Plants don’t get the admiration they deserve.
I mean, sure, lots of people enjoy gardening. Some may even have a favorite flower. But when it comes time to learn about plant science, most people tune out. In all my years of teaching, I have yet to witness a student get excited about reaching the portion of our biology course when we learn about plants.
Plants just don’t seem to be very exciting. Well, looks can be deceiving. Plants are much more fascinating than we give them credit for.
Without plants, life as we know it wouldn’t exist.
Plants are able to capture energy from the sun and use it to convert carbon dioxide and water into sugar in a process called photosynthesis. Plants and other organisms capable of performing photosynthesis form the basis of nearly every food chain on the planet.
Plants are able to keep track of the seasons.
Have you ever stopped to consider how a plant knows when to grow and when to go dormant? How plant bulbs know when to emerge in the Spring, and what triggers a tree to drop its leaves in the Fall? How plants know the optimal time to produce flowers? Like humans and animals, plants have circadian rhythms: internal “clocks” that read and respond to environmental cues.
Plants have developed tricky ways to attract pollinators.
Like all living things, plants need a way to pass their genes from one generation to the next. Flowering plants (angiosperms) rely on pollination to reproduce. To solve the problem of attracting pollinators to their flowers, members of the plant kingdom utilize a variety of clever methods.
Plants can communicate, both with other plants and with other organisms.
Recent studies have shown that plants can communicate with each other, even over vast distances. How? If injured or under stress, plants can release chemical messages into the air (in the form of Volatile Organic Compounds, or VOCs), and these messages can be transmitted to other plants up to a mile away.
Plants can also communicate to each other underground through what has become known as the Wood Wide Web. The roots of many plants share a symbiotic relationship with fungi by forming mycorrhizae. In this relationship, plants provide the fungi with food (made through photosynthesis) while the fungi provide the plants with nutrients acquired by decomposing organic matter. Impressively, the vast networks of mycorrhizae can connect plants to each other, akin to the way the World Wide Web connects people who may be far apart. Studies have shown that plants can use the mycorrhizae to transmit chemical signals (warning nearby plants of insect attack, for example) and to distribute resources (sugar or nutrients) from plant to plant.
The more I learn about plants, the more fascinated I become.
Have you ever stopped to wonder how water is transferred from a plant’s roots to the rest of the plant’s tissues? Consider a tree: water needs to travel from beneath the ground to reach leaves and branches that may be hundreds of feet above the soil. How does that happen?
And how does the sugar produced within the leaves during photosynthesis get to other parts of the plant so that it can be broken down for energy needed to fuel cellular processes?
Let’s explore a cool aspect of plant science that is fun to experiment with: the plant vascular system.
The Plant Vascular System
I’m sure you’re familiar with the human cardiovascular* system: the series of vessels responsible for transporting necessary substances to and from all of the cells of the body. You may not have known that plants also use a system of tubes to transport essential components throughout the plant. This is called the plant vascular system.
Xylem and Phloem
There are two major types of transport vessels within the plants vascular system: xylem and phloem (pronounced zy-lem and flo-em).
Xylem tubes carry water and minerals obtained from the roots of the plant to the tissues above ground. Phloem tissue transport the sugars produced in the leaves to the tissues in the rest of the plant. How does this transport take place?
Within xylem, water and minerals travels in one direction–against gravity–as it is transported from the roots up and throughout the plant. What drives this process?
Have you ever placed a corner of a paper towel in water, and watched how the water is instantly absorbed and transferred across the expanse of the towel? The water is wicked from the wet part of the towel to the dry parts. This happens because water molecules are naturally “sticky”: the molecules stick to each other, and they stick to other objects.The movement of water through xylem occurs in much the same way. The water in the wet part of the xylem within the roots is wicked up to the dryer parts farther up the plant.
Plants continually lose water through their leaves. In order for photosynthesis to occur, leaves need access to sunlight, water, and carbon dioxide. How do they get the carbon dioxide? The answer may surprise you.
Each leaf has tiny holes called stomata (stoma is the singular form). The word stoma comes from the Greek word for mouth, and perhaps knowing that will help you remember the purpose of leaf stomata. It is through the stomata that leaves “breathe” Leaves don’t actually breathe in the same way we do, but they do perform gas exchange. Through stomata, leaves take in carbon dioxide and release oxygen.
