There are very few plants on Earth with leaves that aren’t some shade of green.
From the rainforest to the desert, flashes of jade, sage, and forest greens can be found as long as you’re outside.
In a world where plants and animals are constantly evolving to adapt to their environments, why is the color of leaves so consistent?
We’ll cover the science behind the color, shape, and other characteristics of leaves.
Why Are Leaves Green?
Leaves are green because the plant’s cells are filled with a green chemical called chlorophyll.
Chlorophyll is vital to a plant’s survival as it’s used to absorb energy from sunlight.
The longer answer has more to do with physics than biology.
Visible light can be separated into different wavelengths.
Each wavelength appears to us as a different color.
Light with the longest wavelength is red, while light with a shorter wavelength appears purple.
When light shines on an object, the object will absorb most colors of light while reflecting only some.
The light that is reflected travels into our eyes, and we observe the object as being that color.
For example, a strawberry absorbs all but the very shortest wavelengths of light.
This light is reflected, so the strawberry appears red.
Red light is the most useful to plants as they grow, so the chlorophyll absorbs it.
Green light is not useful to plants, so it is reflected.
Because the green light is what reflects off the surface of the plant and travels back to our eyes, it is the color we see when we look at a plant.
How Do Plants Use Chlorophyll?
Chlorophyll’s job is to absorb sunlight.
Plants require sunlight to create their own energy.
Without chlorophyll, a plant would be unable to take in the light it needs to grow.
Plants use stored energy from sunlight along with water and carbon dioxide to complete a process known as photosynthesis.
In the process of photosynthesis, a plant takes in water and carbon dioxide from the air and soil around it.
It also uses chlorophyll to take in any available sunlight.
The plant uses these three components to create oxygen and glucose, which is a form of energy.
The glucose is used to grow (similarly to how a human uses food for energy and growth), while the oxygen is released into the air.
Photosynthesis is not just crucial to the survival of plants.
All life relies on photosynthesis to produce energy for the rest of the food chain.
For example, let’s take a look at a dandelion.
The dandelion creates its own energy through photosynthesis.
Later, the dandelion may be eaten by an herbivore, such as a mouse.
Most of the energy in the dandelion is transferred into the mouse.
When the mouse is later eaten by a snake, the snake gets energy from the mouse.
This continues through the food chain until it ends with an apex predator.
An apex predator does not have any natural predators and will likely die of natural causes.
When this happens, the energy is consumed by decomposers, such as bacteria or worms.
These decomposers will then return the energy into the soil, where it will once again be used by plants.
At each step in the process, however, energy is lost as heat.
The energy that is returned to the soil is not the same amount of energy that was initially created by the dandelion.
Therefore, plants must continually create new usable energy through photosynthesis.
Why Do Some Leaves Change Color In Autumn?
If you have ever driven through the Northeast or Mountain West in the autumn, you’ve likely seen many leaves that weren’t green.
It’s very common for leaves on trees and bushes to change to yellow, orange, or red in the fall.
Eventually, they turn brown and fall off the tree.
If these leaves have chlorophyll, what causes them to change color?
In fall and winter, less sunlight is available for plants to convert into energy.
When this happens, the chlorophyll that is in the leaves begins to break down.
As the amount of chlorophyll lessens, so does the amount of green pigment in the leaves.
The yellow and orange colors of the leaves are revealed.
Red leaves are created when sugars get trapped in the leaf.
As the leaves stop creating energy for the tree or bush, the tree stops providing them with water.
This protects the tree from dying.
The leaves eventually dry up and become brittle.
At this point, they fall off the tree.
Some places do not get colorful leaves in the fall.
The color changes will only occur if the climate is right.
In places like California and Florida, it stays sunny year-round, so there is no reason for the trees to lose their leaves.
In humid places, the trees will not cut off the leaves’ water supply.
Places that are dry and cool are more likely to see the yellows, oranges, and reds of autumn.
You will see these color changes in trees like aspens, maples, and oaks.
Evergreen trees, such as pines, firs, and spruces do not change color, even though they are often found in the same environments as trees that do.
Why Don’t All Leaves Change Color?
Trees that change color in the fall belong to the category of deciduous trees.
Deciduous trees have large, broad leaves.
These leaves require a lot of sunlight, and therefore, energy to grow.
Coniferous trees, on the other hand, have leaves that look like small needles.
For example, Christmas trees are usually some type of coniferous tree.
These needles have a small surface area, so they don’t require as much sunlight to grow.
However, this also means that they don’t help the tree with energy production nearly as much as bigger leaves.
Because of this, the tree hangs onto its needles for several growing seasons.
Over many years, the needles contribute enough energy to the tree to make their growth worth it.
Each type of conifer tree keeps its needles for a different length of time; some needle lifespans are as short as two years, while others stay on their trees for up to fifty years.
Because conifers don’t require as much sunlight to grow, they can live in places where deciduous trees can’t survive.
A deciduous tree’s leaves only stay on the tree for a single growing season, so they need a lot of sunlight in order to efficiently contribute to their tree’s life.
Since many evergreen trees grow in very cold, windy conditions, they invest their energy in the toughness of their needles.
The needles have a waxy coating to keep water in during the winter months.
With this feature, the trees are able to keep photosynthesizing during the winter (although at a slower rate than in the summer).
They are also small to keep the branches from getting overloaded with snow.
Because the needles can’t hold much snow, heavy snowfall is unlikely to break any branches on a coniferous tree.
Why Do Deciduous Leaves All Look Different?
