Dispersal by the elements
Seed size is a compromise between small size for dispersal and large size for germination and establishment. Wind-dispersed seeds may be tiny, often with specialized germination requirements, or have an attached sail. Some other plants have a flexible stalk but no adaptations in the seed. Many fresh water and coastal plants have seeds dispersed by water.
Fruit and seed eating
Seeds of fleshy fruited plants are dispersed internally by vertebrates and many have a resistant seed coat. Fruits relying on specialist frugivoresoften have nutrient-rich hard fruits, whereas smaller sugar-rich fruits are typical for opportunist frugivores. Some nutrient-rich seeds, particularly of trees, are hoarded by rodents and birds which leave some to germinate. Ants disperse some seeds a short distance mainly into safe germination sites.
Other modes of dispersal
Seeds with burs or hooks are effectively dispersed externally on mammals or birds. Some plants have explosive fruits that scatter the seeds. Many plants have no obvious adaptation for dispersal but high winds, floods and transport in mud on animals’ feet must be very important for long-distance dispersal. Some rainforest trees have almost no dispersal powers.
The measurement of dispersal
Effective dispersal is hard to measure. Initial dispersal is mainly close to the parent with rapid tailing off, but effective dispersal requires the seed to land at a safe germination site. The floras of oceanic islands have many bird-dispersed seeds, with fleshy fruits on wet islands, mainly burs on dry islands, and few wind-dispersed seeds, suggesting that bird dispersal is the most effective for long distances.
Seeds vary in their dormancy from days to centuries. Many plants of mature communities in predictable climates have no or little dormancy, whereas pioneer plants and those in unpredictable climates may have long dormancy. Dormancy may be (i) innate with all seeds requiring some treatment before germinating; (ii) enforced in unfavorable conditions; or (iii) induced by initial unfavorable conditions and then requiring particular conditions to germinate. Dormancy is caused by a thick seed coat or chemical constitution and is broken by abrasion or heavy rain or temperature extremes.
Variation in seeds
Some species produce seeds of constant weight but in others it varies considerably. Larger seeds are associated with dry conditions and small seeds with increased dormancy. Some plants produce more than one type of seed which differ in dispersal and dormancy characteristics.
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.
The large differences in seed size between species reflect differences in their modes of dispersal and germination. Seeds may be dispersed by wind,water or animals. 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 requirements involving mycorrhizal fungi or, for the parasites, a host plant, and the seed has few stored nutrients. Other wind-dispersed seeds or small fruits have a cottonysail, 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 vertebrates, usually birds or mammals, ingest the seeds andthey 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 specialist frugivorous birds frequently 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 damage the seeds. This is compensated for by large numbers of fruit produced with much lower nutrient investment in each.
Some nutritious seeds, particularly those of trees, are themselves a food source for vertebrates, particularly rodents and some birds such as jays and other members of the crow family. Some of these animals store the seeds, often burying them as a long-term food supply, after which they may not need them or they may forget where they have put them or may even be killed before they return.
Ants can be important seed dispersers and seeds dispersed by ants often have a nutrient-rich body attached to them which the ants remove once the seed is in the nest, before discarding the seed to a refuse pile or leaving it outside. The distances of dispersal are short and the main advantage is thought to be that the seeds cease to be so vulnerable to predation and are taken to a good germination site.
Seeds may be transported externally by vertebrates. Some plants have seeds with spikes or hooks formed by the fruit or other floral organs which attach themselves to fur (or clothing), such as those forming burs or awned grass fruits . Some of these may be dispersed mainly by one particular species of mammal, and may be adapted in some way to it. This can be most effective as a dispersal mechanism with long distances being covered and some colonizing oceanic islands.
Some plants have an explosive fruit which throws the seeds out, such as some legumes and balsams, although the distances covered are likely to be short and the seeds may have other mechanisms of dispersal as well, e.g. by ants.
For many seeds with no obvious form of dispersal, a chance strong wind or flood may transport them for some distance and many are transported inmudadhering to the feet of mammals or birds. Many of these seeds can remaindormant for months or years increasing their chances of dispersal. Frequently,dispersal by these methods will be distant and may be the most important for the future of the species, but is the least easily studied.
Some plants, particularly rain forest trees, have large seeds with no clear dispersal mechanism and almost no dormancy. In these, dispersal is extremely limited and they often have a restricted natural distribution and do not colonize islands.
True effective seed dispersal is notoriously hard to measure since, if a seed can lie dormant, chance dispersal may happen at any time over a long period. Initial dispersal is easier to measure. Studies on plants that are not dispersed by animals suggest that the great majority of seeds land close to the parent with a rapid falling off of numbers with distance (Fig. 1), but this is unlikely to reflect the effective dispersal unless the plant is in a new area and can colonize densely. A seed must be dispersed to a safe site for germination to be effective and, for many, this will not be immediately beside the adult. Safe sites may be common or infrequent and, if infrequent, at least a few seeds must disperse a long way. Colonizing species in particular must have effective long-distance dispersal.