During the day when the sun is shining, the stomata of leaves are kept open to allow gas exchange so that photosynthesis can occur. You can watch the process of stomata opening in this video.
While the stomata are open, water loss occurs from the leaves in a process called transpiration. In fact, transpiration of water from plants back into the air is actually a part of the water cycle!
The water loss from leaves due to transpiration is what drives the movement of water through the xylem through wicking action, as shown in the following video.
Unlike the one-way transport that occurs within xylem, movement of sugars and other compounds through phloem can occur in either direction (up or down). This process, called translocation, delivers sugars from the leaves (the source) to the tissues of the plant that need energy to grow (the sink). Translocation depends on a series of cells within the phloem and requires an expenditure of energy.
Within plants, xylem and phloem tissue exist side by side in what is called vascular bundles. These vascular bundles are organized in different ways depending on the part of the plant. Learn more in the following video.
Hands-On Activities to Study Transpiration and the Plant Vascular System
View Transpiration in Living Leaves
It’s easy to view transpiration for yourself. All you need to do is take a clear, sealable plastic bag and use it to enclose a single green leaf. Make sure that the entire leaf is inside the bag, and that the bag is sealed around the leaf stem as closely as possible to prevent water loss.
Depending on how hot and sunny the location of your plant is, you should begin seeing water vapor accumulate within the bag in no time.
You could take this experiment further and compare how much water transpires from different types of leaves or at different times of day.
View Leaf Stomata Using a Microscope
Speaking of leaves, would you like to view stomata?
All you need is a microscope, blank slides, and items you likely already have at home. While it’s possible to see leaf stomata directly using a microscope, you can create a permanent impression of leaf stomata using a simple procedure.
You can find the instructions for this easy lab here.
Using Celery to View the Vascular System
If you’d like to get a good look at vascular bundles, all you need is some celery stalks (with leaves attached), a glass, water, and food coloring.
Take a stalk of celery and cut off a small slice at the bottom to remove any hardened tissue. Place the celery in a glass containing water (approximately ⅓ cup) and food coloring of your choice. (You’ll want to go heavy on the food coloring if using liquid food coloring. You may not need as much if you’re using food coloring gels).
Within an hour or so, you may notice that the celery leaves have started to change color as the colored water makes its way up the stalk.
After a few hours, remove the celery from the water. Cut a slice off of the bottom of the celery and examine the slice. You should be able to see that as the colored water traveled up through the xylem, it left color behind in the vascular bundles.
If you make a thin cut lengthwise along the celery stalk, you may be able to get another view of the vascular bundles.
Create Your Own Colored Flowers
As long as you have those glasses full of colored water, why not use it to make something pretty?
Most grocery store floral departments sell white (or light-colored) flowers. Buy some, cut a bit off the stems (to remove hardened tissue) and place them in the colored water. The colored water will make its way up the xylem and into the flower petals.
I’ve done this over the years with carnations and chrysanthemums. I’ve seen others have success using white roses as well. You can take this further by cutting the bottom of the stem of a single flower lengthwise and placing each end in a different color of water. This will allow you to create a flower with petals that are two different colors.
I hope I’ve started to convince you that there’s more to plants than meets the eye.
In fact, the reason that leaves change color and fall each Autumn is related to the plant vascular system and transpiration.
It is estimated that up to 95% of a plant’s water loss occurs through transpiration from stomata. Recall that stomata open to allow gas exchange to occur so that photosynthesis can happen.
As the days shorten and temperatures drop in the fall in winter, conditions for photosynthesis are no longer conducive. To prevent water loss through leaf stomata, plants seal off the leaves from the rest of plant. Without access to water and minerals from the soil, the leaves stop conducting photosynthesis. Over time, chlorophyll—the bright green pigment that drives photosynthesis—begins to break down, and the other leaf pigments are revealed. Those brilliant yellows and oranges that we associate with fall foliage are actually present in the leaves year-round, but are masked by the bright green chlorophyll.
If you’d like to learn more about the Science of Autumn Leaves, I have created a free, self-paced, online mini course. It contains instructions for a fun, hands-on experiment that uses simple paper chromatography to separate the different pigments in leaves. By comparing the pigments present in green, red, yellow, and orange leaves, students can visualize what happens inside a leaf during the fall color change. You can access the free course here.
*The cardiovascular system is also referred to as the circulatory system