Braiding Sweetgrass

with a remarkable physiology to endure drought. Like the mosses
with whom it shares the rocks, lichens are poikilohydric: they can
photosynthesize and grow only when they are wet, but they are not
able to regulate their own water balance—their moisture content
mirrors the moisture in the environment. If the rock is dry, so are
they. A rain shower changes everything.
The very first drops splatter hard against the rigid surface of rock
tripe, which instantly changes color. The mud-brown thallus
becomes sprinkled with clay-gray polka dots, the tracks of
raindrops, which deepen over the next minute to sage green, like a
magic picture developing before your eyes. And then, as the green
spreads, the thallus begins to move as if animated by muscle,
stretching and flexing as the water expands its tissue. In a matter
of minutes it is transformed from a dry scab to tender green skin,
as smooth as the inside of your arm.
With the lichen restored, you can see how it got its other name.
Where the umbilicus anchors the thallus to the rock, the soft skin is
dimpled, with little wrinkles radiating about its center. It looks to all
the world like a belly button. Some are such perfect little navels that
you want to kiss them, like a little baby tummy. Some are kind of
saggy and wrinkled, like the old woman whose belly carried those
babies.
Since the navel lichen grows on vertical surfaces, the top will dry
out faster than the bottom, where moisture collects. When the
thallus starts to dry and its edges curl up, a shallow water-holding
trough forms along its lower edge. As the lichen gets older, it
becomes asymmetrical, the bottom half as much as 30 percent
longer than the upper, a legacy of lingering moisture that permitted
it to keep photosynthesizing and growing after the top half was dry
and still. The trough can also collect debris, the lichen equivalent of