywhere and why aren't all land areas covered in them? The reproductive strategy of a pteridodphyte is not particlarly efficient or targeted and is very much a hit and miss affair. Although a mature plant may produce many millions of spores that may be dispersed hundreds of kilometers by favourable winds, a spore must land in a suitable microhabitat in order to germinate into a sexual prothallus. The viability of spores is anything from a few days to a few months; spores that are green and contain chlorophyll are short-lived. The environmental conditions surrounding the prothallus must allow fertilization, and once fertilization is complete, conditions must allow development of the sporeling. As the sporeling grows out of the microhabitat of the prothallus, it must grow into an environment capable of supporting the mature plant, the requirements of which are quite different.
The gametophyte generation, or prothallus, in ferns and their allies is nearly always short lived and inconspicuous. Germination from the spores, the prothallus is mostly only a few mm across, photosynthetic, generally simple in structure, without vascular tissue and often only one cell thick for the most part, tapering to many cells thick at the centre. They may be attached to the substrate by fine multicellular root hairs or rhizoids, and often resemble thallose liverworts. The prothallus has a single set of chromosomes (i.e. haploid) and is the sexual part of the life cycle. The sexual organs are microscopic or nearly so. The male organs or antheridia produce and shed numerous motile sperm or antherozoids. These antherozoids are armed with hair-like or whip-like cilia or flagellae and are able to swim through water; they do not travel great distances and are only released when free water is available. Only a continuous film of water is needed. They antherozoids are chemically attracted to the developing flask-like female organs or archegonia where fertilization of the single egg cell embedded in each archegonium takes place. On a given prothallus in some species the antheridia sometimes mature before the archegonia to reduce the risk of self fertilization
The fertilized egg cell or zygote contains a double set of chromosomes, one from the egg cell, one from the antherozoid, and is thus diploid. It repeatedly divides mitotically, becoming an embryo, developing in to a sporeling, then into juvenile plant and eventually into a mature diploid spore-bearing sporophyte. The embryo and sporeling is initially parasitic and dependent on the gametophye, but rapidly develops roots of its own and becomes an independent photosynthetic plant as the prothallus withers and dies.
Antheridia and archegonia may be borne on the same prothallus (monoecious), or in the case of heterosporus species, on separate male and female prothallia (dioecious).
Gametophytes have an ecology of their own which may be quite different to that of the sporophyte. They may be superficial or subterranean, and in some megasporous taxa, gametophyte development takes place within the spore wall. While the more robust sporophyte may be able to exist in a wider range of conditions, it may be the habitat requirements of the gametophyte that determines the distribution of a pteridophyte species. This is one of the reasons that ferns are most commonly found in moist shady places. Some ferns have adapted to arid environments, but they still need wet periods to produce gametophytes and complete fertilization.
The sporophyte generation in ferns and their allies is long-lived compared to the gametophyte. It is mostly larger and structurally more complex in that it has a vascular system of xylem and phloem, often associated structural supporting tissue and is mostly organized into highly specialized stems, leaves and roots. The aerial parts are protected by a waxy cuticle and are thus able to withstand greater degrees of exposure than the gametophyte; stomata are present allowing the exchange of gases between internal chambers and the external environment. The sporophyte has a double set of chromosomes (i.e. diploid) and is the asexual part of the life cycle.
At maturity the sporophyte develops specialized structures of varying complexity on the leaves called sporangia in which the spores are produced; a number of sporangia may be aggregated into structures called sori. Spores consist of a single cell surrounded by a durable cell wall; they are produced by meiotic divison in which the number of chromosomes is halved and are hence haploid. Spores are minute, non-motile and often produced in large numbers; they are shed by rupturing of the sporangial wall and can be dispersed long distances by wind or water. When mature and under appropriate conditions the spores are dispersed widely into the environment, and if they land in a suitable place germinate to create the next generation of gametophytes, completing the cycle.
Most pteridophytes produce spores of only one size (homosporous), but a significant number of species produce spores of two distinct sizes (heterosporous): minute microspores and relatively very large megaspores. Microspores produce the male antheridia and megaspores produce the female antheridia.
In addition to the sexual gametophyte-sporophyte life cycle, some of pteridophytes have developed various vegetative means of propagation to increase the extent and number of their population. This is advantageous where seasons are unreliable or the environment is otherwise not conducive to gametophyte production, and in some cases it may just be advantageous to generate numerous genetically identical individuals.
A number of quite unrelated species produce vegetative buds or bulbils that are capable of producing roots and new plants. These bulbils are leaf derive structures and are produced in leaf axils or at various places on the leaf surface. Plants may chain along as successive bulbils take root, establish themselves and produce fronds and bulbils of their own. Proliferating bulbils can be found in Asplenium, Camptosorus, Diplazium, Polystichum, Ampelopteris, Huperzia and a number of other genera.
Some species with long creeping rhizomes can extend over large areas and it may not be obvious that plants on one side of a population were physically derived from plants on another. Large brakes or open areas can be covered by scrambling ferns with creeping underground rhizomes in this manner; Gleicheniaceae genera (Gleichenia, Sticherus, Dicranopteris) and bracken (Pteridium) are good examples. Similarly populations of climbing epiphytes may be increased in extent and mass as rhizomes branch, break and regrow; examples include genera of Polypodiaceae (Pyrrosia, Lemmaphyllum, Microsorum). The floating ferns Salvina and Azolla are striking examples of propagation by stem fragmentation.
Proliferation is possible from root stolons, tubers and similar structures. This occurs in some species of Nephrolepis and Blechnum.
More of a survival strategy than a means of propagation, some ferns of arid areas are able to dry out almost completely without actually dying. Crisp and brittle to touch, they resuscitate rapidly when rains come and continue their growth. A number of species of Cheilanthes behave this way. Commonly called resurrection ferns.