The importance of plants in everyday life cannot be understated. They provide oxygen, food, shelter, shade and countless other functions.
They also contribute to the movement of water through the environment. Plants themselves boast their own unique way of taking in water and releasing it into the atmosphere.
TL;DR (Too Long; Didn't Read)
Plants require water for biological processes. The movement of water through plants involves a pathway from root to stem to leaf, using specialized cells.
Water Transportation in Plants
Water is essential to the life of plants at the most basic levels of metabolism. In order for a plant to access water for biological processes, it needs a system to move water from the ground to different plant parts.
The chief water movement in plants is through osmosis from the roots to the stems to the leaves. How does water transportation in plants occur? Water movement in plants occurs because plants have a special system to draw water in, conduct it through the body of the plant and eventually to release it to the surrounding environment.
In humans, fluids circulate in bodies via the circulatory system of veins, arteries and capillaries. There is also specialized network of tissues that aids the process of nutrient and water movement in plants. These are called xylem and phloem.
What Is Xylem?
Plant roots reach into the soil and seek water and minerals for the plant to grow. Once the roots find water, the water travels up through the plant all the way to its leaves. The plant structure used for this water movement in plants from root to leaf is called xylem.
Xylem is a kind of plant tissue that is made out of dead cells that are stretched out. These cells, named tracheids, possess a tough composition, made of cellulose and the resilient substance lignin. The cells are stacked and form vessels, allowing water to travel with little resistance. Xylem is waterproof and has no cytoplasm in its cells.
Water travels up the plant through the xylem tubes until it reaches mesophyll cells, which are spongy cells that release the water through miniscule pores called stomata. Simultaneously, stomata also allow for carbon dioxide to enter a plant for photosynthesis. Plants possess several stomata on their leaves, particularly on the underside.
Different environmental factors can rapidly trigger stomata to open or close. These include temperature, carbon dioxide concentrate in the leaf, water and light. Stomata close up at night; they also close in response to too much internal carbon dioxide and to prevent too much water loss, depending on the air temperature.
Light triggers them to open. This signals the plant’s guard cells to draw in water. The guard cells’ membranes then pump out hydrogen ions, and potassium ions can enter the cell. Osmotic pressure declines when the potassium builds up, resulting in water attraction to the cell. In hot temperatures, these guard cells do not have as much access to water and can close up.
Air can also fill the xylem’s tracheids. This process, named cavitation, can result in tiny air bubbles that could impede water flow. To avoid this problem, pits in xylem cells allow for water to move while preventing gas bubbles from escaping. The rest of the xylem can continue moving water as usual. At night, when stomata close up, the gas bubble may dissolve into the water again.
Water exits as water vapor from the leaves and evaporates. This process is called transpiration.
What Is Phloem?
In contrast to xylem, phloem cells are living cells. They make up vessels as well, and their main function is to move nutrients throughout the plant. These nutrients include amino acids and sugars.
Over the course of the seasons, for example, sugars may be moved from the roots to the leaves. The process of moving nutrients throughout the plant is called translocation.
Osmosis in Roots
The tips of plant roots contain root hair cells. These are rectangular in shape and have long tails. The root hairs themselves can extend into the soil and absorb water in a process of diffusion called osmosis.
Osmosis in roots leads to water moving into root hair cells. Once water moves into the root hair cells, it can travel throughout the plant. Water first makes its way to the root cortex and passes through the endodermis. Once there, it can access the xylem tubes and allow for water transportation in plants.
There are multiple paths for water’s journey across roots. One method keeps water between cells so that the water does not enter them. In another method, water does cross cell membranes. It can then move out of the membrane to other cells. Yet another method of water movement from the roots involves water passing through cells via junctions between cells called plasmodesmata.
After passing through the root cortex, water moves through the endodermis, or waxy cellular layer. This is a sort of barrier for water and shunts it through endodermal cells like a filter. Then water can access the xylem and proceed toward the plant’s leaves.
Transpiration Stream Definition
People and animals breathe. Plants possess their own process of breathing, but it is called transpiration.
Once water travels through a plant and reaches its leaves, it can eventually release from the leaves via transpiration. You can see evidence of this method of “breathing” by securing a clear plastic bag around a plant’s leaves. Eventually you'll see water droplets in the bag, demonstrating transpiration from the leaves.
The transpiration stream describes the process of water transported from the xylem in a stream from root to leaf. It also includes the method of moving mineral ions around, keeping plants sturdy via water turgor, making sure leaves have enough water for photosynthesis and allowing the water to evaporate to keep leaves cool in warm temperatures.
Effects on Transpiration
When plant transpiration is combined with evaporation from land, this is called evapotranspiration. The transpiration stream results in approximately 10 percent of moisture release into the atmosphere of the Earth.
Plants can lose a significant amount of water through transpiration. Even though it is not a process that can be seen with the naked eye, the effect of water loss is measurable. Even corn can release as much as 4,000 gallons of water in a day. Large hardwood trees can release as much as 40,000 gallons daily.
Rates of transpiration vary depending on the status of the atmosphere around a plant. Weather conditions play a prominent role, but transpiration is also affected by soils and topography.
Temperature alone greatly affects transpiration. In warm weather, and in strong sun, the stomata are triggered to open and release water vapor. However, in cold weather, the opposite situation occurs, and the stomata will close up.
The dryness of the air directly affects transpiration rates. If the weather is humid and the air full of moisture, a plant is less likely to release as much water via transpiration. However, in dry conditions, plants readily transpire. Even the movement of wind can increase transpiration.
Different plants adapt to different growth environments, including in their rates of transpiration. In arid climates such as deserts, some plants can hold onto water better, such as succulents or cacti.
About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction and fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.