Seeds have adaptive significance because they provide an efficient way for plants to reproduce themselves, and they serve as a mechanism for colonizing new areas.  Numerous dispersal strategies have evolved in response to survival needs, and plant dispersal is obviously related to plant distribution.  However, the dispersal strategy of a species sometimes fails to explain its geographic distribution.  For example, the adaptation for seed dispersal of Jewelweed or Touch-me-not (Impatiens biflora) is an explosive seedpod.  When the mature seedpod receives the slightest touch, there is an explosive discharge that flings the seeds outward for a distance of 8-10 feet.  If the larger distance is used, it can be seen that 528 years would be required for this species to travel one mile, and a distance of 100 miles would require 52,800 years.  However, the Wisconsin ice sheet was at its maximum only 11,000 years ago, and since that time Jewelweed has immigrated northward a distance of 300 miles or more.

It is clear that some factor other than an explosive seedpod was involved in this dispersion.  It is probable that the seeds were transported in mud on the feet of migratory birds.  For many, perhaps most, species seed dispersal may be accomplished by more than one agent.  However, in the evolution of dispersal mechanisms, natural selection has favored local dispersion.  Long distance dispersal, although an important aspect of plant geography, is apparently the result of chance.

Seed Size and Number

There is great variation among species in the size and number of seeds produced.  Size variations range from 0.000002 g in Rattlesnake Plantain (Goodyera repens) to 27000 g in the Coconut Palm (Lodoica maldivica).  Rattlesnake Plantain is in the Orchid family, which has the smallest known seeds.  Seeds from plants of this family have incompletely developed embryos and no nutrient tissue, and in order to germinate they are dependent upon a saprophytic association with a species of fungus.  The germination of less than 1 seed in 10,000 is not unusual for some species of the family.  The seeds of most other species are characterized by a well developed embryo, a store of nutrient material and a higher rate of germination.

In contrast to the variation in seed size and weight between species, there is great uniformity within a species.  The constancy in the weight of seeds is reflected in the use of the "grain" as the basic unit of weight in some measurement systems.  The weight of the seed of the Carob tree was used as a measure of gold described by a term which translates from Greek as "carat" or "Karat".  Seed size is an important factor in the survival strategy of the species, and thus constancy has been favored by natural selection.

The number of seeds produced by some species is phenomenal.  W. C. Muenscher has listed the number of seeds on a single plant in one season for the following species growing in central New York:

Tumble Mustard (Sisymbrium altissimum) 511,208

Pigweed (Amaranthus retroflexus) 196,405

Horseweed (Conyza canadensis) 243,375

Black Nightshade (Solanum nigrum) 178,000

Purslane (Portulaca oleracea) 193,213

In most plants the number of seeds matured is more than are necessary for survival of the species.  Those that escape seed predation eventually find their way into the soil.  Several studies have indicated large numbers of seeds in the soils of various habitats.

Habitat Type__________________ # Seeds/m² _

Arable Land 34,000 - 75,000

Early Successional Fields 1,200 - 13,200

Forest Stands 200 - 3,300

In contrast to seed size, there may be great variation among plants of the same species in the number of seeds produced.  Seed fecundity may depend on the vigor of the plant, successful pollination and environmental conditions during seed development.

There is a direct relationship between the size of the seed and the amount of stored nutrients it contains.  Seed production requires an expenditure of energy by the plant, and since most species have a limited amount of photosynthate that can be used in producing seeds, species with large seeds often produce relatively fewer seeds than species with small seeds.

Dispersal By Wind

Of all the agents by which seeds are disseminated, wind and animals are the most important.  Wind is less efficient then animals for two reasons:  (1) wind force is highly variable and unpredictable, and it may not be present at the optimal time for dispersal; (2) dispersal by wind is random with many seeds falling in areas unsuitable for germination.  Despite these shortcomings, anatomical modifications of fruits and seeds for dispersal by wind are common among seed plants.  In some plants the entire fruit is dispersed while in others the ovary ruptures to release seeds that are modified for wind dispersal.

Thin membranous wings are effective adaptations for dispersal by wind.  Examples are the fruits of maples, ashes, elms, birches and the Tulip tree.  Among Gymnosperms, winged seeds effect the dispersal of firs, hemlocks, larches, pines and spruces.  In the temperate deciduous forests of North America, 35% or more of the woody plants are dispersed by wind.  Winged diaspores are more abundant among woody plants than among herbaceous plants.

