Time To Spread Out

We plant flowers in our gardens because they are attractive but plants have flowers for one thing, to reproduce, to make seeds. Having attracted pollinating animals or caught pollen passing on the wind, the plant then has the problem of how to spread the seeds. Some plants have mechanisms to catapult the seeds, but many plants again rely on animals or the wind.

Many plants such as brambles wrap their precious seeds inside succulent fruits stuffed with sweet sugars. The fruits are eaten by birds and small mammals, often attracted by key colours such as red or purple indicating the fruits are ripe. The seeds themselves have a protective coat that defies digestion and allows them to pass all the way through the gut to be dropped some time later in the creature’s poo which often acts as a fertiliser for the new plant. Even humans can play the role, sewage works often have large numbers of tomato plants growing in the sedimentation tanks in the summer.

Not all fruits are sweet and juicy, some such as oak acorns are packed with food and which are collected as a winter store and buried in the ground by squirrels and jays. Inevitably, some of the acorns are forgotten and so the jay has actually been a gardener planting the seed that is inside the acorn.

Some plants grow a fruit with a tasty oil packed addition called an elaiosome that is loved by ants. The ants take the fruit back to the nest to feed the oil to their young. The seed, which is now safely underground, is not eaten and will be ready to grow. This is how the highly invasive three-cornered leek manages to spread around our hedgerows.

Other plants such as burdock, cleavers and agrimony have fruits that stick on the outside of the animals. Their fruits aren’t juicy, they have bristles on the outside and these will catch on passing animals to be taken far away. In 1948, while walking his dogs Swiss engineer George de Mestral noticed the fruits of burdock clinging to his clothes and his dogs. This inspired him to replicate the hooks and the loops in his clothing fabric in a material he called velvet hook, in French velour crochet, shortened to Velcro.

Many plants produce very light seeds that can be dispersed by the wind. Most have a form that helps catch the slightest breeze. Dandelions and their relatives the hawkweeds and thistles, as well as the willowherbs have feathery extensions to the seed coat.

Trees in the Acer genus such as the sycamore, as well as hornbeams and limes (not the citrus ones) have winged fruits. Usually these are lopsided and they spin in the wind like mini helicopters, slowing down the drop from the tree canopy to make a longer journey. Fruits from the Norway maple trees on the triangle are often blown as far as the cricket pitch. The real master of this adaptation is the birch. The seeds are so light that they can blow for more than a kilometre and birch trees are often one of the first to colonise newly exposed ground.

Some plants are self sufficient, having mechanisms to catapult their seeds some distance. Usually the seed pod dries in a way that builds tension and then it snaps open. If you walk through a stand of Himalayan balsam in late summer seeds will be pinging in all directions at the slightest touch to the ripe pods. The champion is the dynamite tree Hura crepitans (translated as the crackling tree) from the Amazon jungle. The fruit looks like a small pumpkin with 16 segments. When it is ripe it literally explodes sending the seeds from 30-100m.

When there is a job to be done, nature will find a way.

Ed Dolphin