I’ve learned that the white streaks on many conifer needles are, on close examination, actually close clusters of little white dots or patches. So what are they? Collectively, they’re called stomatal bloom. Each is the wax that lines a stoma, or opening in the surface of the leaf. Stomata allow the exchange of gases between the interior of the leaf and the air outside; in other words, they are how a tree breathes, excretes, and conducts photosynthesis. They are completely fascinating and also beautiful. I’ve come across this particular photo in a few different places on the web, so I am not the only one who finds it arresting:

The underside of a leaf of Tradescantia zebrina. The majority of the surface is made up of epidermal cells with the occasional stoma – a pore in the leaf which can open and close to control gas exchange, primarily to mimimise loss of water vapour while still taking up carbon dioxide. In this species the stomata cells are green (due to chlorophyll) while the epidermal cells are red in colour due to additional pigmentation. Photo by Zephyris under the Creative Commons Attribution-Share Alike 3.0 Unported license.

The leaf pictured above is from Tradescantia zebrina, a.k.a. Wandering Jew, a somewhat problematic common name that some people, I was amused to discover, have proposed replacing with Wandering Dude. Wandering Dudes are common houseplants and not trees. So back to my wax-lined, whitish stomata.

Many of the needles I’ve been drawing have stomatal bloom, resulting in streaks or stripes whose number reveals the pattern of stomata on the tree in question, and therefore making a handy way for would-be tree identifiers to tell various conifers apart. I have already developed a keener eye for the many species of these trees and the differences among them, due to this project, but I can’t remember which ones are which. Today’s might be easier to identify, because the underside of the leaf has such a broad swath of stomatal bloom that it has been dubbed the Pacific silver fir. Unfortunately, its natural range begins in the very northwest corner of California and heads north from there, so I’m not likely to see it on a walk around Palo Alto, but I’ll keep my eye out for the silvery undersides that might indictate Abies amabilis far from their mountain home.