Another common adaptation for dispersal by wind is a tuft of hairs or a plume that functions as a parachute.  Although this mode of dispersal is found in willows and aspens, it is more common in herbaceous plants, and open vegetation.  According to one estimate, about 16.5% of American plants have fruits that are dispersed by plumes.  These include the cattails and members of the composite family such as Aster, Dandelion, Goldenrod, Thistle and Wild Lettuce.  Among the plants that have seedpods which rupture to release plumed seeds are the milkweeds, the dogbanes, and some members of the evening primrose family such as Fire Weed and Willow Herb.

In the tumbleweeds the whole plant serves as the diaspore.  These plants have a highly branched, bushy growth habit which gives them a globular shape.  When seeds are mature the plant becomes detached at the base and rolls with the wind, scattering seeds as it goes.  Some native members of the goosefoot and amaranth families are of this type, as are the introduced species Tumble Mustard and Russian Thistle.  Tumbleweeds are most abundant in the plains and deserts of western North America where they may be blown for miles.  Fencerows with long piles of tumbleweeds are not an uncommon autumn sight along many western highways.

The sizes of seeds will obviously influence the extent of their dispersal by wind.  The dust-like seeds of the Orchid family were referred to earlier.  In some species that produce more than 3.5 million seeds per seedpod, the slightest breeze will result in wide dispersal.  At the other end of the size scale, the larger the seed the greater the wind force necessary for its dispersal.

The seeds of many open vegetation herbaceous plants have no distinct anatomical modifications for dispersal.  The seeds either fall to the ground or are cast outward as the plant sways in the wind.  Seeds dispersed in this way are called wind ballistics, and the force of the wind will determine the distance of dispersal.

In some plants raindrops may influence seed dispersal.  The force of raindrops falling on seedpods or branches may depress these structures causing them to rebound with enough force to eject the seeds.  These seeds are referred to as rain ballistics.

Rain and wind ballistic mechanisms operate throughout the winter and may be observed in plants characteristic of early successional fields.  These include members of the mint family such as Motherwort and Heal-all, some members of the pink and snapdragon families, and evening primrose.  During winter a hard snow surface may enhance this method of dispersal by providing a smooth surface over which seeds may slide or roll for considerable distances.

Dispersal By Animals

Most seed ecologists agree that seed dispersal by animals is the most effective means for the greatest number of angiosperm species.  Dispersal by animals, especially vertebrates, has at least two advantages over wind.  First, in contrast to the erratic variations in wind, migratory birds and mammals move at predictable intervals.  Over long periods of time, this could exert a selective pressure in favor of plants with seeds that are mature during animal migrations.  Second, since animals usually move from one favorable environment to another, the seeds they are transporting are likely to be deposited in an area that is suitable for germination and growth.  In temperate deciduous forests more than 60% of the tree species have seeds dispersed by vertebrates, and in one beech-maple forest in New York 67% of the herbaceous plants produced animal-dispersed seeds.  Seeds are dispersed by animals in three ways:  (1) by ingestion; (2) by adhering to the outer surface of fur, feathers or feet; and (3) by transport for a stored food reserve.

Through the ingestion of fruits, seeds are transported in the intestines of animals.  By one estimation frugivores (fruit-eaters) account for the dispersal of about 12.5% of the seed plants in northeastern North America.  Birds are the most important of these, but mammals and reptiles are also fruit-eaters.  In more than 70% of the plants that have bird disseminated seeds, fruit ripening coincides with the onset of autumn bird migration.  During this season birds need reserves of energy for sustained periods of flight.  The fruits with high lipid content are eaten first and seldom persist into the winter.  Energy rich fruits, then, are a survival strategy of plants competing for dispersal by migratory birds.  Plant fruits that persist into the winter months are starchy.  These species have a lower energy investment in fruit production, and they take advantage of a time when food is scarce and birds have less choice in what they eat.

Sometimes seed accidentally enter and pass through the digestive tracts of browsing or grazing animals.  Such passage whether in browsers/grazers or in fruit-eaters often facilitates germination by softening hard seed coats.  W. C. Muenscher reported on the germination success of the seeds of 40 species of herbaceous plants after passage through the digestive tracts of the horse, cow, swine and sheep.  The seeds of the following species germinated after passage through each of those animals:

Catnip (Nepeta cataria)

Common Chickweed (Stellaria media)

Curled Dock (Rumex crispus)

Horse Nettle (Solanum carolinense)

Lambs Quarters (Chenopodium album)

Ox-eye Daisy (Chrysanthemum leucanthemum)

Pigweed (Amaranthus retroflexus)

Rough-fruited Cinquefoil

Tall Buttercup (Ranunculus acris)

Teasel (Dipsicus sylvestris)

Yarrow (Achillea millefolium)

Seeds or fruits adapted for dispersal by adhesion have hooks, spines, or a sticky surface.  These are usually herbaceous plants characteristic of temperate deciduous woodlands.  Examples include Enchanters Nightshade (Circaea sp.), Avens (Geum sp.) Bedstraw (Galium sp.), Tick-trefoil (Desmodium sp.) and Agrimony (Agrimonia sp.).  However, this type of dispersal is not restricted to woodlands and may be found in abundance in open vegetation and wetlands.

