NATURAL SCIENCE: This passage is adapted from the essay “The Fen That Rose at Noon.”
At Lumen Fen, a shallow wetland enclosed
by pale dunes, the ground appeared to
breathe. Broad carpets of stitch-reed lay
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flat on the water at dawn. Near noon, some
carpets rose ten or twelve centimeters,
carrying snails, fallen leaves, and even
small stones upward. By nightfall they
settled again. The rise was too broad to
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be a wave and too regular to be random.
From shore, the motion looked like a tide
arriving in the wrong direction.
I joined botanist Mara Venn there after
Ilya Sorn, the fen's caretaker, reported
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the movement. Ilya suspected a buried
spring. He proposed that a daily pulse of
groundwater bowed the reed carpets upward
and that any air beneath them was trapped
only after they rose. Venn reversed that
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explanation. The reeds grew through a mesh
of threadlike algae, and she believed
oxygen produced by the algae gathered
inside the mesh. In her view, expanding
pockets of gas lifted the carpets; the
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water underneath merely filled the space
they left.
To test both ideas, we selected six
similarly sized carpets. Beneath each one,
we placed a pressure gauge that would
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detect a surge from below. We pushed
slender measuring stakes through the
carpets so that a change in height would
tilt them. Clear collection funnels caught
gas escaping from three carpets, while the
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other three were covered by identical
frames that held removable shade cloth.
Finally, I released a narrow ribbon of
harmless violet dye under every carpet. A
spring pulse should move that dye before
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the reeds rose, whereas gas forming within
the mesh might lift the reeds first.
The next morning, tiny beads appeared
among the algae soon after sunlight
reached the fen. At first, the stakes
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remained upright. As the beads joined into
silvery pockets, several stakes began to
lean, showing that their carpets were
swelling. Only then did the violet ribbons
widen beneath them as surrounding water
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flowed into the growing space. The
pressure gauges registered no surge.
Finally, the upper surfaces visibly
lifted, and gas began spilling into the
funnels. The order did not fit Ilya's
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prediction that water pushed upward first.
The carpets differed with age. Young ones,
woven from loosely crossing shoots,
released bubbles through many small
openings and scarcely moved. Mature
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carpets contained dense layers of dead
stems beneath their living tops. Those
layers held about three times as much gas,
and the mature carpets rose much farther.
Yet age alone was insufficient: even an
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old carpet remained low on an overcast
day. Its structure supplied storage, but
some other process had to supply the gas.
The shade experiment identified that
process more clearly. Shaded sections
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produced few bubbles and stayed nearly
flat, while neighboring sections beneath
clear frames rose normally. When we
removed the cloth at midday, the shaded
sections lifted forty-seven minutes later.
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Evidently, renewed light did not raise
them at once; the algae first had to
produce enough oxygen to fill the internal
spaces faster than it escaped. We also
pierced one raised carpet with hollow
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needles. It sagged within minutes as gas
hissed out, although the pressure gauge
below it remained steady.
Rising altered more than the carpets'
height. When a mature mat lifted,
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oxygen-rich water entered temporary
chambers underneath. Silver water-beetle
larvae moved into those chambers and fed
among the dangling algae. At dusk, gas
production slowed, bubbles escaped through
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the reeds, and the carpet descended. The
larvae departed before the chambers
flattened. During six weeks of
observation, the same pattern returned on
bright days, though the youngest carpets
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began participating only after their stems
thickened and accumulated a deeper layer
of debris.
The fen, then, was keeping two clocks. The
quick clock was daily: sunlight encouraged
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oxygen production, stored gas increased
buoyancy, and darkness allowed the carpets
to sink. The slower clock measured the
carpets' development from leaky young
meshes into mature structures capable of
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holding large pockets. Our tests strongly
favored Venn's explanation, though they
did not show that underground springs were
absent from the fen. What looked like
water lifting plants was chiefly a
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community of plants and algae lifting
itself—and briefly constructing habitat as
it did so.