What is a leaf?
Leaves come in all shapes and sizes, but they all have the same purpose. They make food out of thin air. Green plants are the only living things that can actually create their own food. This process is called photosynthesis.
The word “photo” means light and “synthesis” means combine two or more things and make something new.
Leaves use the energy from light to combine water and carbon dioxide to make a simple kind of sugar.
Photosynthesis takes place in special plant cells called chloroplasts. These cells interact with light to break water down to its basic parts (hydrogen and oxygen.) Carbon dioxide enters the leaf through tiny openings called stomas. A chemical reaction in the chloroplasts turns all these ingredients into sugar.
This sugar is used to help the plants grow. Any extra is transported and stored in the plant’s roots, stems, seeds, and fruits. Animals can take advantage of this stored sugar by eating plants. The sugar is then turned into energy and helps animals, like humans, grow.
What part of a plant do we eat?
Activity: Leaf sun tatooing
Light is one of the things needed for photosynthesis. Here is an experiment you can do to see the effect of light on leaves.
What you’ll need: Some thin (but not clear) plastic, like the lid of a plastic food tub
A tree, shrub or house plant
Use the scissors to cut some shapes out of the plastic. Try different shapes like circles, squares, stars and triangles. make these shapes about half the size of the leaves on the plants you will be using.
Carefully paperclip the plastic shapes to several different leaves. Put some on the top surfaces and some on the bottom, but don’t put two shapes on the same leaf.
After four days carefully remove the plastic shapes from the leaves.
* What happened to the leaf surface that was covered?
* Did the same thing happen on the top of the leaf as on the bottom?
- Why do you think these things happened?
- Do you think there was any photosynthesis taking place underneath the plastic shapes?
How long can you hold your breath?
Another important function of leaves makes it possible for you to breathe. When animals breathe in, their lungs separate the oxygen from the air. Animals need oxygen to survive. When they breathe out they release carbon dioxide.
If there wasn’t some way to make more oxygen we wouldn’t be able to breathe.
Plants are the magic machine that makes new oxygen. Leaves absorb carbon dioxide, the same stuff that animals exhale, use it to make sugar, and then release oxygen into the air.
What this means is that plants need animals to make carbon dioxide and animals need plants to make oxygen.
This even works under water. Plants that live in the water make oxygen that fish need to survive.
Leaf Cross Section
If you cut a leaf in half and looked at the cut edge under a microscope it would look something like the drawing above.
The cuticle is a waxy substance that prevents water loss. Epidermis cells are the skin of the leaf and are usually clear. This lets light pass through to the mesophyll cells. These cells contain the chloroplasts where photosynthesis takes place.
The stoma is a very small opening where carbon dioxide enters the leaf and oxygen is released. These openings are surrounded by guard cells.
Xylem and phloem cells are long and skinny. They work like straws to transport food throughout the plant.
Leaf shape and grouping
The shape and other characteristics of leaves make it possible to identify different plants. If leaves were not different we wouldn’t know which plants to eat, which ones are safe to be around and which ones to avoid.
The edges of leaves can be toothed, smooth, or lobed.
Leaves also come in a wide variety of shapes. Some are rounded, others are long and narrow. Still others look more like a triangle or even have finger-like points.
The toothed edges and grouping of three leaves together help us identify this plant as poison ivy. Poison ivy secretes an oil that will make you itch.
Why do leaves change color in the fall?
All leaves have color pigments all year long. During the summer, when photosynthesis is taking place, the chlorophyl that make leaves look green is very active and overshadows the other pigments.
In the fall the days get shorter and the nights get longer. The relationship between the length of daylight and dark is called photoperiodism. When the photoperiod changes it signals changes in plants. In the fall the plant stops the photosynthetic process and stores energy to produce seeds or save energy to survive the winter.
The production of chlorophyl stops and the green pigments go away. This makes it so the other pigments in the leaf are easier to see. Carotenoid pigment makes yellow and orange leaves, anthocyanin makes leaves red.
The most brilliant fall colors occur when the fall has a combination of warm sunny days and cold, but not freezing nights. This results in a lot of sugar production during the day, but at night the sugar doesn’t move out of the leaves to the rest of the plant. This combination, lots of sugar and lots of light, makes the leaf produce more anthocyanin, resulting in more brilliant red colors.
An example of how leaves change color
When bananas are picked they are still quite green. This is because they are still full of chlorophyl. Once picked they will stop making chlorophyl and the green color will fade away, just like the green fades when a leaf changes color in the fall. The result is that the banana turns yellow because the carotenoid pigments are easier to see.
Why do leaves fall off in the fall?
Photoperiodism also signals the creation of a special group of cells called the separation layer. These cells are at the base of the leaf petiole where it joins the plant stem. Once this layer is complete the leaf drops off.
Trees drop their leaves in the fall to reduce moisture loss and damage from wind and snow loads.
Once leaves are on the ground, they are broken down by bacteria, fungi, earthworms and other organisms. The decomposed leaves provide nutrients and become part of the compost layer that absorbs and holds rainfall. Even dead leaves are beneficial. In nature, nothing goes to waste!
Activity: Leaf rubbing
For this activity you will need to gather a couple of leaves from several different trees. Get permission from the tree owner before you pick the leaves. You will also need some crayons and some thin white paper.
Place one of your leaves on a clean flat surface and put a clean sheet of paper on top of the leaf. Hold the leaf and paper as still as possible, then rub a crayon over and around the leaf. The leaf veins will show up better if you put the leaf upside down before rubbing with the crayon.
This will give you a colorful impression of the leaf showing the outline, texture, and veins. Make rubbings of several different leaves using different colors of crayons to make your own colored leaf collection.
Some plants don’t drop their leaves in the fall. They are called evergreens. Most evergreens have long skinny leaves called needles. These needles are covered with a heavy wax-like coating and the fluids inside contain a special anti-freeze substance. Evergreen leaves can live several years before they fall and are replaced by new needles.
Pine Fir or Spruce
All these trees are evergreens and will have needles. How can you tell them apart?
Pines are prickly and paired - Pine needles will feel sharp and prickly. Their needles also grow in groups of two or more from a common base.
Spruce are sharp, square and single - Spruce needles grow as a single needle and feel sharp and stickery. They are also square in cross section and roll between your fingers.
Firs are flat and furry - Fir needles are flat in cross section and if you rub your hand across the needles they usually wont feel stickery or prickly.
Leaf collecting can be a fun learning experience, but remember that many plants are on private property and you should get permission before gathering leaves. National Forests can be great places to see different leaves, especially in the fall when leaves change colors.
It is illegal to gather plants from National Forests without the proper permits. Please check with property owners and administrators before gathering leaves.
For the best collections, leaves and flowers should be pressed and dried. There are commercial presses available, but a simple and effective press can be made with materials you probably have at home.
Place wax paper or plastic under the press to protect the surface. Place several layers of newspaper on top of the plastic or waxed paper. Sandwich the leaf between two sheets of clean white paper. Place several more sheets of newspaper on top of this and repeat for as many leaves as you want to press. Put something flat and hard (like a piece of plywood) on top of the newspapers and then put something heavy (like books or bricks) on top of the board.