Henry S. HornTree seeds that are dispersed by the windhave parachutes, wings or sails to slow theirdescent. This keeps them in the airflowlonger, allowing them to travel farther. Butthe assumption has been that a seed fallingin a forest is doomed to travel only a shortdistance, because the wind is impeded as itpasses among the trees.

To travel far, a seed must rise above theforest canopy on an updraft whose velocityexceeds the rate of fall of the seed in still air.If the updraft is part of a coherent rollingeddy, the seed might ‘surf the wave’ to agreat distance. To guess how far a seedmight get, it becomes important to know thesizes and lifespans of coherent wind eddies,but it seems that no one has made theappropriate measurements.

My colleagues and I have producedcomputer models that predict winddispersal over long distances, but it has beendifficult to convince others that our modelsare realistic (Nathan et al. Nature 418,409–413; 2002; Div. Distrib. 11, 131–137;2005). A visit to The Gates, Christo andJeanne-Claude’s temporary art installationin New York’s Central Park this Februaryallowed me not only to visualize coherenteddies, but also to measure their sizes andlocal lifespans. The measurements camefrom 57 photographs that I took with adigital camera on the afternoons of 24, 25and 27 February 2005.

The Gates in Central Park, New York City,1979–2005 comprised 7,500 gates, around 4 metres apart and 5 metres high, followingthe line of the paths through the park.Saffron-coloured fabric panels were hungfrom the top of each.

When there was no detectable wind oronly a light breeze, the fabric of The Gateshung vertically with minimal flutter. The panels billowed out to within 20�of the horizontal for winds recorded near the ground at 2 to 5 metres per second(www.cdo.ncdc.noaa.gov/ulcd/ULCD). I measured the ‘footprint’ of a coherent eddy by counting a consistent number of

contiguous billowing gates and calculatingthe distance they span. Footprints spanningat least 12–13 gates (about 45 metres) werecommon at wind speeds of 2 to 5 metres per second. From timed sequences ofphotographs of sets of gates, I recorded local lifespans of 32 to 57 seconds.

These records are biased toward shorterlifespans, as I chose to photographsequences only when the gates werechanging orientation rapidly. Some eddieslasted longer than 100 seconds, and thetheir footprints tended to move along a lineof gates at scales of about 100 metres.

Seeds that are kept aloft by updrafts in a45-metre coherent eddy, for 50 seconds, ina horizontal wind of 5 metres per second,could travel at least 0.25 kilometres. This isfarther than we thought, even though themeasurements behind the calculation are allsubstantial underestimates.

Thanks to Christo, Jeanne-Claude and TheGates, I now have direct quantitativeobservations, in moderate winds, of the

coherent eddies that are crucial to long-distance dispersal of seeds and other bioticagents. Praise is also due to Christo forhaving anticipated realistic sizes for thewind eddies that drive The Gates in hisconceptual drawings, which were madebefore construction of the work itself.

Of course, the aerodynamic presence ofThe Gates is part of the landscape that mayinteract with the generation andpropagation of coherent eddies. Somedetails are likely to be peculiar to CentralPark, and even to the installation itself.Nevertheless, the general pattern and itsspatial and temporal scales are highlysuggestive of features to be expected in anatural landscape. So the seed can indeedfall far from the tree.Henry S. Horn is professor of ecology and evolutionary biology and associate ofthe Princeton Environmental Institute atPrinceton University, Princeton, New Jersey08544-1003, USA. He is the author of The Adaptive Geometry of Trees.

NATURE|Vol 436|14 July 2005 BOOKS & ARTS

179

that a lot of suffering, war, and conflict couldbe eliminated if we could agree to live by themmore consciously.”

This conclusion would follow if our univ-ersal moral sense had been implanted by anall-knowing and all-loving God. But biologicalevolution is a notoriously amoral force. Innatemoral universals would have been shaped bythe selective advantages that arise from caringfor our kin and cooperating with our neigh-

bours, but nothing in our genes tells us thatslavery is wrong, or that men and womendeserve equal rights. Such insights emergethrough individual and group processes thatengage all of our faculties, including our innatemoral sense, but also the capacity to appreciateabstract arguments, formulate analogies, learnfrom experience, take other’s perspectives andso on. Much of moral progress consists ofusing reason to override our gut feelings.

An excellent illustration of why ethics doesnot reduce to instinct comes from Gazzaniga’sown treatment of issues such as stem cellresearch and euthanasia. This shows ethicalreasoning at its best — rooted in commonsense but also informed by a sharp, inquisitivemind and a deep appreciation of the facts. ■

Paul Bloom is in the Department of Psychology,Yale University, 2 Hillhouse Avenue, New Haven,Connecticut 06520-8205, USA.

Eddies at The GatesAn art installation hints that, even in a forest, wind may disperse tree seeds farther than expected.

SCIE

NC

E IN

CU

LTU

RE Seeing ghosts: a 58-second sequence showing the waxing and waning of a 25-metre eddy.

H. H

ORN

14.7 Books 177 NEW PJ 8/7/05 2:38 PM Page 179

Nature Publishing Group© 2005