We can infer the effectiveness of different dispersal types by examining the floras of remote islands. One remarkable fact is that, although some island plants are wind-dispersed and probably arrived by wind, the number is quite small and wind dispersal is rarer than on continents. This is underlined by the
fact that the most remote islands have the fewest wind-dispersed species. The commonest means of seed dispersal on oceanic islands is by birds and they appear to be the most effective long-distance seed vectors. Many plants on wetter islands have small fleshy fruits. On dry islands there are many burred plants, perhaps through shortage of water for fleshy fruits, and colonization appears to have come through external transport by birds. Sea drift is important for coastal plants. Chance events such as storms probably play an important part in driving the birds there since successful colonization needs to have happened only once in every few thousand years on average to account for the present floras, though much of it is likely to have happened in the early years.
Seeds vary considerably in their ability to go dormant. Dormancy in some can last only a few weeks or even days whereas in others it can last centuries. There are some relationships with climate and biome. Dormancy may be caused by the presence of a thick seed coat or by inhibitory compounds that can be broken by appropriate conditions . Trees of the tropical rainforest characteristically do not have dormant seeds and these will die after a few weeks, though there are exceptions. Some mangrove trees show no dormancy whatever and have seeds that germinate before they disperse from the trees; these possess a radicle that plants itself in the mud on release from the parent. In temperate zones many seeds go dormant for a few months but no longer. In contrast, ephemeral plants in deserts, many agricultural weeds and many pioneer species everywhere have seeds that can remain dormant for many years. In general, plants from a predictable climate and environment have less dormancy than those in unpredictable climates and the dominant plants in a mature community have less dormancy than colonizers.
Dormancy may be innate, in which case the seed requires a period of dormancy even in good conditions for germination . This is most clearly seen in many temperate plants that have a requirement for chilling before they will germinate, known as vernalization, and for seeds of fleshy fruits which require passage through a gut. Since many mammals and some birds across the world have become extinct, this latter requirement has proved to be a most unfortunate adaptation for some plants: one near-extinct plant from Mauritius has seeds that require passage through the gut of a dodo; some tropical American plants have seeds adapted to the extinct giant ground sloths. Other animals can sometimes provide a substitute but often germination is less and dispersal patterns will be different.
Dormancy can be enforced on seeds in unsuitable conditions such as waterlogging or lack of light and many seeds can be kept dormant in a cold dry atmosphere for much longer than in other conditions, a most useful fact for plant nurseries. Dormancy can be induced following an initial unfavorable period. These seeds will germinate immediately on release from the parent, e.g. in light or dark, given good conditions, but following a period in unfavorable conditions, they will germinate only in the light. There may also be annualcycles in the degree of dormancy of particular species, often giving maximal germination in particular seasons.
Seed size and weight can be remarkably constant in some species despite different sizes of adults growing in different conditions; the ‘carat’ measure in jewellery is named after the seeds of the carob tree, Ceratonia, because they were used as jewellers’ weights. Other species vary more depending on conditions experienced by the adult, with the weight frequently declining through a flowering season or varying depending on daylength or growth conditions. Smaller seeds frequently show greater dormancy than larger ones and plants growing in dry places often have larger seeds than those in wet places, perhaps because rapid growth is important to avoid drought. In some species there may be up to a 100-fold difference in weight between seeds on one plant. In grasses and composites, and perhaps others, the position of the seed within the inflorescence determines its size, shape and dormancy, and some species have two totally different types of seed (Fig. 2). In these, the seeds with the greater dispersal powers have less dormancy and it seems that these species can disperse in time and space using the different types of seeds.
Seeds that remain dormant in a soil build up a seed bank. Normal soils contain 100–100 000 seeds m–2 of surface, the higher numbers present in soil that is frequently disturbed. Usually, a seed bank consists largely of pioneer species and in disturbed conditions most species will be the same as in the standing vegetation. In mature communities, the seed bank is quite different in species composition from the existing vegetation and there may be almost no overlap in the species present since many species typical of mature plant communities have no or little dormancy. A full description of a plant community should include the seed bank. A seed bank will be constantly shifting and be different in constitution at different times of year, depending on short-term dormancy differences and timing of seed production. In most soils the viable seeds are nearly all near the surface although ploughing will bury some deeper.
Dormancy creating such a seed bank allows dispersal of seeds in time as well as in space. The seed bank of a species with potentially long-lived seeds can consist of the seeds from many different generations of the plant and, when favorable germination conditions arise, these generations will mix. This mixing will prevent any short-term genetic change from occurring.
A seed bank can be retained on the plant itself in a few places, particularly where fire is a regular feature of the community. Some conifers and plants of Australian savannahs, such as some Eucalyptus trees and other genera, retain dormant seeds in dry cones or fruits on the plant. They can remain there for decades until a fire breaks the dormancy and they germinate.
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