Instant Notes: Plant Biology

The large differences in seed size between species (Topic D3) reflect differences

in their modes of dispersal and germination. Seeds may be dispersed by wind,

wateroranimals. Small seeds are likely to travel further than large ones and

can be produced in larger numbers at a similar cost to the plant, but larger seeds

contain more nutrient stores for germination and rapid establishment. Seeds are

a compromise between these opposing selection pressures.

Some seeds that are mainly wind dispersed are tiny, such as those of orchids,

heathers and some parasitic plants, and rely solely on their size for effective

dispersal. These plants often produce enormous numbers of seeds but many

have specialized germination requirementsinvolving mycorrhizal fungi (Topic

M1) or, for the parasites, a host plant, and the seed has few stored nutrients.

Other wind-dispersed seeds or small fruits (Topic D4) have a cottony sail, such

as many members of the daisy family, willows, Malvaceae such as cotton and

others. Other seeds are attached to a papery winged fruit (known as a samara)

which can be caught by the wind. Many trees such as the maples and birches

with larger seeds than those with cottony sails, have these. Many others have no

particular organs to aid wind dispersal but the seeds may be transported at least

a short way by the wind, particularly if the fruiting stalk is long and flexible.

Some have dry fruits opening at or near the tip so seeds will mainly be shed

when there is a powerful wind.

Many freshwater plants have seeds dispersed by water, in contrast to pollen

which is rarely water-dispersed, and many coastal plants have seeds that are

partially resistant to salt water so they can drift with ocean currents. Many

species that occur by coasts are widespread in distribution and colonize islands

easily.

Animals disperse seeds in several different ways. In those plants with fleshy

fruits(Topic D4) vertebrates, usually birds or mammals, ingest the seeds and

they may pass through the digestive tract. Many of these seeds have a thick

resistant seed coat and may germinate more freely after passage through a

vertebrate gut. Passage through a gut can take from a few hours to days or even

weeks and dispersal may be most effective, as shown by the fact that there are

many fleshy-fruited plants on oceanic islands. Some fleshy-fruited plants have a

seed that is discarded or regurgitated rather than ingested. Fruits dispersed by

specialistfrugivorous birdsfrequently have a different chemical composition

from those attractive to opportunists, with much more protein and fat and often

a larger seed. Opportunist frugivores take softer sugar-rich fruit and some

Fruit and seed
eating

Dispersal by the
elements

Normal soils contain 100–100 000 seeds m–2. In a frequently disturbed

community these will overlap with species present, but in a mature

community they may overlap infrequently or not at all, consisting mainly

of pioneer species. The seed bank will vary seasonally with fruiting

seasons and degree of dormancy. In seeds with long dormancy,

generations will overlap. A few plants in fire-prone areas can retain

dormant seeds on the plants for decades.

Related topics The seed (D3) Seed development, dormancy and

Fruits (D4) germination (H4)

Regeneration and establishment (L3)

Seed banks

188 Section L – Reproductive ecology