Bryophytes
Did you know that all land plants evolved from a slimy green algae, most likely to have come out of fresh water?[1] The most primitive group of land plants on earth are bryophytes, which have incredibly been around for 400 million years or more. The bryophyte plant group consists of mosses, liverworts and hornworts. Their primitive nature means that they require water to reproduce – this is the reason why they are only found in damp habitats.[2]
Bryophytes have a two-stage lifecycle/ an alternation of generations. Firstly, there is the sporophyte stage. The sporophyte structure consists of a seta (foot) and a stalk that is topped by a capsule (you can see the stalks and capsules clearly sticking up in the blog image, which is of a moss). The capsule contains many separate single cells called spores. Spore cells contain two complete sets of chromosomes (they are diploid), like the cells in our own bodies, and these cells give rise to new diploid individuals when dispersed from the capsule. These new individuals that grow from the spores are called gametophytes and are the next life stage.[2]
The gametophyte structure is the green photosynthetic part that we would typically recognise (again, you can see this in the blog image as masses of green tiny ‘leaves’). It is attached to the substrate by threads (rhizoids). Gametophytes are either male or female, producing either sperm or eggs. These germ cells are haploid and so contain only one set of chromosomes. Splashes of water transport the sperm from the male to the female gametophye and allow the sperm to swim to the egg. When the sperm fertilizes the egg they form a diploid individual; a sporophyte. The sporophyte grows from the gametophyte and starts the cycle again![2]
So how do the spores spread? The capsules of the sporophytes disperse spores when the capsule dries out, causing the elaborate peristome teeth of the capsule to move, and the seal of the capsule (the operculum) to shed. The wind then carries the spores away.[3] A type of moss called ‘Sphagnum’, which covers more than 1% of the earth’s surface, has a more impressive trick. As wet, spherical Sphagnum capsules dehydrate in the sun, the pressure inside them increases and they start to become more cylindrical. At some point, the pressure causes the capsules to burst. Since the capsule is cylindrical and pressurized at this critical moment, 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)![4] It exerts an internal pressure of 4-6 atmospheres, a pressure equal to that of the "huge tyres of heavy trucks".[5]
Photograph: Bryophyte, © Kate Dey, 2014
[1] 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 21st February 2014.
[2] 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.
[3] Glime, Janice M. 2007 Bryophyte Ecology. Volume 1. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Accessed 21st February 2014 at http://www.bryoecol.mtu.edu/ .
[4] 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 21st February 2014.
[5] Crum, H. A. 1973. Mosses of the Great Lakes Forest. Contrib. Univ. Mich. Herb. 10: 1-404.
