I’m speaking with a 60 year old Black Locust, Robinia pseudoacacia, who grows in the village parkway in Wilmette, Illinois.
Gil: Mr Black Locust I couldn’t help but notice the beautiful flutes and ridges in your bark. I had to stop and talk. Why is your bark so different from other trees, like the smooth bark of a Beech for instance or the white bark of a Birch?
Mr. Black Locust: Well let me first clear up one misconception you have concerning me. I am not a Mr. neither am I a Ms. My species is monoecious which means that I am both male and female. Now you're probably wondering how I get cross pollination to insure genetic variation so that my species is able to withstand the challenges of diseases and climate variation. Right? Well I send my male pollen to other Black Locust trees and my flowers accept other trees’ pollen first. If all else fails and I don't receive other trees’ pollen in my flowers, then I am self pollinating as well. Pretty neat, aren't I? So you may call me Blackie.
Gil: Please excuse me Blackie. Would you mind talking to me about your bark?
Blackie: You should know that tree bark is not like people bark, or skin, as you like to call it. You should know the difference, because people are always doing things like nailing signs to our trunks, or wrapping wires around us, or weed whacking our bark to get the grass cut. It’s not a laughing matter, because it hurts! Unlike human skin, tree bark never heals. Once it has been damaged, it remains damaged for our whole life.
Gil: I thought trees healed over their wounds!
Blackie: This is a misconception that people have because they anthropomorphize. That means that you people think trees, and every thing else, are like you. Now when you cut your skin it heals, which means that damaged tissue is replaced by new healthy skin cells. This is important because your skin protects you from attack by weather, diseases or insects, not so with trees. Our bark is dead, mostly.
Let me make a comparison. All trees (except Palm trees) grow like coral. Every season they add living cells on top of old mostly dead cells. Our cambium layer, which is just below the bark, is the only 100% living part of us. The rest is partly or completely dead.
Gil: What? How can that be? You mean you're only alive in that little layer just under your bark? The rest of you is mostly dead?
Blackie: Yep! Let me try to simplify for you poor, thick headed person.
I have many different parts, some living, some completely dead that work together to form a wonderful me.
So, let’s start with the my cells. They look like long, thin straws, that connect from my roots to my crown. These pipe shaped cells on the inside of my cambium, called sapwood, carry water and minerals from the soil to my crown. The cells on the outside of my cambium, called phloem carry food from the leaves and branches to the rest of me.
Every season my cambium cells grow and divide adding new growth onto my trunk and branches making me wider and taller.
Gil: Okay, that still doesn't explain the dead part.
Blackie: My heartwood, the dark central inside is completely dead, and like coral is the hard structure on which I build new layers. The next layer, the lighter color sapwood is formed 6 to10 years from the present and it contains cells that are used for storage transport and strength. Those cells are only partly alive. The next layer is the cambium layer that is where all the action is. It divides constantly, adding to the girth of my trunk, covering old wounds and adding to my height. The cells are wider in the spring when the sunlight and water allow big growth and thinner in the fall and winter when growth is slower. That is why you can count the rings of a dead tree and see how old it was.
Gil: Wait! I thought you stopped growth when you lose your leaves and go dormant in the winter.
Blackie: That’s what all you people think, but Im sorry, you’re wrong! I slow down a lot but I never stop growing, as long as I am alive. Even on cold days there are little green chloroplasts in my cells that are constantly converting the sun’s energy into sugar. It is what keeps me alive. Remember this, when it comes to plants, wherever there is green there is life! Those chloroplasts would not be green if they were not getting some sunlight and converting it to food. Even under my bark!
If you scratch the bark on a dead branch it looks grey or brown. In the coldest winter when my leaves are all gone you can scratch a live branch, “ouch” and you will find green, which means that I am actively photosynthesizing the sun’s energy. Try that out on your science teacher and see if they don't insist that you are wrong.
Gil: You've talked about everything but the bark. What about it?
Blackie: I’m getting there, you people are always in such a hurry! We trees are much more patient about things.
You have to understand that my cambium makes everything in me. It is a very thin microscopic layer that creates the phloem on the outside of itself. My phloem is important because it carries all the food from my leaves to the rest of the me. Finally on the outside comes the bark. It’s also important because the bark protects me. The bark is a little alive but mostly dead and it is hard like your finger nails so it can protect me from harsh weather or being hit.
Now to the part where you have to stretch your mind to understand. So my cambium adds a new layer on to my old, mostly dead center which is called the xylem. I expand every year just under the bark. At the same time my cambium creates the phloem which eventually becomes the bark which, of course is my outermost layer, like your skin. The bark has to expand every time I add a layer underneath it and it is tough stuff. So what keeps me from exploding with that tough mostly dead bark surrounding me when I’m growing bigger every day?
Well when a snake or a Cicadae bug gets too big for its skin, it splits its old skin, climbs out of it, and forms new skin. I can’t crawl out of my bark because then I’d be unprotected, but my bark does expand and split. It doesn't happen too fast so you impatient people don’t notice it. Every tree is a little bit different. A Beech tree’s bark looks like elephant skin. Birch bark peels off in sheets like paper, and my bark comes off in lines and flutes just as you described when you stopped to talk. Did I answer your question?
Gil: Actually you gave me a lot more questions to think about. Do you have any questions for me?
Blackie: Yes. Why do you run around so fast everywhere. You don’t stop and let the wind caress your branches or the stars spin and amaze you. Don't you enjoy the wonderful world you live in?
Gil: Well I’ve got to get going, I’ve got so much to do today. It was nice talking, see you later, Blackie.