1. Feeling the Difference Between Outer Leaves in Sun and Inner Leaves in Shade
When you look up at a large tree, the leaves on the outer branches receive plenty of sunlight. The leaves closer to the trunk, on the inside, receive much less. Even on the same tree, the amount of light each leaf receives is very different. [1]
In educational materials from the Ecological Society of Japan, "sun leaves" are defined as leaves on the edge of the tree, especially those facing south or east where light is strong. "Shade leaves" are those inside the tree, facing north or in middle layers where light is hard to reach. These two types differ in thickness, internal structure, and how they perform photosynthesis. [1]
So, what about their color? Do shade leaves look like a darker green than sun leaves? This is the question we will investigate.
2. Why Sun Leaves Are Thicker: Their Layers of Cells
If you cut a leaf vertically, you can see a layer of long, column-like cells near the top surface. This is called the palisade tissue, and it is a crucial site for photosynthesis. [2]
According to the abstract of a doctoral thesis from Osaka University, researchers grew the plant *Chenopodium glaucum* (a type of goosefoot) in bright and dark environments. They found that sun leaves in the bright environment were thicker than shade leaves in the dark. The palisade tissue had two layers of cells in sun leaves but only one layer in shade leaves. The number of cell divisions in the thickness direction was about 7 times higher in sun leaves than in shade leaves. [2]
This suggests that sun leaves are thick because cells divide more in the thickness direction as the leaf grows, creating more layers. However, this result comes from one species grown under artificial light conditions, so it is not a number that applies to all trees. [2]
Thick leaves have another difference: the amount of chlorophyll, the green pigment. The result changes depending on how you count it.
3. Chlorophyll in Sun and Shade Leaves: Why Measuring by Area or by Weight Changes the Result
A research team in Germany compared sun and shade leaves from four tree species: maple, beech, linden, and fir. They reported that thick sun leaves have more chlorophyll per unit of leaf area. However, thin shade leaves have more chlorophyll per unit of dry weight. [3]
The reason becomes clear when you consider thickness. Thick leaves are packed with more material, making them heavier for the same area. Therefore, even if sun leaves have more chlorophyll per area, dividing by their higher weight makes the number smaller. Shade leaves, being lighter, end up with a higher number per weight. [3]
Another study on ginkgo and beech found that sun leaves had more chlorophyll and carotenoids (yellow to orange pigments) per area. Photosynthesis was also faster in sun leaves. For beech, sun leaves averaged 8.5 micromoles per square meter per second, while shade leaves averaged 1.8. In beech, the sun leaf value was about 4.7 times that of the shade leaf. [4]
Whether one has "more" depends on the standard used. So, what does our eyes see?
4. How the Top and Bottom Sides of One Leaf Differ
The top side of a leaf (where light hits) has palisade tissue, while the bottom side has spongy tissue. The Japanese Society of Plant Physiologists explains that even within a single leaf, the top side has "sun-type" chloroplasts (less chlorophyll, more Rubisco enzyme) and the bottom side has "shade-type" chloroplasts (more chlorophyll). [5]
The change from sun-type to shade-type is continuous. If you grow a leaf with light coming from the bottom, this relationship reverses. This suggests that chloroplast differences are determined by "which side received light," not "where the leaf is located." [5]
If a single leaf has both a side facing the light and a side away from it, comparing outer and inner leaves of a tree to say one is "darker" or "lighter" might be a bit too simple.
5. Why a Leaf's Visible Greenness Depends on More Than Chlorophyll
The Japanese Society of Plant Physiologists notes that the visible darkness of a leaf's color is not determined only by chlorophyll. It is also influenced by other pigments like carotenoids and anthocyanins, as well as differences in leaf surface shape. The numbers from instruments that measure greenness (SPAD values) match visual appearance only under certain conditions. [6]
Results also vary by tree type. A general comparison table for C3 plants states that chlorophyll per area is "similar" between sun and shade leaves, but chlorophyll per chloroplast is higher in shade leaves. [7] Since the four tree species and ginkgo/beech studies reported higher amounts in sun leaves, it is best to accept a range of results from "slightly more" to "similar." [7]
In the case of *Fukugi* (a tree in Okinawa), the expected traits—small, thick sun leaves and large, thin shade leaves—were not clearly observed. Chlorophyll per fresh weight was 2.24 milligrams per gram for sun leaves and 1.98 milligrams per gram for shade leaves. [8] Note that this uses fresh weight, unlike the dry weight in previous studies, so these numbers cannot be directly compared. [8]
"Visual greenness" changes depending on how you count and the tree species. This is why looking with your own eyes helps you understand the reasons behind what you see.
6. How to Compare Leaf Thickness with Your Fingers
If you want to try this, choose a branch at a reachable height when leaves are green. Pick one outer leaf and one inner leaf. Gently pinch them with your fingers to compare thickness. Then, hold them up to the sky to see how light passes through. The difference in thickness may be related to the number of layers in the palisade tissue. [2]
Even if they look different in darkness, you cannot determine if this is due to chlorophyll differences by eye alone. Other pigments like carotenoids and surface shapes are also involved. [6]
SAFETY NOTE: The story of leaves turning red in autumn is separate from the green differences in summer. When observing, avoid areas near roads or ponds. Choose only one leaf and do not pick too many.