Mosses and liverworts, simple plants?

by Jan Parmentier

with photographs by the author

Mosses (Bryophyta) are quite interesting, simple green land plants with leaves and a stem and always without roots. In many mosses, the leaves are only one cell thick, except for the midribs, which are sometimes present. So the leaves are easy objects for the microscope. The plant is normally attached to the ground by delicate, colourless or brown threads, the rhizoids.

There are two major groups in the Bryophyta: Mosses (Musci) and Liverworts (Hepaticae).

Most mosses are found in areas with a humid and a cold to moderate warm climate. In the tropics, mosses are found especially in the mountains. In Europe, the south western part of Ireland is a paradise for mosses.

Mosses can reproduce asexually, by means of small clusters of cells or plates of tissue which break away and germinate to become new plants. Especially liverworts do this.

The normal, sexual method of reproduction however, involves special organs, the antheridia and the archegonia. These organs are the interesting parts for the microscopist who is interested in the biology of mosses. In the Musci the antheridium is the male organ, a delicate sac in which the male gametes are formed. It has a greyish or brown colour and an ovoid or globose form. It is a spectacular sight to see the male gametes, with two flagella, escape under the microscope from the antheridium. These antheridia are normally accompanied by numerous short filaments of cells, the paraphyses (see right image).

The archegonium is easy to recognize, with a shape like a little bottle or flask. So look carefully with a hand lens among wet patches of mosses, archegonia and antheridia are often found in special cups of leaves.

paraphysis with archegonium

I always try to study first some of the common microscopic objects in detail, especially their biology and then try to determine the names of the species that are more difficult to find. So I studied very common mosses to see some details of their reproduction.

The male gametes, escaping from the antheridia, need water to reach the egg in the archegonium. After fertilization, the egg develops in most cases into a spore-containing capsule on a stalk called a seta. Capsule and seta together form the so called sporophyte. At maturity, the capsule sheds the spores as a fine dust. The spores can be held back in wet weather by a mechanism whereby the teeth of the capsule close it.

Mosses can have one or two rows of teeth, (images left and below), an important aspect for the determination.

In the leafy liverworts, the antheridia generally occur in a packet-like swelling, the androecium, which develops on the lower portion of a modified leaf. The sporophyte develops from the archegonium. The seta is very delicate, often white and glassy, grows very fast and perishes after the shedding of the spores. The capsule is black, often globose or ellipsoid (see below).

In most species the capsule (sporangium) peels open in four sections (see above), exposing the spores and the elaters, cells with helically arranged moisture absorbing wall thickenings. (see below).

These cells are sensitive to slight changes in humidity, causing a twisting action that aids in dispersing the spores. The elaters are initially attached at both ends to the sporangium. Upon drying, one end of each elater snaps loose from the center of the sporangium, spreading the spores.

Marchantia polymorpha, a more complicated liverwort, is common in flowerpots in green houses, on moist bricks in gardens and on badly drained soils. On its leaves we can see small cups, (gemma cups) with small oval pieces of tissue, which can be spread by rain drops and become new plants (see right).

gemma cups

the male umbrella

This dioecious liverwort is known immediately by the male and female "umbrellas". These umbrellas carry the male and female receptacles. The numerous sporogonia develop on the underside of the umbrellas; each capsule contains spores and elaters.

the female umbrellas

You may call mosses simple plants, but important biological processes are easily studied by looking carefully at these sometimes fascinating plants.

Footnote: This is the classification adopted until recently and found in many books available on mosses and liverworts. The Phylum Bryophyta (mosses, liverworts and hornworts) has recently been split by taxonomists into three separate phyla: Bryophyta (mosses), Hepatophyta (liverworts) and Anthocerophyta (hornworts) e.g. see 'Margulis and Schwartz' in 'Further Reading' below.