More on mosses

Mosses have a complex and very interesting reproductive cycle: individual plant specimens are either male or female (although some have both male and female portions). The male plants produce gametes in the antheridium, which looks rather like a pepper-pot lid:

Antheridium on male moss plant

These male gametes swim towards the female gametes, which are held in the archegonium on the female plants. Fertilisation takes place and the offspring grows upwards, forming the seta ('stem') and eventually the spore-bearing capsule. This sporophyte looks like it is part of the original plant, but is actually parasitic on it, so when you see a spore capsule forming like the one below, it isn't one plant with green leaves and a spore capsule, it's the mother (leaves) and child (spore-producing sporophyte).

Emerging spore capsule on Polytrichum moss

Here's a shot of last year's capsules:

Old capsules of Polytrichum commune

The specimens I used for these photographs are Polytrichum commune, which is one of our larger mosses. The setae regularly reach over 8 cm. long, which makes them an ideal subject to use for initial moss studies, since everything is large enough to be seen with the naked eye.

The fact that the male gametes swim towards the females gives us one reason that mosses flourish in damp places.

Moss life-cycle

I have been trying to decipher the life-cycle of mosses, and believe I now have it worked out. For so-called 'primitive plants', mosses have a complex life-cycle.

We'll start with the sexual generation, with male antheridia and female archegonia, which are usually - but not always - borne on separate plants. The following shot shows the cone-shaped antheridia of Polytrichum commune.


These produce the male gametes which swim (through water, hence the need for humidity around mosses) towards the female archegonia, which are tiny pockets on the stem of the female plant:
Fertilisation takes place, resulting in the sporophyte, or asexual spore-bearing generation, which grows as a parasite on the female shoot. So although the sporophyte - which consists of the seta (capsule-bearing 'stem') and the capsule (spore-bearing container) - appears to be part of the female plant, it is only 50% hers.

This shot shows the immature sporophytes of Polytrichum commune.


And these are the mature sporophytes with open capsules: