The double life of a bryophyte
The most primitive group of land plants on earth are bryophytes, which have incredibly been on this planet for 400 million years or more. The bryophyte group consists of mosses, liverworts and hornworts. Like all land plants, they evolved from slimy green algae, most likely to have come out of fresh water. As a primitive group of land plants, bryophytes have retained their ancestor's dependency on a covering of water to reproduce. Therefore, bryophytes are only found in damp habitats. Bryophytes have a two-stage lifecycle, or in other words their generations alternate. These two stages/generations are called gametophyte and sporophyte. The gametophyte structure reproduces sexually and possesses haploid cells (cells which contain single sets of unpaired chromosomes), whereas the sporophyte structure reproduces asexually and possesses diploid cells (cells which contain pairs of chromosomes). The advantages of these two reproductive strategies are that sexual reproduction creates genetic diversity within a population and asexual reproduction produces large numbers of individuals quickly. The dominant stage of bryophytes is the gametophyte. This means that what we would typically recognise as the main bulk of a bryophyte (including the mass of tiny leaf-like structures) is the gametophyte. In contrast, the sporophyte is a long thin structure that pokes out above the main plant mass, depends on the gametophyte for nutrition, and is not always present (see image). Let’s start with the gametophyte stage. As previously mentioned, gametophytes possess haploid cells, and they are also either male or female, producing either sperm or eggs. These germ cells are haploid like the gametophyte. Splashes of water transport the sperm from the male to the female and allow the sperm to swim to the egg. When the sperm fertilizes the egg a diploid individual forms and grows to become a sporophyte.
Sporophyte structures consist of a seta (foot) and a stalk that is topped by a capsule (see image). They possess diploid cells, like the cells in our own bodies. The capsule contains many single cells called spores, but these spores are haploid. The new individuals that grow from the spores are haploid gametophytes, and so the cycle starts again. So how do the spores spread? The capsules of the sporophyte stalks disperse spores when the capsule dries out causing the seal of the capsule (the operculum) to shed. The wind then carries away the spores. In this way, bryophytes effectively disperse their offspring which can increase chances of survival. Many mosses have teeth-like structures (peristome teeth) around the opening of the capsule which bend outward as they dry out, until a large enough gap is created through which the spores can leave. Moss species can often be identified by the character of their peristome teeth. A type of moss called ‘Sphagnum’, which covers more than 1% of the earth’s surface, has a more impressive trick. As the wet, spherical Sphagnum capsules dehydrate in the sun, the pressure inside them increases and they become more cylindrical. When the pressure reaches a critical level it causes the capsules to erupt. The spores get launched ballastically in a mushroom cloud of air and rise at least 10cm (which is pretty good going for a tiny spore)! The explosion exerts an internal pressure of four to six atmospheres, a pressure equal to that of the "huge tyres of heavy trucks".
Photographs taken by author, ©Kate Dey, 2014.
Crum, H. A. 1973. Mosses of the Great Lakes Forest. Contrib. Univ. Mich. Herb. 10: 1-404.
Glime, Janice M. 2007 Bryophyte Ecology. Volume 1. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Accessed 21 February 2014 at http://www.bryoecol.mtu.edu/.
National Geographic, (2001), All Land Plants Evolved From Single Type of Algae, Scientists Say, [online], Available at: http://news.nationalgeographic.co.uk/news/2001/06/0604_wirealgae.html Accessed 21 February 2014.
Nature, (2010), Moss releases its spores as ‘mushroom clouds’, [online], Available at: http://blogs.nature.com/news/2010/07/moss_relases_its_spores_as_mus.html Accessed 21 February 2014.
Royal Botanic Garden Edinburgh, (2014), Bryology (mosses, liverworts and hornworts), [online], available at: http://www.rbge.org.uk/science/cryptogamic-plants-and-fungi/bryology Accessed 19 February 2014.
