The dark, eye-shaped marks scattered across birch bark are lenticels, small pores that let the living wood underneath breathe. Trees need to exchange gases just as animals do, and because birch bark is unusually airtight, these openings are the trunk’s main channel for pulling in oxygen and venting carbon dioxide. They look like eyes because each lenticel is an elongated, lens-shaped slit stretched horizontally by the expanding trunk, set against bark so strikingly white that the dark pore tissue stands out at a distance. The story behind these markings touches on everything from chemistry to climate survival.
What Lenticels Actually Do
A tree trunk is a living cylinder. Beneath the outer bark, cells are respiring around the clock, consuming oxygen and releasing carbon dioxide. In many broadleaf trees, the outer bark is porous enough that gases diffuse through it without much trouble. Birch bark is different. Its outer layer, the phellem, is built from multiple sheets of heavily wax-impregnated cells. Each individual sheet in birch is only about 150 micrometers thick, but the waxy portion of that layer is dense enough to create a serious barrier to gas movement.1Journal of Experimental Botany. Survival strategies of plants during secondary growth: barrier properties of phellems and lenticels towards water, oxygen, and carbon dioxide Stack several of these sheets together and you get the papery, peelable bark birch is famous for, but also a trunk that would essentially suffocate its own cambium if not for the lenticels punched through it.
Each lenticel is a patch where the bark-producing tissue generates loosely packed cells instead of the tightly sealed, waxy ones that make up the rest of the phellem. These loose cells have air spaces between them, creating a path for oxygen to reach the living tissues inside and for carbon dioxide and water vapor to escape. The lenticel tissue is darker because it lacks the white pigment that dominates the surrounding bark, so each pore reads as a dark dash or oval against a pale background.
Why the Eyes Are So Conspicuous on Birch
Lenticels exist on virtually every woody tree species. Cherries have them. Apples have them. But on most trees, the outer bark is brown or grey, so the lenticels blend in. On birch, the contrast is extreme because the background bark is packed with a compound called betulin, a white crystalline triterpene that can make up a remarkable share of the bark’s dry weight. In Turkestan white birch, for instance, betulin accounts for roughly 82% of the bark’s extractable compounds.2European International Journal of Multidisciplinary Research and Management Studies. Study of Extract Substances of White Birch Bark That concentration of a single white compound gives birch bark its almost painted appearance. The lenticels, which are made of a different kind of tissue without that betulin loading, look nearly black by comparison. It is pure visual contrast that turns ordinary breathing pores into “eyes.”
As the trunk grows in diameter, the outer bark stretches tangentially. The thin-walled cells in the outermost layers get pulled and compressed as newer layers form beneath them.3Heritage Science. The interaction of water with archaeological and ethnographic birch bark and its effects on swelling, shrinkage and deformations This lateral stretching is what gives each lenticel its horizontal, slit-like shape rather than a circular one. On a young sapling the marks are small and subtle. On a mature birch trunk, years of circumferential stretching have widened them into the broad, dark dashes that look so strikingly eye-like.
Why Birch Bark Is White in the First Place
The whiteness is not just cosmetic. It serves a thermal function. White-barked birches extend to the northern limit of tree growth, and their pale bark helps reduce solar damage during winter and early spring by limiting how much the trunk heats up in direct sunlight.4bioRxiv. Chemistry and Structure of Birch Bark Support Passive Radiative Cooling In cold climates, sunny winter days can warm the south-facing side of a dark-barked trunk well above freezing while the shaded side stays frozen. When the sun sets, the warm side refreezes rapidly, and these freeze-thaw cycles crack the bark and damage the living tissues underneath. By reflecting most incoming sunlight, white bark keeps the trunk temperature more uniform and avoids the worst of that damage.
Research suggests the bark’s reflective properties go beyond simple color. The layered structure of birch phellem, with its repeating sheets of suberized cells and air gaps, supports passive radiative cooling. The bark does not just bounce light away; it also radiates heat efficiently in the thermal infrared range. For a tree that routinely endures temperatures far below freezing, this combination of reflectivity and radiative emission is a genuine survival strategy, not an accident of pigmentation.
Bark Photosynthesis Through the Lenticels
Lenticels do more than just let the trunk breathe. They also supply carbon dioxide to a thin layer of green, photosynthetically active tissue buried just beneath the outer bark. This tissue, called bark chlorenchyma, cannot photosynthesize as aggressively as a leaf, but it does something valuable: it recaptures a large fraction of the carbon dioxide that the trunk’s own respiration produces. In young birch trees exposed to full sunlight, illumination reduced the trunk’s apparent respiration rate by about 65%. In shade-grown birch, the reduction was even greater, around 81%.5Acta Biologica Slovenica. Light-modulation of bark photosynthesis in birch (Betula pendula Roth.) Separate measurements at a standard temperature found that birch bark chlorenchyma could offset dark respiration by up to 68% under saturating light.6Journal of Experimental Botany. Temperature dependency of bark photosynthesis in beech (Fagus sylvatica L.) and birch (Betula pendula Roth.) trees
This means a birch trunk is not purely a metabolic drain on the tree’s carbon budget. On a sunny day, the bark is quietly recycling a substantial portion of the carbon dioxide that the wood produces through respiration, converting it back into sugars. The lenticels are the gateway for this process, letting in enough carbon dioxide and oxygen to keep the bark chlorenchyma functional while the white outer layers let light through to power the reactions. It is an elegant bit of multitasking for a structure most people think of as just dead outer covering.
