That title comes from a poem by Dinah Maria Mulock Craik. One morning not long ago, I opened the front door of the house and was hit by a smell. It was delicious. I just couldn’t place it. I sniffed and sniffed and finally decided it was the smell of green. I went to the computer and started looking up what makes things smell green, and there was this charming poem by Craik.
That was all well and good, and described how I felt about it, but I wanted to know WHY. The science behind that smell. So first I needed to know, exactly, what is a smell? It turns out that smells exist mostly in our heads. “Molecules exist in the air, but we can only register some of them as smells,” reports smell expert Avery Gilbert. According to brainfacts.org, “smell begins at the back of the nose, where millions of sensory neurons lie in a strip of tissue called the olfactory epithelium. The tips of these cells contain proteins called receptors that bind odor molecules.” We have about 450 different types of these receptors, and each is activated by different molecules. What we think of as a single smell is actually a combination of molecules acting on a variety of receptors.
So, when I opened the front door, many different molecules combined to make the scent that I was smelling. But my brain couldn’t figure out what the smell exactly was. Those molecules binding with those receptors sent electric signals to my olfactory bulb, which then relayed that signal to other areas of my brain for processing, such as the piriform cortex, and the thalamus (which tries to marry smell with taste), and then to the hippocampus and amygdala, adding a layer of memory to what I was smelling. You’ve experienced that, haven’t you? Smelling something and having a very clear memory? That comes from the hippocampus and amygdala, which are key regions for learning and memory.
On top of that, this was just after dawn, and it seems that everyone’s smell sensitivity has a circadian rhythm. Your sense of smell may be different at different times of day. Also the amount of pollution can change the smell of the air, so smells in the morning are different than smells later in the day. Smells also move by diffusion. I wonder, do the smells collect in the air and sink overnight? Who knows?
Anyway, what are the exact molecules that are attaching to my receptors and causing me to smell this certain scent when I open my front door? This is what I see outside: Flowers. Trees. Green leaves. Wood chips. Soil. The grass across the street. The concrete of the sidewalk and road.
Certainly flowers have a distinct odor. Right now in the garden there are two dominant flower smells - one from the Mock Orange Tree, and another from the sweet peas. Both are very strong and sweet, and perfume the air around them, which can move, as we said, by diffusion. It’s hard to find any data on the smell of green leaves. I’m probably smelling ‘Green Leaf Volatiles’ which are organic compounds that are released when plants suffer tissue damage. This is happening all the time - snails and slugs munching the leaves overnight, people mowing their grass, or using a string trimmer to cut weeds, deer eating the tops of all the stuff I have planted outside the fence, birds ripping leaves for nests and for eating. Then there is the smell of the soil, which is very strong. It is actually not the soil we smell but the bacteria and microbiology in the soil, processing and releasing minerals and organic matter. The wood chips are breaking down, and again that’s the smell of well-oxygenated decomposition by the soil life.
So all of those compounds are moving through the air by diffusion to my little freckled nose, where they all attach themselves to different receptors, which send an electric signal to my brain, causing me to smell. That’s the scientific explanation. But how about this: Maybe all these smells were simply created for my pleasure? So that when I open the front door in the morning, I start the day with a smile on my face. Yeah. Let’s go with that one.