The shape and size of leaves depend on the amount of sunlight and water available to the plant.
A leaf can only grow as large as the accessible water will allow.
If there isn’t much water around, the plant will be unable to grow large leaves.
On the other hand, the plant does not want its leaves to grow to be too large.
A leaf will take in all the sunlight it can.
If it grows too big, it will take in too much sunlight, and the plant could overheat.
Therefore, between the sun and water supply, the plant must find a happy medium in terms of leaf size.
When it comes to shape, millions of years of evolution help determine what a leaf will look like.
There is strategy involved in nearly every aspect of the leaf’s shape.
For example, leaves with round edges are better at collecting the sun’s rays.
Leaves with sharper angles can shade themselves, so plants that need less sunlight are more likely to have these kinds of leaves.
Plants that grow in areas prone to severe wind will likely not grow broad, round leaves, since these leaves are more likely to catch the wind and rip off.
Long, thin leaves have an easier time getting water to all the cells of the leaf.
However, too much water can also be a problem, so leaves in water-rich environments may develop a “drip tip.”
Drip tips are small extensions at the end of a leaf that release excess water.
Some plants, such as the weeping fig, can even lose their leaves when their environment changes, and then grow them back in a more suitable way.
What Are The Veins In Leaves For?
Most leaves have patterns of veins that reach from the stem of the leaf to the tip.
Some leaves have more than others.
They are called veins because they work very similarly to human veins.
There are several purposes for the veins.
The first is to provide structure.
If you look closely at a leaf, you will see thick, long veins that reach to the ends of the leaf, and thinner, shorter veins that connect the larger ones.
Each of these veins helps to keep the leaf from drooping.
This allows the leaf to gather as much sunlight as possible.
Without the veins, the leaves wouldn’t be able to hold themselves up, and would therefore be unable to gather energy for the plant.
The second purpose of the veins is to move water, energy, and nutrients throughout the leaf.
For this purpose, there are two types of veins.
The first type of vein is called the xylem.
Xylem move water and minerals from the plant’s roots to the tips of its leaves.
The other type of vein is called phloem.
These veins move nutrients that are made in the leaves to the rest of the plant.
The plant can tell where the nutrients are needed, and the phloem can move them there.
The network of veins in a leaf looks different depending on the plant’s needs.
Plants with denser vein networks are better able to withstand water loss.
Distance between veins is related to how well the leaf supplies nutrients to the plant.
In each leaf, veins serve a vital purpose.
Leaves also have pores on them, called stomata.
The stomata are the part of the leaves that allow water and nutrients in and out.
Without them, there would be nothing for the veins to transport.
How Do Leaves That Aren’t Green Photosynthesize?
Not all plants have green leaves.
However, for many of these plants, photosynthesis is still important.
If chlorophyll, the green pigment found in most plants, is important for photosynthesis, how do plants without green leaves survive?
The answer to this question is that other pigments are also involved in photosynthesis.
In fact, even green plants have these other pigments.
That’s why deciduous leaves turn yellow and orange in the fall.
The other pigments can capture light that chlorophyll does not.
Most plants have more chlorophyll than any other pigment, so they appear green because green is the color that is reflected the most.
However, some pigments reflect red, yellow, and orange light.
Another pigment, called anthocyanin, is not involved in photosynthesis but gives plants a reddish-purple hue.
Japanese plum trees are a good example of one of these plants.
When it comes to these plum trees, chlorophyll is not entirely absent.
There is still some amount of chlorophyll in their leaves.
There is just more anthocyanin, so the trees’ leaves do not appear green to the human eye.
The red leaves still use chlorophyll for photosynthesis, and in fact, can trap the sun’s light just as well as green leaves.
If red plants can photosynthesize just as well as green ones, why are most plants green?
When light is dim, green leaves photosynthesize more efficiently than any other color.
Therefore, in some places, it is easier for green plants to survive than red ones.
You may find that in certain environments, there are more plants with red leaves than you’d see elsewhere.
Some scientists hypothesize that red leaves are a signal to animals that the plant contains some kind of toxin.
The plant may use its red leaves to avoid being eaten.
How Do Plants Without Leaves Photosynthesize?
When it comes to plants that seemingly do not have any leaves of any color, photosynthesis may seem impossible.
Surely, the plant cannot contain any chlorophyll if it does not grow leaves, right?
You may be thinking of a cactus when considering this question.
Cactuses, along with other desert plants, do not have the large, broad leaves found in areas with more moisture.
This is because leaves require a lot of energy and water to grow.
It would be inefficient for desert plants to grow large leaves in a place with so little water.
Leaves also tend to lose water through their pores, meaning they would be a liability to the plant during the long stretches without precipitation.
Cactuses, instead, have developed a waxy skin that keeps water inside, allowing them to store the water for a long time.
The spines and pads on cactuses are a form of leaf modification.
They have pores on them, called stomata, just like leaves do.
However, the stomata on cactuses and other desert plants are closed during the day.
They are only open at night.
This allows the plant to take in nutrients without losing a lot of water to the heat of the sun.
The stomata can take in sunlight during the day while they are closed.
Then at night, they open to allow the physical components of photosynthesis (water and carbon dioxide) into the plant.
The spines do not take much energy or water to grow, so they are efficient for cactuses.
They also help provide the plant with protection.
Without them, animals could chew into the waxy surface of the cactus and get to the water stored inside.
The spines keep most animals away, although some species of birds, rodents, and bugs have adapted to get past them.
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