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 byalternating 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

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.

Gametophyte

The gametophyte generation, or prothallus, in ferns and their allies is nearly always short lived and inconspicuous. Germination from the spores, the prothallus is