Autumn is the time when many plants prepare themselves for the long cold winter. Apart from shedding leaves, there is one important job to be done to secure its future – seed dispersal
Nature has devised some pretty cunning tricks to use wind, water, birds, mammals, insects and others to complete the task of seed dispersal. Colour plays a vital role in deciding who is going to do what. In very general terms, birds see bright colours, mammals (other than humans) are colour blind but have an enhanced sense of smell, even slugs and ants have a role to play in getting seeds as far away from the parent plant as possible.
I first met the mango in the Far East, and have long been puzzled as to why such a delicious fruit should be so difficult and messy to eat. Nature’s answer is easy: the pulp was never meant to be divorced from the seed. The perfect seed carrier here is the elephant. Its gullet is wide enough to swallow the mango whole. The stone then travels through digestion complete, to be deposited many miles away, beautifully prepared for germination. As well as eating seeds and fruit, some animals collect and bury them to eat later but forget where and the seed germinates in situ.
Seeds such as foxglove are minute and are easily blown about by the wind. Larger wind-dispersed seeds are generally heavier and need features such as parachutes or wings to help keep them aloft. I now blush at the huge populations of dandelions that I have wantonly distributed in my time! As gardeners, we know that sycamore and ash seeds are unbelievably successful with their helicopter spinning blade structures.
Plants such as burdock and stick willy have hooks to which the seed is attached. These hooks easily get caught up in the fur of mammals as they pass by the plant. At some point the hooks will release their payload and if the conditions are right, the seed will germinate.
Some plants, mainly in the pea family, distribute their seeds by violently ejecting them so that they fall well away from the parent plant. As the pods dry in the sun tensions are set up in the wall of the pod, eventually causing it to split. As the two halves spring back they flick out the seed. Gorse is a very good example of this. Sitting near gorse bushes on a hot day in summer can be like sitting near a firing range as the exploding pods sound almost like gun fire.