Spore dispersal in Bryophytes

SPORE DISPERSAL IN BRYOPHYTES

The sporophyte generation is short-lived. It comprises a capsule which produces spores by meiosis and a stalk which holds this aloft the gametophyte.

The spores once released are dispersed by air currents and, once they settle somewhere moist, germinate. This recommences the gametophyte generation. The spore first produces a filamentous stage called a protonema. These cells are full of chloroplasts.

Late (left) and earliest stages (right) of a moss protonema.
This short-lived phase develops into the familiar gametophyte of the moss or liverwort.

Capsule Structure

The liverwort and moss capsules are very different. In essence, the liverwort capsules is simpler, containing spores and a second cell type called elaters. The moss capsule has several tissues.

L.S.
Liverwort capsule & stalk

calyptra (cap)
operculum (lid)
annulus (ring)
peristome
columella
spores
air-space

apophysis

seta (stalk)

L.S.
Moss capsule

.	Moss	Liverwort
1. Internal tissues	generally complex	2 cell types - spores & elaters
2. Stalk	stronger, slow-growing	grows rapidly (hours!) once spores ripe
3. Photosynthetis	in apophysis of some mosses	No
4. Stomata	on apophysis of some mosses	None
5. Conducting tissue	in stalk of some mosses	None

Liverwort Spore Dispersal Mechanisms

Under dry conditions, the liverwort capsules splits into valves or segments, exposing the spores and elaters. Elaters undergo hygroscopic movements causing spores to be flicked into the air (e.g. Pellia)..

exposed spore/elater mass at centre of open valves
spores (red/purple) and elaters (blue)

Elaters are dead water-filled cells. Two forces are key here - cohesion between water molecules and adhesion between water molecules and the elater wall.

Under dry conditions water is lost and the elater walls are pulled inwards.

The elater walls have spiral bands of thickening so that as more water is lost the elaters assume a twisted form. The water is in a state of tension. The water "wants" to remain as one cohesive mass. It also "wants" to adhere to the elater wall.

More water is lost. This shifts the balance. The attractive forces between the water and elater are overcome. The elater snaps violently back to its original shape. The water remains as a cohesive mass.

Cephalozia is an extreme example. The elaters are coated in spores and are attached at one end to the capsule wall.

the capsule splits open
violoent hygroscopic movement of elaters detaches them from the capsule wall, flicking them and spores into the air

Riccia is at the opposite extreme. It is an aquatic, thallose liverwort whose capsule remains embedded in the gametophyte thallus - it has no seta! The spores are few in number and large and are possibly animal-dispersed.

Moss Spore Dispersal Mechanisms

With the exception of one group of mosses (Graphite mosses), all moss capsules have an operculum and peristome.

The general dispersal mechanism is as follows. Under dry conditions,

the calyptra (remnant of the archegonium) drops off
the operculum is shed as a result of water loss by the annulus
the peristome teeth bend outwards
spores fall out of the capsule and are carried by air currents

The peristome teeth are triangular two-ply structures which operate like trap-doors. One layer tends to readily absorb or lose moisture while the other has little affinity for water. What this means is that as water is lost one side of the peristome teeth shrinks while the other does not. This results in a bending of the teeth outward.

Here are some interesting variations on this theme.
(Diagrams after Ingold, 1971)

*Funaria* Tips of the curved peristome teeth fuse in a central disc. Wet Teeth elongate and slits between teeth disappear. Dry Teeth shrink and gaps develop between teeth, allowing spores to sift out.
*Polytrichum* The central disc or epiphragm is here very large and the peristome teeth are tiny.The peristome do not respond to moisture. Wet Epiphragm loose, teeth not stretched, gaps between teeth disappear. Dry Epiphragm taut, teeth stretched, gaps develop between teeth, allowing spores to sift out. Photo by Alan Hale
*Tortula* Peristome teeth long and hair-like. Wet Teeth elongate and are tightly wrapped around each other. Dry Teeth shrink, disentangling, allowing spores to sift out.
*Sphagnum* Grows in temperate swamps ('bogs'). Dead plants provide peat moss. Capsule contains air and spores. No peristome. As the capsule dries, it shrinks, compressing the air inside (up to 5 atms!). The operculum is shot off and spores with it. This actually makes a popping noise.
*Splachnum* This grows on dung and the capsule with its broad, skirt-like apophysis resembles a flower! The peristome form a fringe at the top of the capsule. The columella sticks out of the top, covered in sticky spores. Flies are attracted to the capsule by its smell. The spores stick to the flies' feet and are soon dispersed to another load of #@%& !

Click here to find out more about moss capsules.

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