Some seeds are so thin and light they can be carried by the wind. Some can float for long distances on water currents, as they contain air. Some are expelled as the shells crack open. Seeds are propagated in other ways, too. Some plants reply on animals to transport their seeds, and they become like close partners!
Birds are the most important "partners" of seeds. To attract birds to propagate seeds, many plants bear fruits that are of suitable structures, colours, sizes and numbers for birds.
Tasty berries contain sugar and water that reward the birds that propagate the seeds for them. Most fruits are brightly coloured – yellow, orange, or red, while some are black, to notify birds they are ripe. The sizes are important too. In general, if fruits are too small, birds may find it hard to satisfy their appetite; if they are too big, birds cannot eat them. The sizes and shapes of the fruits fit the needs of the birds that do the work of propagation.
Some fruits have only one big seed, making it hard to swallow for a bird that eats it, which may have to eject the seed near the mother plant. Some fruits have numerous small seeds that can be eaten along with them. These seeds often have tough shells to protect them from being digested inside the birds' stomachs. After a while, the seeds will be discharged with the birds' faeces, and so may be propagated over long distances.
Ants are also important partners of plants. Studies show that, worldwide. 11,000 angiosperm species rely on ants for propagation. Most of them bear capsules, which eject the seeds when they ripen. Ants transport the seeds to relatively distant locations. To attract ants, there is white elaiosome on the seed surface, containing rich fatty acid. This is tasty food for ants.
After the ants transport the seeds to the ant nest and consume the elaiosome, the seeds are dumped in the ant colony's "waste dump". This dump has suitable humidity and temperature for seed germination. As other "rubbish" decomposes, it becomes nutrients for the seedlings.
Much like birds, many mammals are attracted by delicious fruits, and later expel indigestible seeds. There are other interesting ways of seed propagation. The short memory of some mammals is taken advantage of. For example, squirrels and rats collect nuts that do not readily rot and have high energy content, before winter begins. But they may forget the whereabouts of their collected food. These forgotten nuts may germinate in places far from the mother plants.
Seeds of herbaceous plants are tiny and often mix with soil upon falling on the ground. When animals step on the soil, the seeds may be attached to their feet and then travel to distant places. Some seeds develop special structures such as hooks, spikes and sticky secretions that help them become attached to animal fur and skin.
We may find many seeds on our clothing after a visit to the countryside. Next time, before leaving the countryside, brush your clothing so the seeds can return to the wild instead of entering the city. You too can become a partner of seeds!