It’s that time of year when people are considering travel. Of course, 2020 has panned out very differently to recent years; some of us may be reading this in the local paper whilst on our holiday – welcome to our beautiful county of Carmarthenshire! Or perhaps we are planning trips with our families to visit those friends and relations we’ve been apart from for so long. Some of us may decide to stay put this year and keep close to home. Whatever your destination, it is unlikely to be as far reaching or unusual as some of the travels our plants and animals go on.
We are all familiar with the migration of animals like the swallows who will soon be lining up on the telephone wires, ready to head off to Africa – or the inevitable Manx Shearwaters that turn up inland when blown off course on their mammoth 10,000 km return journey from Skomer to South America. I wonder if they are surprised to hear the Welsh language all summer in Wales and then again over winter in Patagonia?
But have you ever considered the traveling habits of plants? How do they get around? Diversity is essential for life – whether that’s the diversity in the ecosystem that ensures we have oxygen and clean water; or the diversity of cultures, neurologies, and races that makes our world a more interesting and productive place. Plants migrate in order to promote species diversity and ensure survival. But how do they do this without legs or wings?
There are two ways that plants disperse their seeds. Autochory, which means spreading seeds by their own means, and allochory – spreading seeds with outside help.
Autochory involves a few familiar techniques. One of the most common uses gravity (known as barochory). At this time of year, the horse chestnuts are beginning to ripen in their hard, prickly seed cases. They’ll either drop when they are ready, or when the wind or a successful clunk from a thrown stick brings them falling to the ground, where the protective seed case opens to reveal a lovely shiny conker. Far more impressive in my opinion is the method known as ballochory – this is the method the ballistic plants use. Gorse is a familiar example, and I delight in those walks on the coast path on a hot summer’s day when the gorse pods dry out in the afternoon sun and with a tell-tale ‘pop’ their seeds are flung out far and wide with a delicious accompanying aroma of sweet vanilla and coconut.
Although trees can’t move themselves around, they certainly make use of animals that can. This is where the various types of allochory come in useful. Unsurprisingly, the use of animals to transport seeds is known as zoochory. I’ve noticed that the berries are beginning to ripen on the bushes and trees, and whether it’s a blackberry being eaten by a hungry badger, or a rowan berry being gobbled up by a blackbird, they share a similar fate. The tasty berry is enjoyed by the creature and the inedible seed passes through their digestive system and is deposited elsewhere – in a handy dollop of fertiliser. We have a friendly squirrel in our village that likes to hang about our oak tree. I’ve been keeping an eye out for it burying acorns in our lawn to keep it going through the colder months. Of course, many of the seeds cached by jays and squirrels go on to take root and grow into new oak trees and help our forests grow and spread.
Not all plants offer their animal hosts a tasty reward. My dog frequently comes in covered in barbed or sticky seeds that have clung to her fur and get rubbed off elsewhere. Some of these will grow into plants, but most get picked off by me – much to her annoyance! They are incredibly sticky and persistent.
The wind helps seeds disperse too. Spring can feel a nightmare time for hay fever sufferers. Pollen is produced in large amounts by trees like the hazel. They take advantage of the lack of leaves and produce catkins full of pollen that can be blown by the wind and hopefully come into contact with female plants. Taking advantage of the wind conserves energy. Instead of putting energy into providing tempting nectar for pollenating insects, they use the energy of the wind instead.
Other plants that make use of anemochory include the maples and sycamores with their ‘helicopter seeds’. Rather than falling due to gravity and bouncing or rolling to their new location, these seeds travel quite spectacularly using the lift from the wind.
Some seeds and fruit can travel for thousands of miles using hydrochory. Palm trees can grow from seeds that have travelled the ocean currents; and closer to home, foxgloves often grow near rivers where their seeds fall and float further downstream to grow in new locations. Some seeds are fragile and lightweight, whereas others have air pockets that keep them buoyant.
Finally, humans have been responsible for dispersing seeds for as long as we have been on this planet. Whether through trading in prehistoric times; or deliberately cultivating in agriculture; or accidentally carrying them with us when traveling the globe. Often humans are responsible for those ‘invasive’ species that were introduced as an exotic or decorative plant but ended up unmanageable and unwanted. Us humans have also been responsible for reintroducing plants, saving species, developing seed banks and ‘seed bombing’.
Whatever your travels this summer, why not take a look at the sedentary plants and trees when you are next outside and reflect on the journeys their species may have been on too.