Seed dispersal

Seed dispersal is the way seeds get away from the parent plant to a new place. "Dispersal" means to spread or scatter.

The basic idea is as follows. Plants cannot move after they have put down roots. It follows that it is an evolutionary advantage to get their seeds away from the parent plant. If the seeds take root nearby they will compete with each other and the parent plant. Also, a species is more likely to survive when its members are widely spread. This is because local disasters still leave plants in other places.^[1]

From the first land plants in the Silurian period for 300 million years to the Lower Cretaceous, virtually all transport of spores and seeds was done by mechanical means. In fact, for most types of plants both fertilisation and dispersal was done by wind. If not wind, then water was the medium.

A great change took place with the appearance of flowering plants in the Cretaceous. The story of flowers and insects is one of the best examples of co-evolution. The gut contents, wing structures, and mouthparts of fossilized beetles and flies suggest that they acted as early pollinators.

The association between beetles and angiosperms during the Lower Cretaceous period led to parallel radiations of angiosperms and insects in the Upper Cretaceous. The evolution of nectar in Upper Cretaceous flowers signals the beginning of the mutualism between hymenopterans and angiosperms.^[2]

What is true of fertilisation is also true of dispersal. Spores, the tiny products of lower plants, are almost always dispersed by wind. So are many seeds. Some seeds, and their later development, fruits, are obviously adaptations to a world full of animals. If they are dispersed by being eaten, it is advantageous for them to be nutritious and good to eat. So spores, seeds and fruits may get dispersed mechanically or by animals:

• Mechanical means:
  • Spores and seeds dispersed by wind are light, and get blown easily. An extra step is when the spores or seeds are blown out with force.
  • Spores and seeds may be fired out by force in some cases.
  • Seeds and fruits dispersed by water can float.
• Zoological means:
  • Many fruits are eaten by animals, and the seeds are dropped after the fruit is digested.
  • Some fruits are carried on the fur of mammals.

1. ↑ Ridley, Henry N. 1930. The dispersal of plants throughout the world. Ashford, Kent: Reeve. ISBN 0-85393-004-X
2. ↑ Stebbins G. Ledyard, Jr. 1974. Flowering plants: evolution above the species level. Harvard.