Dissemination Of Seeds
A plant seed is a unique genetic entity, a biological individual. However, a seed is in a diapause state, an essentially dormant condition, awaiting the ecological conditions that will allow it to grow into an adult plant, and produce its own seeds. Seeds must therefore germinate in a safe place, and then establish themselves as a young seedling, develop into a juvenile plant, and finally become a sexually mature adult that can pass its genetic material on to the next generation. The chances of a seed developing are generally enhanced if there is a mechanism for dispersing to an appropriate habitat some distance from the parent plant.
The reason for dispersal is that closely related organisms have similar ecological requirements. Consequently, the competitive stress that related organisms exert on each other is relatively intense. In most cases, the immediate proximity of a well-established, mature individual of the same species presents difficult environment for the germination, establishment, and growth to maturity of a seed. Obviously, competition with the parent plant will be greatly reduced if its seeds have a mechanism to disperse some distance away.
However, there are some important exceptions to this general rule. For example, the adults of annual species of plants die at the end of their breeding season, and in such cases the parent plants do not compete with their seeds. Nevertheless, many annuals have seeds that are dispensed widely. Annual plants do well in very recently disturbed, but fertile habitats, and many have seeds with great dispersal powers. Annual species of plants are generally very poor competitors with the longer-lived plant species that come to dominate the site through the ecological process of succession. As a result, the annuals are quickly eliminated from the new sites, and for this reason the annual species must have seeds with great dispersal capabilities, so that recently disturbed sites elsewhere can be discovered and colonized, and regional populations of the species can be perpetuated.
Plants have evolved various mechanisms that disperse their seeds effectively. Many species of plants have seeds with anatomical structures that make them very buoyant, so they can be dispersed over great distances by the winds. Several well-known examples of this sort are the fluffy seeds of the familiar dandelion (Taraxacum officinale) and fireweed (Epilobium augustifolium). The dandelion is a weed species, and it continuously colonizes recently disturbed habitats, before the mature plants are eliminated from their rapidly-maturing habitats. Favorable disturbances for weed species such as dandelions are often associated with human activities, such as the demolition of old buildings, the development of new lawns, or the abandonment of farmland. In the case of the fireweed, it is recently burned forests or clear-cuts that are colonized by aerially-dispersed seeds. The adult plants of the dandelion and fireweed produce enormous numbers of seeds during their lifetime, but this is necessary to ensure to that a few of these seeds will manage to find a suitable habitat during the extremely risky, dispersal phase of the life cycles of these and other aerially dispersed species.
The seeds of maple trees (Acer spp.) are aerially dispersed, and have a one-sided wing that causes them to swirl propeller-like after they detach from the parent tree. This allows even light breezes to carry the maple seeds some distance from their parent before they hit the ground.
Some plants have developed an interesting method of dispersal, known as "tumbleweeding." These plants are generally annual species, and they grow into a roughly spherical shape. After the seeds are ripe, the mature plant detaches from the ground surface, and is then blown about by the wind, shedding its seeds widely as it tumbles along.
The seeds of many other species of plants are dispersed by animals. Some seeds have structures that allow them to attach to the fur or feathers of passing animals, who then carry the seeds some distance away from the parent plant before they are deposited to the ground. Familiar examples of this sticking sort of seed are those of the beggar-ticks (Bidens frondose) and the burdock (Arctium minus). The spherical fruits of the burdock have numerous hairs with tiny hooked tips that stick to fur (and to clothing, and were the botanical model from which the inspiration for velcro, a fastening material, was derived.
Another mechanism by which seeds are dispersed by animals involves their encasement in a fleshy, edible fruit. Such fruits are often brightly colored, have pleasant odors, and are nutritious and attractive to herbivorous animals. These animals eat the fruit, seeds and all. After some time, the animal defecates, and the seeds are effectively dispersed some distance from the parent plant. The seeds of many plants with this sort of animal-dispersal strategy actually require passage through the gut of an animal before they will germinate, a characteristic that is referred to as scarification. Some familiar examples of species that develop animal-dispersed fruits include the cherries (Prunus spp.), tomato (Lycopersicon esculentum), and watermelon (Citrullus vulgaris).
After seeds have been dispersed into the environment, they may remain in a dormant state for some time, until appropriate cues are sensed for germination and seedling establishment. Especially in forests, there can be a large reservoir of viable but dormant seeds, known as a "seed bank," within the surface organic layer of the soil. The most prominent species in the seed bank are often particularly abundant as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire, windstorm, or clear-cut. These early-successional species cannot survive as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire, windstorm, or clear-cut. These early-successional species cannot survive as adult plants beneath a mature forest canopy, but in many cases they can continue to exist in the forest as living but dormant seeds in the surface organic layer, often in great abundance. Species that commonly exhibit this strategy of a persistent seed bank include the cherries (Prunus spp.), blackberries, and raspberries (Rubus spp.).