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Class Level and Curriculum Links
Senior Primary Students
SESE Science:
Strand: Living Things
Strand Unit: Plant and Animal Life
* Understand some ways in which plants reproduce.
Junior Certificate Science
Leaving Certificate Biology:
1.5.5 Organism Adaptations
Seed dispersal on the bog
Seeds, or fruits containing seeds, are dispersed in four main ways: by animals (as food or caught in coats); by wind, by the plants themselves; and by water. The size, shape and colour of the fruit or seed together with other features all reflect the method of dispersal.
Animal Dispersal
Food sources - the juicy fruits and seeds
Fruits and seeds are an important source of food for many animals,
particularly birds. The fruits or seeds are often shiny and brightly
coloured, or at least with a colour that contrasts with their
background, for example black or white. This makes them easier
to find, especially for birds as they see colour well. The colours
of fruits only develop when they are ripe and ready to be dispersed.
Once eaten the fruits pass through the animal's digestive system; the seeds are protected by a hard resistant coat, and are then deposited in the animals's faeces. The faeces may help the seed in its early stages of growth by supplying a small amount of fertiliser.
Bilberry is mainly dispersed by birds. The shiny berries contrast with the leaves making them easier for birds to find.
Crowberry and cranberry have seeds like stones which are surrounded by a fleshy fruit. Birds often carry them away to eat in a safe place. The fruit is eaten and the stone, containing the seed is dropped.
Food Sources - the takeaways
These form an important food source for many animals, especially
mammals, but are too large to pass through the digestive system.
When ripe they drop to the ground where they are easily found
and carried off.
Carried on coats - the hitch hikers
Fruits or seeds many be accidentally carried in mud or the feet
of animals, on human shoes or on vehicle tyres. The plants in
this group have fruits or seeds which are specially designed
to cling or stick to animals.
Wind dispersal
Some seeds, like those of orchids, are so small that they are
carried in the air like dust. However most seeds are larger and
have special structure (wings oar parachutes) to help them stay
in the air longer and travel further.
The winged seeds
Birch fruits are produces in large number in dangling catkins,
which fall apart when the fruits are ripe. The wall of each small
fruit is extend into two wings which help it to float through
the air. Scots pine seeds are produced between the woody scales
of the cone. As the seed ripens, the cone opens, releasing the
winged seeds which spin as they fall and drift away from the
tree.
The parachutes
Another way of helping a seed to stay in the air is to suspend
it below a parachute. A variety of parts of the plant are used
to make these devices. Each small seed of bog cotton has a tuft
of long silky hairs attached to one end of the seed. This hairy
device enable the seeds to be carried in the wind or in air currents
for considerable distances.
Self dispersal
Seeds are either flung away from the plants when the fruit is
ripe (exploders) or shaken out of the fruit (the pepperpots).
The exploders
In many members of the pea family such as vetches and gorse,
the fruit wall contains special thick-walled cells. These cells
force the fruit walls to twist. Eventually the two halves of
the twisted pod are pulled violently apart shooting out the seeds.
The pepperopots
The fruits form a capsule or pot full of relatively small seeds
or spores. The capsules are arranged on stiff springy stems.
When bent back by a passing animal the stem springs forward again
shaking seeds out of the pot. Strong winds may also shake the
seeds out. The hairy cap moss on bogs has a small brown capule
filled with spores. The capsule is protected by a hairy hood
or cap until the spores are ripe. Then it falls off and the spores
can be shaken out of the pot.
Water dispersal
This method of dispersal is important for aquatic plants living
in or near water. Their seeds all have special tissues which
create air pockets within them so that they float in the water.
Floating on currents disperses them to new pools or ditches.
A good example in bogs is bog bean which has flat orange floating
seeds. The seeds eventually sink to the botton of a pool or drain
where they germinate and grow.