Many aquatic and wetlands plants produce small seeds with no special structures for dispersal.  The cosmopolitan distribution of some of these species is the result of dispersal in the mud on the feet of water birds.  This method probably accounts for the very rapid spread of Purple Loosestrife (Lythrum salicaria) in eastern North America.  This means of dispersal was recognized by Charles Darwin who counted more than 500 seeds from the mud on the feet of a wild duck.

Some seeds are dispersed by animals that transport and store them as a source of reserve food.  Most of the cache is eaten but the animal usually gathers more seeds or fruits than are used, or forgets where some of them are stored, and these give rise to the next generation of plants.  Large fruits such as acorns, beechnuts, hickory nuts and walnuts are stored by squirrels and chipmunks.  In one study it was observed that a single jay transported 4600 acorns over a distance of 4 km.

The seeds of many herbaceous plants have an energy rich appendage that attracts ants.  The ants carry the seeds to their nests where they consume the appendage leaving the seeds intact.  In hardwood forests of New York and West Virginia, ants are responsible for the dispersal of 36% and 30% respectively, of the herbaceous flora.  When ants were denied access, it was found that seed predators, chiefly mice, destroyed more than twice the number of seeds of Wild Ginger and Bloodroot as when ants were present.

Dispersal and Succession

In moist temperate areas, any bare space will, over a period of time, be occupied by a series of plant communities culminating in the climax vegetation, a community which maintains itself.  Most of the species that occupy the preclimax communities are fated for local extinction as a result of replacement by other species.  As a result of this transience they have been referred to as fugitive species.  This is an especially appropriate term for early successional species since they must constantly populate new habitats in order to survive.  In these species natural selection has favored large seed numbers, small seed size, and high dispersability.

Early successional species are also called colonizers or opportunists because of their ability to invade and occupy any space that becomes available.  As a consequence of high speed production and wide dispersal they have the ability to bring about a population explosion when conditions are suitable.  The weed species listed above are examples.  An early successional field in winter may be characterized by many remnants of opportunistic species.

For species that have a niche in the climax vegetation there is less demand for vagility.  Consequently, evolutionary emphasis has been on larger seeds with the accompanying reduction in offspring mortality, and lower dispersal capability.  Thus, theoretically, a seed fallout plate located in a climax forest should collect fewer seeds than one located in a successional field.

References

Cook, R.  1980.  The Biology of Seeds in the Soil, Demography and Evolution in Plant Populations, O.T. Solbrig, Ed., University of California Press, Berkeley, Ca.

Cox, D.  1985.  Common Flowering Plants of the Northeast, SUNY Press, Albany, NY.

Fahn, A. and E. Werker.  1972.  Anatomical Mechanisms of Seed Dispersal, Seed Biology: Vol. I Importance, Development, and Germination, Academic Press, NY.

Gleason, H. A. and A. Cronquist.  1964.  The Natural Geography of Plants, Columbia University Press, NY.

Harper, J. L., P. H. Lovell, and K. G. Moore.  1970.  The Shape and Sizes of Seeds, Annual Review of Ecology and Systematics, Vol. I, Eds:  R. Johnson, P. Frank, C. Michner, Annual Reviews Inc., Palo Alto, CA.

Howe, H. F. and J. Smallwood.  1982.  Ecology of Seed Dispersal, Annual Review of Ecology and Systematics, Vol. 13, Eds:  R. Johnson, P. Frank, C. Michner, Annual Reviews Inc., Palo Alto, CA.

Muenscher, W. C.  1936.  Weeds.  The MacMillan Co., NY.

Pijl, Vander L.  1972.  Principles of Dispersal in Higher Plants.  Springer-Verlag, NY.

Richardson, J. L.  1977.  Dimensions of Ecology.  The Williams and Wilkins Co., Baltimore, MD.

Stebbins, G. L.  1971.  Adaptive Radiation of Reproductive Characteristics in Angiosperms, II:  Seeds and seedlings, Annual Review of Ecology and Systematics, Vol. 2, Eds:  R. Johnson, P. Frank, C. Michner, Annual Reviews Inc., Palo Alto, CA.

Stiles, E. W.  1984.  Fruit for All Seasons, Natural History 93(8):43-52.