Seeds can be dispersed in a number of different ways. They may be carried by wind, water or animals. Some plants even shoot the seeds out explosively. Seed size is an important factor.

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The seeds of flowering plants vary in size. Some are as small as grains of salt (e.g Foxglove), while others may be almost the size of golfballs (e.g. Horse Chestnut conkers). The difference in size reflects differences in the amount of food reserves stored in the seed for the benefit of the embryo plant inside. Usually, the larger the seed, the more food reserves it contains. This allows the germinating seed and young seedling more time to grow. It can then become well established before it must begin manufacturing its own food. The longer a seedling has before it must become self sufficient, the greater chance it has of becoming successfully established.

However, there is a down-side to having large seeds. The larger and heavier the seed, the more difficult it becomes to disperse it effectively by wind, or explosive techniques. All of these require light seeds. Seeds such as Foxglove are minute and are easily blown about by the wind. Larger wind-dispersed seeds are generally heavier and therefore require features such as parachutes or wings to help keep them aloft. For example, Dandelion seeds have developed very light and fluffy parachute-like structures. These help the seeds to float in the wind and delays their fall to the ground. This delay allows the seeds to be carried further. The largest and heaviest wind-dispersed seeds, such as Sycamore cannot rely on hair-like parachutes to keep them airborne. They would have to be enormous to be effective. Instead they have developed a wing which causes them to spin through the air like mini helicopters. This again delays their fall.

The biggest seeds of all cannot possibly be dispersed by the wind. Large seeds such as nuts, are a valuable food for some animals. They are therefore often dispersed by animals which collect them to eat. Rarely are all such seeds eaten. Some will usually be overlooked, leaving them to germinate wherever they have been left when conditions are right.