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.
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.
Liverwort capsule & stalk
|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 capsule 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)
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.
|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 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)
Tips of the curved peristome teeth fuse in a central
The central disc or epiphragm is here very
large and the peristome teeth are tiny. This peristome does not respond to
Photo by Alan Hale
Peristome teeth long and hair-like.
Wet Teeth elongate and are tightly wrapped around each other.
Grows in temperate swamps ('bogs'). Dead plants
provide peat moss.
This grows on dung and the capsule with its broad,
skirt-like apophysis resembles a flower!
to find out more about moss capsules.
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now completed our look at the Bryophytes.
Click the button to move on to look at the Pteridophytes.
© C. M. Sean Carrington 1997