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Monday, 2 April 2012

The way seeds spread

Putting down roots is good for plants but having roots means that plants can't move around, to find new and better places to live. The successful plants are the ones with seeds that travel somehow.

I once worked in a lab during a hot Canberra summer and each afternoon, wattle seeds would bang against one of the windows as seed pods broke open in the hot sun, curling back suddenly, sending seeds flying, but none of the seeds would have gone more than six metres from the tree which was three metres from the window.

Other plants have seeds that 'fly' away from the tree. Australian she-oaks and hakeas have winged seeds, and so do pines, but none of these would get very far from the parent tree. Maybe they carry a little further when the wind blows, but dandelions have a better way. Each seed is like a small parachute, so that even if it lands, wind gusts may pick it up and carry it further along.

A prize of £40 (forty pounds) was paid to a Mr Thomas Woolley for introducing a "New Rare Plant", some time after 1849. He displayed his plants at the Australasian Botanic and Horticultural Society summer Exhibition in November 1850, so it may have been in 1850, but it could have been no later.
Public domain 

Woolley's choice was the dandelion, regarded as a useful plant for treating many illnesses, including scurvy. By 1864, the dandelion's parachute seeds had spread it far and wide and it was no longer rare. In fact it was all around Sydney, "growing wild in the bush on the North Shore, both in flower and seed, and it is now growing wild and in great luxuriance about Braidwood", said a report of the happy Acclimatisation Society.

Bathurst burrs came from South America, either in contaminated seed or maybe on imported livestock. Plants were seen on the banks of the Nepean River, west of Sydney in the late 1830s and soon after, they crossed the Blue Mountains and got onto the Fish River, probably on horses' tails. An 1844 flood carried burrs down into the Macquarie River valley and after that the burr's clinging ability took it all over Australia.

Incidentally, and as a side issue, one of my hobbies is chasing down the earliest appearances of assorted Australian terms in print and recording these in a public database, and I recently added 'Bathurst burr' to the collection.
Three different magnifications of the barbs on a cobbler's peg seed.

It is called Early Australian instances of Australian language. It can also be accessed through a permanent tinyurl that is easier to recall:

If you have ever walked across weedy ground, you have probably seen how 'cobbler's pegs' attach to your clothing, but take a closer look at one of these clinging seeds and you may be amazed. To get a range of seeds like that, go and roll in some grass, or drag an old woollen garment through some weeds or undergrowth and see what attaches itself.

Other seeds are sticky. The cassia (Senna pendula) is a common weed in bushland across Australia, and it spreads because its seeds get sticky when they are wet. I suspect that birds eat some of the seeds, but fly away with other seeds sticking to them, seeds that later dry and fall off.

Other plants have seeds that can survive being eaten by birds, and some of them even have fruits that are designed to help attract birds to get them to swallow the seeds. By the time the seed passes through the bird, it is going to drop out a long way from where it was swallowed.

Now see if you can work out what tricks evolution has played to spread the seeds in your garden.

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