As a general matter, animals move, plants don't.
That's why this time of year plants launch their seeds into the moving air, catching springtime breezes, or take advantage of sparrows, pigeons, seagulls, sheep, shaggy dogs, and attach their seeds to moving animals.
You and I, of course, are moving animals, and I think you'll be surprised by the number of seeds we matter-of-factly and totally unconsciously carry with us as we wander about. The numbers are a little startling.
Case in point: men's pants.
In the 1930's a British botanist, Edward Salisbury, bought himself some trousers, put them on and went for a walk in the country. In his classic book, Weeds and Aliens, he mentions that those trousers had cuffs (or, as he delightfully calls them, "turn ups").
When he got home, he removed his pants and carefully brushed off only those items that had gathered on the cuffs, which, with a few sweeps of his brush, now plopped onto a waiting newspaper.
Salisbury then looked, counted, and found "hundreds" of little seeds that he knew hadn't been there before. They had, he assumed, used his trousers to hitchhike from their parent plant to...well, to wherever Mr. Salisbury was going.
But he didn't stop there. He then planted all the seeds he found to see if they would germinate after their ride. And they did. From a single sweep harvested from one pair of cuffs, Salisbury raised more than three hundred plants, "comprising over twenty different species of weeds."
"Because of his great mobility," Salisbury wrote (projecting from his personal data set), "man is probably the most active agent — though usually an unconscious one — for external transport of seeds."
Seeds On Other Parts Of Us?
Trouser wool, we will all concede, is a good seed attractor. But what about something simpler, more natural? What about...mud?
When you walk across wet, muddy ground, mud attaches to your footwear, then dries, and eventually flakes off. Those flakes, it turns out, are also startlingly rich with seeds.
We know this because Sir Edward (our trouser-wearing botanist was eventually knighted for his discoveries) once visited a series of English churches and carefully swept the dust balls he found in the pews into sample bags.
The Church Dust Experiment
He wrote that he selected churches that had long paths from the street, so that worshippers who'd collected mud on their shoes would have more time for that mud to dry before they settled into their seats at church.
Salisbury reasoned that damp mud flakes would attract floating dust and therefore, inside every dust ball he expected to find whatever the mud brought in: namely "propagules" or seeds. And sure enough, when he looked, that's what he found: church dust provided a "very numerous" collection of healthy plants including daisies, chickweed, iron grass and several varieties of meadow grass. From dust!
Trousers Vs. Mud
In a trouser versus mud contest , which is the better seed carrier? I would probably put my money on wool. Wool comes from sheep and if you've ever seen a sheep after a couple of weeks in the field, you know it's a natural seed disperser. Mud, on the other hand, catches everything that falls, so it's not crazy to think there are thousands, literally thousands of seeds in every square foot of wet earth. But they have to get onto shoes to move. So which carries more seeds? Moving wool cuffs? Or moving muddy shoes?
Salisbury, naturally, checked the numbers. He cites a study from another botanist, H. T. Clifford, who just scraped raw mud off peoples' boots and shoes. Clifford, he reports, got 43 different plants from his shoe scrapes. (We don't know if that's a lifetime average or just a one-off, but 43 is, alas, not an impressive number.)
Not when compared to more than 300 plants from one pair of trouser cuffs.
Maybe Sir Edward was the better weed farmer, or H. T. Clifford a terrible mud scraper, but from the very limited data set available, it appears that wool gathered more seeds than mud.
Hooray for cuffs! Friends to the plant world!
Sir Edward Salisbury's major work on weeds and seed dispersal is, The New Naturalist: Weeds & Aliens (Collins, 1961); I learned about it reading Richard Mabey's new book Weeds: In Defense of Nature's Most Unloved Plants, (Harper Collins, 2010); Producer/Illustrator Vin Liota created our walkabout video; NPR's Adam Cole imagined drying mud in church. (Adam always gets the good stuff.)