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.