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The Alternation of Generations

The differences in the shape and mode of life of liverworts, hornworts, mosses (all of which are called bryophytes, but actually only true mosses belong to the Bryophyta); and ferns (which are generally grouped together as pteridophytes - ferns - but clearly include some non-ferny type plants), and the flowering plants; is a close reflection of, and helps to explain, the progressive process by which plants gradually moved onto the land during the Silurian and Devonian periods. The final colonisation of land was rather explosive and had enormous consequences for the environment on earth as a whole.

The relevance to us in West Cork, and to this blog, is that this move happened just at the time that the West Cork bedrock was being deposited as sediment.

In reading the rest of this blog entry, there are two things to remember:-

Mitosis is cell division in which the number of chromosomes is reduced from the full double set - diploid - to just a single set - haploid. Meiosis is just cell replication that occurs with haploid cells. They start with single chromosome sets, and end up with single chrmosome sets.

(There is also a cell division which will reproduce a full diploid cell, and this is vegetative reproduction, using a bit of the existing organisms to create a new organism, a clone. Mammals don't do it, some plants can.)

The second thing is that the generations that alternate are haploid (one set of chromosomes) and diploid (double set of chromosomes, the 'normal' state of affairs). This switching between the two is really just a different way of looking at sexual reproduction, and provides a good way to relate one thing to another. The essetial requirement is to halve the number of chromosomes, so that when two (haploid) sex cells meet, they combine and form a diploid cell. In humans, and mammals generally, the haploid generations exist internally and do not form an apparently different generational organism. But other orgaisms are very different, but dominant in numbers, so this in fact makes us the odd ones out, no matter how strange the other behviours might seem to us. So the Alternation of Generations may seem a bit strange to humans since we consider just our own selves and our diploid bodies as the only generation, each one succeeding the diploid body (or bodies) that formed it. We do not consider those haploid cells - the sperm and egg - as an other generation.

Also remember that spores are not like pollen - pollen is the male sex cell, haploid, one set of chromosomes - whereas spores can be either haploid or diploid, but ready to grow, more like seeds. Spores are as small and easily dispersed as pollen, which is probably why they are thought of as so similar.

True Mosses (Bryophyta) Life Cycle.

The mosses that we generally see are the single chromosome bearing plants, the haploid gametophyte. This means that all the cells within the plant have only one set of chromosomes. There are male plants and female plants, and each of these produce haploid sex cells. Wet conditions are required for the male sex cell to swim, externally, from where the male sex cell is produced (the antheridium - think 'anthers') to the place where the female sex cell resides (the archegonium). (The planetary symbol for Mars, the 'male' planet, is the same as the symbol for male, a circle with an arrow, signifying that the male goes off in search of a female. Or hunting, I suppose. Or fighting. Or to the pub. The female symbol - that for Venus, is a circle with a cross underneath, suggesting planted, or static, or stable.)

When these two haploid cells meet and join, they form a cell with a double set of chromosomes - diploid - and this then germinates and grows into another generation, but remains fixed within the archegonium. So in effect, the new (diploid) plant grows on top of and out of the originating (haploid) plants. These are the spore capsules you might see on the end of a stalk. The spores are produced by cell division and are single chromosome sets, haploid. When the capsule bursts the spores are released, into the wind, and eventually will grow into new gametophyte plants.

The release mechanism can be quite sophisticated. Some sphagnum mosses, that grow on bogs, rely on the capsule drying and constricting such that the air pressure builds up inside. Then, suddenly, pop! The cap pops off and the spores are propelled out significant distances - up to 16 cm. Considering each spore is just about 2 or 3 hundredths of a millimetre in size, they are flung between 5 and 6 thousand times their diameter. See The Secretly Speedy Life of Plants.

Liverwort (Marchantiophyte) Life Cycle.

Liverworts are similar to Mosses in that the main plant we see is the gametophyte haploid plant. On each plant there grows either a female cell bearing organ - archegonium - or a male bearing organ - antheridium. Just like mosses. Or there may be both male and female on the same plant. Either way, the male sex cell has to reach, by swimming through a moist external environment, a female cell inside an archegonium. Some liverworts propel their male sex cells in a sort of 'liverwort sperm ejection' - or ejaculation I suppose. Researchers have not yet identified quite how, but some liverworts squirt out a stream of sperm laden water from the antheidium such that female liverworts up to 1 metre away are fertilised.

When the male cell meets the female cell, they join and form a new diploid generation. The new plants forms in three parts, a basal 'plate' that anchors itself inside the female capsule, a stem, and a spore bearing capsule. The cells inside the capsule divide by meiosis and form haploid spores. The stem grows longer, the capsule is hoisted aloft and bursts open, spreading spores that will then grow into new liverworts. This sporophyte generation, the spore bearing capsule that arises from the junction of male and female cells in the archgonium, is very short lived.

Hornwort (Anthocerotophyte) Life Cycle.

In Hornworts, as in both Mosses and liverworts, the main plant occurs as the gametophyte haploid plant. The gametophyte plant grows to have the appearance of a leathery wrinkled leaf, up to 5 cm in diameter and several cells thick. Once each plant has grown to maturity there grows a female cell bearing organ - archegonium - and a male bearing organ - antheridium, both male and female, on the same plant. In some species there may only be either male or female on each plant. The male sex cell has to reach, by swimming through a moist external environment, a female cell inside an archegonium, and to achieve this they swim when the environment is wet enough. These sperm cells have two tails, they are biflagellate.

As with both mosses and liverworts, when the male cell meets the female cell, they join and form a new diploid generation. However, unlike liverworts, the new plants that form are horn shaped, very long lasting and capable of photosynthesis. But they also form in three parts, a basal 'plate' that anchors itself inside the female capsule, a stem, and a spore bearing capsule. The cells inside the capsule divide by meiosis and form haploid spores. The stem grows longer, the capsule is hoisted aloft and bursts open, spreading spores that will then grow into new hornworts.

So the cycle is very similar for mosses, liverworts, and hornworts.

Ferns (Pteridophyte) life cycle.

Let's finish with the ferns. The ferns - pteridophytes - 'ptera' means feather and suggests the shape of the fronds - are generally visible to us as sporophytes, that is, the diploid generation. These fronds of the sporophytes have the spore bearing bodies on their undersides, but some ferns bear the spores on seperate structures, like the royal fern (Osmunda regalis)which has what could almost be termed a flower spike. When the spore germinates and grows it produces a seperate, independent living gametophyte haploid generation, as a plant. These are generally small and insignificant and go unnoticed. But it is they that bear the haploid producing cells, the male and female cells, that then have to meet to produce a diploid generation. As with the bryophytes, the process of fertilisation has to take place in the environment and thus required the right conditions.