What Happens to the Eyes in Winter
Birch trees in boreal forests face a seasonal problem. In summer, lenticels are open and active, facilitating gas exchange and supporting bark photosynthesis. But in winter, leaving those pores wide open could allow excessive water loss and expose internal tissues to damaging cold and ice. Research on boreal birch species indicates that the bark-generating tissue produces compact cellular plugs that partially close off lenticels toward the end of the growing season, reducing their permeability to water vapor and gases during the cold months.7Nature Communications. Seasonal dynamics of stem N2O exchange follow the physiological activity of boreal trees
This seasonal gating is not absolute. Some gas exchange still occurs, and researchers studying nitrous oxide fluxes in boreal forests found that birch stems continued to exchange trace gases even during the dormant season. Birch wood is diffuse-porous, meaning its water-conducting vessels are distributed relatively evenly across growth rings rather than concentrated in a single band. This structure makes birch wood more gas-permeable than conifer wood even when the lenticels are partially sealed. The winter closure of lenticels is more like turning the ventilation down to a trickle than slamming a door shut.
The Bark’s Built-In Chemical Arsenal
Beyond its structural role, birch bark carries potent antimicrobial chemistry. Water extracts from white birch bark inhibited all eight microorganism species tested, at concentrations as low as about 0.8 to 1.7 milligrams per milliliter. Catechol, a small phenolic compound, was identified as one of the dominant antimicrobial components in these extracts.8MicrobiologyOpen. Antimicrobial activity and chemical composition of white birch (Betula papyrifera Marshall) bark extracts This chemical defense helps explain why birch bark resists rot and fungal colonization on its outer surface, even in the damp environments where birch commonly grows.
The betulin that makes the bark white is also biologically active. It belongs to the pentacyclic triterpene family, and these compounds have been shown to promote wound healing in both laboratory cell cultures and animal tissue models. Birch bark preparations containing betulin and related triterpenes accelerated wound closure in human skin cells and in porcine wound-healing models.9PubMed Central. From a traditional medicinal plant to a rational drug: understanding the clinically proven wound healing efficacy of birch bark extract Indigenous peoples across the Northern Hemisphere have long used birch bark as a wound dressing, and modern clinical work has validated the practice. So the same compound that makes the bark white, creates the contrast that makes lenticels look like eyes, and reflects sunlight to prevent freeze-thaw cracking also carries genuine medicinal properties. It is a single molecule doing a lot of jobs.
A Vulnerable Trunk Beneath a Tough Shell
The bark is impressive, but the wood underneath has a notable weakness. Trees resist internal fungal decay partly through living storage cells embedded in the wood, called ray and axial parenchyma. These cells produce chemical defenses and can wall off infected areas. Birch has comparatively little of this tissue, only about 4 to 10% of its wood volume, putting it in the same low range as Norway spruce. Trees with much higher proportions, like oak and black locust at 35 to 40%, suffer far less weight loss from brown rot fungi.10Frontiers in Plant Science. The Parenchyma of Secondary Xylem and Its Critical Role in Tree Defense against Fungal Decay in Relation to the CODIT Model
This means birch relies heavily on its outer bark as a first line of defense. Once a pathogen breaches the phellem, whether through a wound, a broken branch stub, or a compromised lenticel, the internal wood has limited resources to compartmentalize the invasion. This is part of why birch trees tend to be shorter-lived than oaks or beeches. The outer bark is chemically and structurally formidable, but it is defending a relatively soft interior. The lenticels, as necessary as they are for gas exchange, represent the most obvious potential entry points for pathogens. The seasonal closing of lenticels in winter may serve a defensive role as well as a moisture-conservation one.
Why Other Trees Do Not Have Visible Eyes
Oaks, maples, and pines all have lenticels too, but you rarely notice them. The reasons are straightforward. First, most mature trees develop thick, rough, deeply furrowed outer bark that buries and obscures any lenticel openings. Birch bark stays thin and smooth even on old trunks because it sheds its outermost layers in papery sheets rather than accumulating them into a thick rind. Second, without betulin’s stark whiteness, there is no high-contrast background to make the darker lenticel tissue pop. On a grey or brown trunk, a slightly darker pore is invisible at any distance.
Cherry trees are an interesting comparison. Young cherry bark is smooth and glossy with clearly visible horizontal lenticels, often cited as the closest visual analogue to birch. But cherry bark darkens and roughens with age, eventually losing its lenticel visibility. Birch maintains its smooth, peelable surface throughout its life because the bark keeps generating and shedding thin phellem sheets. Each fresh sheet brings a new coat of betulin-white surface and newly formed, clearly delineated lenticels. The eyes keep appearing because the bark keeps renewing itself.
Birch Bark as a Material
The same properties that make birch bark biologically interesting have made it one of the most widely used bark materials in human history. Its impermeability to water, flexibility when fresh, and antimicrobial chemistry made it ideal for canoes, containers, roofing, and writing surfaces across cultures from Scandinavia to North America to Siberia. The layered phellem structure that blocks gas exchange also blocks liquid water, making birch bark a natural waterproofing membrane.
But birch bark’s behavior changes when it gets wet after being separated from the tree. Water acts as a plasticizer on the stretched outer cells, allowing them to contract back toward their original dimensions. Because the outermost layers were stretched most during the tree’s growth, they contract more than the inner layers when wet, causing the bark to curl with the outer surface rolling inward.3Heritage Science. The interaction of water with archaeological and ethnographic birch bark and its effects on swelling, shrinkage and deformations This curling behavior is familiar to anyone who has picked up a piece of birch bark from the forest floor. It is also a major concern in museum conservation, where archaeological birch-bark artifacts must be carefully controlled for humidity to prevent warping and cracking.
The lenticels on harvested bark remain visible and serve as useful orientation markers. Because they run horizontally on the living tree, a piece of birch bark with visible lenticel lines can always be oriented correctly even after it has been removed and dried. Traditional canoe builders and basket makers used this feature routinely, keeping the lenticels aligned horizontally to maintain the bark’s natural grain direction and maximize its structural integrity in the finished object.