Evolution of plants; part 2, The Bryophytes

‘Evolution of plants; part two, The Bryophytes’…….Sounds almost like something out of Doctor Who….and in some ways it is that weird.

The classification of plants is largely based on the reproductive behaviour. Other characteristics like their structure, the possession of roots stems and leaves, their photosynthetic pigments, cell differentiation, like whether they have an epidermis or specialised cells for water transport, are taken into account but are less important.

To get to grips with the reproductive behaviour  we need to understand the words gametophyte and sporophyte  also the concept of ‘alternation of generation’   it is not that difficult so bear with it. The photo above is of a common and large liverwort called Marchantia polymorha… and yes this is large, well as large as it gets for a liverwort and this is the gametophyte.

Gametophyte means a plant that produces gametes ie the male and female reproductive cells, in human terms sperm and eggs. Mosses and Liverworts both produce gametes which are actually not dissimilar to the human gametes. The male gamete is called an antherozoid, and it is reminiscent of a sperm, it is a single cell with just one set of chromosomes (haploid) and it swims, propelled by two tails  or flagella whereas a sperm has just one tail. The female gamete is called an oosphere and they are produced singly in a flask shaped structure called an archegonium which has some resemblance to the female reproductive system, ie an opening, a neck area which then leads to a rounded vessel in which the oosphere is located. It is of course much smaller.

The antherozoids which incidentally are also known as spermatozoids have to swim from where they are produced, attracted by a chemical stimulant which is produced from the archegonium, they swim in a film of water ( rain water, dew, or splashes from a waterfall) and then find their way through the opening in the archegonium, along the neck region and eventually to the oosphere, where it fuses with the also haploid oosphere cell to produce a fertilised (diploid) cell called an oosphere.  Many parallels here with mammalian reproduction!!!???

So mosses and liverworts are both gametophytes and all their cells are haploid, just one set of chromosomes, no pairs. And they do all the stuff I have just described.  When the oosphere gets fertilised and forms a oospore it is now diploid and it will develop into a sporophyte. The plant  illustrated above is Marchantia  again and the star shaped structures are where the archegonia are located. Other plants will produce the antherozoids and these are produced by little structures which resemble a golf tee. Drops of rain hitting the top of the tee will rebound out and carry the antherozoids a little way a way and start them on their journey to find the archegonium.

Sporophyte means a plant producing spores. – well it is not so much a plant, more of a stalk with a capsule on top. If you have ever looked at mosses closely you will have noticed that sometimes, some of them will have a little yellowish or reddish stalk sticking up with a bump on top. This is the sporophyte, it is pretty much dependent on the gametophyte to supply it with water and nutrients and all its cells are diploid ie double set of  chromosomes per cell. The photo is of a small moss called Funaria hygrometrica which often grows on old bonfire sites. The little pin like structures are the sporophytes.

When the sporophyte has developed the bump at the top is called a capsule, in the case of the mosses it can be quite microscopically complex and indeed beautiful, whereas in the case of the liverworts it is much more simple, just a spherical case.  Inside the capsule the spores are produced. This involves a process which is known as meiosis and the spores end up with just a single set of chromosomes, ie they are now haploid.

The spores get released from the capsule and get blown about by the wind, most will perish but some if they are lucky will land in a damp place and will germinate and develop into the next gametophyte generation and the the whole process that I have just described starts all over again. This next photo is of the more primitive group the Liverworts and it shows the green flat gametophyte generation growing over the rock. What  shows up quite nicely is the sporophyte generation which is the white stalk with the black capsule on top. It is quite unusual to come across this, you do not see this that often.

It may all sound a bit hit and miss but obviously it works as these plants have been around for millions of years, and it involves what is known as  ‘Alternation of Generation’   Gametophyte generation, gives rise to Sporophyte generation which then produces the next Gametophyte generation and so on and so on.

In the next ‘episode’ we will see how this situation links to the ferns and the subsequent evolution through the Horsetails to a group of plants know as Sellaginella and eventually to the Club mosses.

I bet you cant wait!

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