Grasses Ratchet Their Seeds Into Distance Travelers
John Muir said we should not pity plants as prisoners to one spot. In their own ways, they travel the world as we humans do. Anyone who has walked through wild dry grass may have been annoyed at how many foxtails get buried in their socks and how hard it is to get them out. The seeds were not engineered for socks, but for animal fur. Once embedded, they become successful hitchhikers. The shape of the seeds, along with tiny barbs pointing backwards, ensure that the seeds work their way deep into the fur. The seed may find itself the pioneer of a new land far from where it was born.
Charles Wolgemuth [U of Connecticut) talked about this in Current Biology.1 Many plants do more than just cast their fates to the wind; “some grasses, at least, are not so cavalier and have engineered their seed carrying appendages (spikelets) to increase dispersion and facilitate seed burial by converting periodic or random oscillations in the environment into directed motion.” Foxtails may look like dead, dry, inert things, but they can hitchhike, walk along the ground and even bury themselves into the soil. On the ground, they can take advantage of diurnal cycles of moisture and temperature. Their spikelets, called awns, expand and shrink, ratcheting the seed forward. Some of them even have spiral tips that can drill the seeds into the ground.
Wolgemuth noted other instances where nature has used ratchet mechanisms to good advantage: snakes, whose skin moves their undulating motions in one direction, and jellyfish and bugs that can use water or air motions to achieve unidirectional travel. Even inside the cell, molecular motors like myosin, the flagellum and ATP synthase (05/25/2009) ratchet up the random Brownian motion in their environment into linear or rotary motion. His discussion of ratchets in nature led Wolgemuth to speculate about ratcheting as a general principle of biology. Unfortunately, this won him Stupid Evolution Quote of the Week :
Biology has thus repeatedly found ways of producing net work by rectifying fluctuations with ratchets, and it is interesting to speculate on other areas where this mechanism may play a role. Evolution is one directly analogous system and a comparison between it and Brownian ratchets has been drawn previously. Clearly, random mutations in an organism’s genome lead to fluctuations in phenotype. Reproduction can lock in these variations, and natural selection then acts as a ratchet, reducing the likelihood of maintaining a population that is less competent at reproducing while increasing phenotypic populations that are fitter. A more tenuous comparison, though, comes to mind when I consider my own thoughts, which all too often seem quite random. I must consciously work to rectify these thoughts, plucking out the good ones and discarding the bad, in an attempt to construct an understanding of the world about me. Could my own thinking be working by trapping useful ideas from a pool of noise? One of the not-so-useful ideas, right? But, it has been suggested that certain nuclei in the basal ganglia act as a random motor pattern noise generator. If our brains can create noise, maybe they can ratchet it too.
Wolgemuth did not make a distinction between purposeful choice in matters of truth and falsehood, and mindless mechanisms without purpose or goal. He also did not distinguish between physical ratchet mechanisms and conceptual ones (argument from analogy), and ended up personifying biology as an engineer. On fitness as a ratchet for progress, see 10/30/2002, “Fitness for Dummies.”
Tip: To get foxtails out of your socks, don’t try to pull them out backwards. Push them through the cloth in the direction their natural ratchet wants to go.
1. Charles Wolgemuth, “Plant Biomechanics: Using Shape to Steal Motion,” Current Biology, Volume 19, Issue 10, 26 May 2009, Pages R409-R410.
One of the best videos of the motion of grass seeds is in volume one of Moody’s DVD set Wonders of God’s Creation. Time-lapse photography shows the seeds moving around like bugs and even burying themselves in the soil.
It’s a shame Wolgemuth had to turn an otherwise interesting scientific discussion about plant engineering into another shallow Darwinian speculation. If he is unable to separate his own thoughts from noise, why should his readers try to do so?
Tip: To get Darwinian foxtails out of your brain, don’t try to pull them out backwards. Push them through to their logical conclusions, where their blunt pointlessness becomes evident to all.