Braiding Sweetgrass

quite loyal. They always choose Trebouxia as their algal partner.
The alga, however, is a bit more promiscuous, willing to hook up
with a wider array of fungi. I guess we’ve all seen marriages like
that, too.
In their shared architecture, the algal cells are embedded like
green beads in fabric woven of fungal hyphae. If you sliced a cross
section of the thallus it would be like a cake with four layers. The
upper surface, the cortex, feels like the top of a mushroom, smooth
and leathery. It is tightly woven of fungal filaments, or hyphae, to
hold in moisture. The dusky brown coloration acts like a natural
sunscreen that shields the algal layer, which lies just below, from
intense sunlight.
Below the shelter of the fungal roof, the algae form a distinct
medulla layer where the hyphae wrap themselves around the algal
cells, like an arm draped over a shoulder or a loving embrace.
Some fungal threads actually penetrate the green cells, as if they
were long slender fingers reaching into a piggy bank. These fungal
pickpockets help themselves to the sugars made by the alga and
distribute them throughout the lichen. It has been estimated that
the fungi take as much as half of the sugars the alga produces,
maybe more. I’ve seen marriages like that, too, one partner
siphoning off way more than he or she gives. Rather than thinking
of lichens as a happy marriage, some researchers view them more
as reciprocal parasitism. Lichens have been described as “fungi
who discovered agriculture” by capturing photosynthetic beings
within their fences of hyphae.
Below the medulla, the next layer is a loose tangle of fungal
hyphae designed to hold water and thus keep the algae productive
for longer. The bottommost layer is coal black and prickly with
rhizines, microscopic hairlike extensions that help attach the lichen