A Jolly Good Phelloderm
Hidden within the bark of a boxelder is harbored a secret green! When we think of green on plants we think of leaves, and indeed this is where chloroplasts, those tiny organelles in plants that harvest sunlight and water to make sugars, are most abundant and visibly obvious. I tell my students that whenever you see green on a plant it’s telling you “Photosynthesis!” It doesn’t matter if the green is on a fruit or leaf, it’s always caused by the presence of chloroplasts. In the case of immature fruits that are green before they ripen to red or blue or whatever, the fruit is pulling its weight with photosynthetic skin that creates some of the sugars that get stored in their tasty little fruit bodies. This takes the burden off the leaves so that much of their photosynthate can be used for growth.
Green in the bark
But there’s another spot where chloroplasts can be found: the bark. A few trees have twigs that have a characteristic smell when scratched (black cherry twigs, for example, smell like almonds), and today I was scratching black and pin cherries to try and tell the difference by smell (it’s tough). Only the “green” or still living twigs carry the scent. The scratched surface reveals a thin green layer of tissue. This layer, the phelloderm, or cork skin, is produced by the cork cambium and laid down just inside of the cambium. Cork cambium (phellogen), cork (phellem), and phelloderm make up the periderm, which is what you think of when you think of bark. Phelloderm isn’t present on all species and it’s also not present on mature bark.
Phelloderm’s primary function is to photosynthesize. Because it photosynthesizes, phelloderm needs access to light and is only found in young tissues where the cork hasn’t thickened enough to block all light from penetrating to the deeper tissues. Some species maintain thin cork so that even at maturity their bark can photosynthesize. The phellem of aspen, for example, turns to a powder and washes off the trunk, which maintains a thin bark well into maturity.
Why photosynthetic bark?
Photosynthetic bark is more the rule than the exception, at least in the far north. In colder climates, photosynthetic bark is a huge boon in both the fall and the spring. Unpredictable temperatures that oscillate back and forth from above to below freezing pose a significant risk to the leaves of our deciduous plants. So they drop their leaves and weather out the winter without their primary photosynthetic structures. It’s a gamble trying to hold leaves long enough to get a leg up on the competition, but if you wait too long then your leaves can die on the twig (more on this next). Having phelloderm allows a tree to take advantage of warm, sunny days without risking frost damage to the sensitive tissue of young leaves.