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What is a bryophyte?

Bryophytes are a group of plants that include mosses, liverworts and hornworts.

Mosses may be small, but they may also be as complex as flowering plants. They have stems with leaves, and there is just about as much variation in the form and size of these plants as there is in the flowering plants. The 20,000 species range from being microscopic to over a metre; they may be upright, or creeping and much branched. They may grow in streams or deserts, on mountain tops or in sea spray, from the antarctic through tropical rain forests to the arctic, and in fact just about anywhere except in the sea itself.

Photo: Michael Lüth Tortula ruralis

Liverworts may be leafy and very similar to mosses (although the fruit looks quite different). Or they may form flat plates of apparently leafless tissue, in which case they are called thallose. Hornworts look like thallose liverworts, but have fruit that is unlike that of either mosses or liverworts. It is probable that mosses, liverworts and hornworts are not at all closely related, being united only by sharing their peculiar life-cycle.

Photo: Michael Lüth Saccogyna viticulosa

 

Photo: Michael Lüth Preissia quadrata

 

Mosses and Liverworts have been called up-side-down or rôle-reversal plants. The green and often leafy part underneath that we would think of as the moss or liverwort itself, is equivalent to tiny parts within a flower, or to a small, rarely seen part of the fern. The part that is equivalent to all of the flowering plant or fern that we normally see, is the fruit of the moss or liverwort.

As in all plants, and indeed animals, these two parts, or generations, alternate with each other in the life-cycle. The spores produced by the moss fruit will germinate into green leafy plants. These plants produce gametes, or eggs and sperm, and the resulting embryos grow up into new fruits. But what makes bryophytes different from all other plants, is that the fruit or spore-bearing generation remains semi-parasitically attached to the green gamete-bearing generation, and never becomes independent. The truth about this strange life cycle was not finally established until 1851!

Bryum bicolor

The gamete-bearing generation is the green plant that most people think of as the main plant. This is the part that traps light energy and converts it into food for both generations.

One of the most attractive and flower-like structures of mosses is found on the fruit. The fruit consists of a fruit stalk whose foot remains embedded in the moss plant, and a spore-producing capsule. When the spores are ripe, the lid falls off, but the release of spores is usually controlled by a fringe of flexible teeth round the capsule mouth.

Photo: Hattori Botanical Laboratory Moss Peristome

These are the peristome teeth (meaning ‘around the mouth’) and they are as varied and beautiful as any flowers, but very much smaller.

Moreover, they move as they are watched, since small currents of air cause changes in humidity, making the teeth flex in and out. These movements assist in the most effective distribution of spores, flicking out small quantities when air conditions are right, and stopping the process when they are not. Look at a ripe moss capsule with a hand lens, and your breathing will make the teeth flex.


 
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