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The Sex Life of Pteridophytes

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Let us venture back to the Palaeozoic Era (about 570 million years ago), at the start of which plant life began. These simple plants were limited to algae which gradually evolved into non-flowering plants. It is one of these groups which still grow in our countryside and gardens which have given me much pleasure, and sometimes disappointment, as I try to cultivate them in my little plot of land.

What are they? ............................... They are THE FERNS.

Possibly the most fascinating period during the life of these plants is the reproductive one. As they are non-flowering they do not produce seeds but instead rely upon microscopic objects called spores. If you look at the back of the fronds of most ferns a growth can be seen which may be any sort of shape - round, horse shoe shaped or straight lined, which are called sori. These sori in some ferns may be covered with a membrane or in others may be naked. Inside the sori there are other spherical objects called sporangia. They may be seen faintly by the naked eye and they contain the spores which are so small that up to 20,000 could fit into a cubic millimetre. One amazing thing about theses spores is their variety of shapes. Spores may be seen under a strong microscope and their shape depends on the specie of the parent plant. It is these tiny objects which are planted by the fern enthusiast.

To start, the spores have to be collected. The easiest way is to get a frond with ripe sori (normally showing black, green or yellow round the edges) and place it between two sheets of duplicating paper, sori side downwards. After a couple of days the sporangia will have fallen onto the paper and ejected their spores. Carefully remove the top sheet and the frond, and a dusty looking outline of part of the frond will be left on the paper – this being the sporangia shells and the spores mixed together. To remove the spores from the dross it is best to carefully turn the paper over and let the dross fall off (do not try to blow it off!) - most of the spores will cling to the paper. Hold the paper spore side down fairly tightly on the top of a flower pot full of well sterilised compost and tap the paper with a finger to release the spores. Place the pot in a sealed plastic bag and leave for nature to act (germination can take from a few weeks to a few years). Keep things warm but not in sunlight.
Professor Manson of Leeds University once told me that some Ophioglossum spores she planted did not show anything resembling a fern for 24 years – but that is another story. I managed it in 8 years but I was lucky enough to have a suitable fungus present in the growing medium.

The first thing to happen which is visible to the naked eye is the growth of round or heart-shaped green filmy looking things which are only one cell thick round the outside and slightly thicker towards the centre. There could be several hundred in the pot and each one is supported by a bunch of hairs. This is the first generation growth and it is sexless. There are many millions of these growing in the wild, on rocks, brick walls – especially those built with lime mortar, on trees and on the ground, but they are too small to be noticed by anyone but the most experienced fern hunter. Most are eaten by slugs, snails and other members of the animal kingdom. This filmy growth, resembling a liverwort, is called a prothalli or prothallium. After a while, depending on conditions, things start to happen and sex organs

develop on the underside of the prothalli. These are the antheridium which are male organs and the archegonia which are the female counterpart. The male organs develop at the hairy root like end of the prothalli and the female at the other end. When the time is right – after a few weeks or a few years – the male organ releases spermatozoids which have to swim through an almost invisible film of water to a female organ. The archegonia is bottle-shaped with a rather long neck. At the bottom of this neck a minute central cell has formed called the oosphere. The sperms enter the neck and one of them fertilises the oosphere. This is the end of the first generation as the growth resulting from the union begins to develop. It has not produced any roots yet so it continues to take nutrients from the prothalli. The prothalli has finished its short life and just rots away, leaving the young offspring called a sporophyte to grow into a mature fern. I have seen this fertilisation happening on a film and it is very interesting to see the sperms, which are free swimming, travelling like all animal and human sperm to their ultimate goal. Some ferns do produce in other ways, such as baby ferns actually growing on the parent fronds and then falling to the ground to root – but it quite certain that the spore method has stood the test of time ever since the Palaeozoic Era .

-- Jack Bouckley