Bryophytes: mosses and liverworts
Collectively called Bryophytes
(the Greek bryon is a tree moss or oyster-green, phyton is a plant), mosses and liverworts are ancient plants dating back almost 500 million years – way before the dinosaurs, in what geologists call the Ordovician Period when the first sea urchins were developing in the seas, some of the older volcanic rocks of the Lake District and Wales were erupting and the Skiddaw slates were being deposited.
Mosses and liverworts are found only on land and in freshwater, and are unique among complex land plants in lacking the lignified vascular tissue that all ferns, conifers and broad-leaved plants (collectively called "higher plants") have. For this reason they can't conduct large amounts or water or sugar solution up and down their stems, so can't grow very tall. Many can survive long periods of drought, and rehydrate and start to grow again when properly wetted. Bryophytes reproduce and disperse through tiny spores rather than seeds, and also vegetatively by fragmenting, and by special structures called gemmae.
Like ferns, bryophytes have two life stages, which differ in the number of chromosomes In their cells. In “higher” plants and most animals, the dominant or main life stage is diploid – with two copies of each chromosome, one from its female and one from its male parent. A haploid cell or organism has only a single set of chromosomes. To reproduce sexually, diploid organisms first need to halve the chromosome count, producing haploid eggs and sperm. When these come together and fuse, the diploid chromosome count is restored and an embryo can form.
In mosses and liverworts by contrast, the dominant life stage is the haploid phase called the gametophyte
, because it produces gametes - eggs and swimming sperm. Fertilised eggs grow into the diploid or sporophyte
stage which is small and remains attached to the parent female gametophyte. It is confined to producing the next generation of haploid spores. When these germinate they grow into the next haploid gametophytes. The structure containing the spores is called a sporangium (plural sporangia), often held on a slender upright ‘stalk’ called a seta, which allows the minute spores to be released above the ground making for better dispersal.
This two-stage life cycle is sometimes call "Alternation of Generations" In some algae both life stages are equally prominent. In the ferns it is the sporophyte, in the bryophytes the gametophyte, and the sporophyte is usually very small and inconspicuous.