Most plants are only able to move by dispersal of their seeds or spores from the parent plant to a new location, where a child plant can germinate, take root and grow. Dispersal allows plant populations to get larger, and to reach new locations and habitats. This is especially important if the population has filled its local habitat patch, or the environment is changing (for example as a result of climate change) and the population needs to move somewhere with more favourable conditions.
Plants have evolved many different strategies and structures to give them the best chance of dispersing their seeds to appropriate environments. Some plants encase their seeds in fruit to entice passing animals to eat them. After carrying the seeds in their gut, the animals will deposit them onto the ground some distance away in their faeces. Other plants rely on the wind to disperse their seeds.
When a plant releases seeds, each one travels a different distance, depending on the journey that it takes after leaving the parent plant. If two wind-dispersed seeds fall off a tree, one may be released on a still day, fall vertically, and land close to the parent. The other may be carried metres or even kilometres by the wind. Because of this variation, dispersal distances are usually thought about in terms of probabilities, which define the dispersal kernel. For example, what is the distance from the parent plant within which half the seeds are likely to land, with the other half travelling further? What is the probability that a seed will end up as far as 100 metres away from its parent plant?