The Life Cycle of a Moss

Spores of a moss can endure years of draught, travel hundreds of kilometers and wait silently for the day they germinate and create a new generation of their species.

First, what exactly are mosses? Mosses belong to the botanical division of the Bryophyta, which means that they are plants lacking a vascular system (the tracheid) which is used to pump water and nutriments through the body of trees and higher plants like flowers. The lack of this irrigating system accounts for the generally very small size of these plants: If they were bigger there would have to be a way in which water could be transported to the higher growing parts of their body.

As with every plant, the life cycle of a moss can be subdivided into several generations. Another characteristic of mosses which makes them stand apart from most other plants is the fact that they are mostly haploid, which means that their cells only have one set of chromosomes, in opposition to diploid cells, which possess a pair of identical chromosomes and which make up the great majority of higher plants excluding algae.

Germination of the Spore

The first generation in the life cycle is the haploid spore, a tiny seed with nearly no additional nutriments as reserve for the germination process, which is why spores begin germinating more slowly and need better conditions than usual plant seeds. This spore eventually develops into a protonema, an either filament or ball-like structure lying on the earth.

Growth of the Mature Plant

From the protonema grows a gametophore, a long stem with tiny leaves attached to it. The tips of these gametophores are where the sexual organs of a moss are located. Female gametophores carry the so-called archegonia, pear-like balls holding the female reproductive cells or eggs, linked by a canal called “venter” to the outside of the archegonium. Male gametophores carry the so-called antheridia, longish bags full of sperm-cells.

(In the case of monoicious a moss species, both the female and the male sexual organs are carried by one single plant.)
Sexual Reproduction Through Fertilization

The actual fertilization and thus sexual reproduction of a moss can only take place in the presence of water, as the male sperm swims to the archegonia and there enters a venter to swim all the way down to the egg cell, where they fuse and produce a diploid sporophyte: This generation possesses the sum of the two single chromosome sets of both the sexual cells and thus has a complete pair of them, which makes it diploid.

The sporophyte eventually grows by cell division and pushes it’s way out of the venter where it starts maturing, which takes from 3 to 6 months until it is fully developed: It now is a capsule closed by a cap and sheathed by the remaining bit of the venter, the calyptra. In this capsule, special cells produce haploid spores through meiosis, the process by which a cell generates two new cells each one having half the number of chromosomes of its diploid parent.

As soon as the spores are mature, the calyptra dries up and falls off and the spores fall out and get carried away by the wind or animals. The whole process repeats from here on.

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