Life With Sex!

The Sex Life of Cryptogams

Many lichens and bryophytes produce sexual spores which are formed after the uniting of chromosomes from two individuals. Lichen and bryophyte spores are very small, mostly between one hundredth and one tenth of a millimetre in greatest length. Bryophyte spores are often more or less spherical in shape, though there are many species with angular spores. Lichen spores vary greatly in shape, from spherical to thin and thread-like.

A multi-celled lichen spore of an unnamed species.

Lichens in the genus Pertusaria produce their spores in a cylindrical structure called an ascus. This species has eight spores in each ascus.

A cluster of spores from the thallose liverwort Fossombronia wattsii.

Spores of the thallose liverwort Asterella drummondii.

Lichens

The disc- or cup-like structures and the wavy lines you see on various lichens are where the fungal partner produces its spores. Only the fungal partner produces spores, so the spores need to land near an appropriate photobiont to form a new lichen. Often this will happen after the spores land near a free-living photobiont, but the fungal spores from some lichens are able to germinate on another lichen and then hijack that lichen’s photobiont by killing the original fungal partner.

The cup-like structures, called apothecia, on a foliose lichen are where the fungal partner produces its spores. The asci line the apothecia.

Bryophytes

The leafy or thallose part of a bryophyte, which constitutes the bulk of the plant that you see, produces the sexual propagules — male sperm and female egg. Bryophytes need water to carry the sperm to the egg and once the egg has been fertilised the spore capsule develops. Depending on the species, a spore capsule may contain anywhere from a few dozen to many hundreds of tiny spores. While leafy liverworts and mosses look rather similar, they have quite different spore capsules. Moss spore capsules have an opening at one end, through which the spores are released. Leafy liverwort spore capsules either split open along the whole length or simply fall apart to release the spores.

In hornwort capsules the spores mature from the top down. The top of the capsule browns off and splits open to release the mature spores in that area. Then the spores from a little lower down mature and more of the capsule turns brown. The capsule opens a little more to allow the next lot of mature spores out, and so the process continues.

The spore capsules of a moss in the genus Bryum. While the spores are maturing there is a cap, or operculum, over the mouth of the capsule. The cap falls off when the spores are mature. In this species a narrow band of tissue joining the cap to the capsule unzips to release the cap — you can see this happening in a capsule near the centre of the photo.	Many mosses have one or two rings of teeth around the capsule opening. With changes in humidity these teeth curl in or out of the opening, picking up and releasing spores. You can clearly see the teeth in this electron microscope photograph of the mouth of a moss in the genus Hypnodendron.
	The thallose liverwort Fossombronia intestinalis bears its spherical black spore capsules atop flimsy transparent stalks.

The thallose liverwort Asterella drummondii seems to have quite complex capsules which look like umbrellas on black stalks. However each umbrella is not a single capsule but a compound structure holding four capsules, each of which is spherical.

This photo shows a fertile hornwort in the genus Phaeoceros.

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