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Plant Biology - How Plants Drink

Updated on July 9, 2013
watergeek profile image

Susette has a Masters degree in wise use of natural resources. She leads the Green Council and writes for The Sustainable Business Review.

Irrigation Water Soaking Down
Irrigation Water Soaking Down | Source

Most people are aware of the crucial need of plants for water, but not of all the different ways in which water helps a plant. Since plant biology lectures don't normally approach their physiology from the point of view of water, then this article should provide a new look for many people.

Here are some of the ways in which water contributes, both inside and outside of a plant, to its overall health:

  • Water inside the plant transports nutrients to cells and wastes away from them.

  • Helps create energy to fuel plant activities (photosynthesis).

  • Contributes to turgidity, keeping the plant sturdy.

  • Makes nectar that attracts bees and birds to help pollinate.

  • Cools the plant via transpiration.

  • Moistens the air to help distribute scent.

  • Supplies extrafloral nectaries that protect the plant against insects.
  • Rainwater washes the outside of the plant clean.
  • Softens the soil so roots can penetrate down and out.

  • Softens the seed cover so a seed can take root.

Cross Section of Stem
Cross Section of Stem | Source
Cross Section of Root Tip
Cross Section of Root Tip

Absorption of Water by Plants

The first five functions above require a plant to be able to absorb water inside of itself. Plants obtain water through their root systems via osmosis. When the ground around a plant's roots is saturated, the roots suck water in by absorbing it into the root cells, then transporting it up into its stems (or trunk), out into the leaves, and out the rest of the way into the air as transpiration (plant sweat). This process happens more often with some types of plants, and hardly at all with others, depending on the type of plant and the atmosphere in which it grows.

Here is another way to look at it. Ninety percent (90%) of a plant's water evaporates from the leaves, so the plant is continually being dried out. Dry cells pull water in from more saturated areas like wetter cells below and, further down, the wet soil. As a plant dries out then, it creates a subtle suction that pulls fresh water up from the roots to slake its thirst, as long as water is available down there.

A Plant's Internal Watery Functions

Transportation: As water is subtly pushed and pulled into and out of a cell membrane it moves nutrients into the cell and pulls wastes out of it. The nutrients, including nitrogen, have come from the soil. Wastes are transported to the leaves to be discarded into the air, if they are gaseous, or they stay in the leaf structure to be discarded when the leaf grows old.

Simple Photosynthesis Diagram
Simple Photosynthesis Diagram | Source
Orchid Roots (Note the turgidity of some roots, while others are flaccid.)
Orchid Roots (Note the turgidity of some roots, while others are flaccid.) | Source
Painted Lady Butterfly Feeding on Nectar
Painted Lady Butterfly Feeding on Nectar | Source

Photosynthesis: 6H2O + 6CO2 ----> C6H12O6 + 6O2 Water plus carbon dioxide - six molecules of each - make one molecule of sugar, with six of oxygen left over. The sugar is the plant's food, the oxygen its discards. This is the formula for photosynthesis. You can see the integral role that water plays, and also how plants produce the oxygen that we breathe.

The trigger for this conversion is sunlight. Sunlight is absorbed by the plant's chlorophyl - its green pigment - which starts the chemical conversion from water and carbon dioxide to sugar and oxygen. The plant creates its sugary energy in this two stage process, discarding leftover oxygen into the air.

Turgidity: Water provides volume that bolsters the cells and walls of a plant's stems, helping them to stand up straight. Water flows up a series of tubules called "xylem vessels" on its way to the leaves. It fills all the plant's cells, creating pressure against cell walls and making them stiff. Without water most plants become flaccid or wilt.

Nectar Production: Nectaries located inside flowers produce a sweet-tasting, insect attractant called nectar. Nectar is a combination of water and sugars made by the plant. This nectar becomes food for insects like bees and tiny birds like hummingbirds. The plant benefits from their accidental brush against plant pollens as they feed on the nectar. When they fly away they carry the pollen with them and deposit it on the next plant they feed on. The pollen is what plants use to fertilize other plants, so they can make seeds. Without this sweet nectar, most plants would not be able to reproduce.

Transpiration: Water that leaves the plant via transpiration serves the same purpose as sweat in a human body. It removes gaseous wastes (including oxygen) and helps cool the plant. Stomata are little openings in the plant leaf, like air pockets, that let used air and water evaporate out into the atmosphere and let carbon dioxide in to be utilized by the plant in photosynthesis. The stomata are flanked by neighboring "guard cells," which open and close, depending on how much water is in them.

When the plant is dry and there is no water in the guard cells, they automatically close, preventing the plant from transpiring, so it can retain its moisture. When the air is hot outside, the plant and its water heat up too, which makes the water travel to the surface, where some of it enters and opens the guard cells to let the rest of it out as cooling vapor.

Indoor Plants Providing Humidity
Indoor Plants Providing Humidity
Softening the Soil Prior to Planting
Softening the Soil Prior to Planting | Source

External Functions of Water for Plants

Leaf Cleaning: Rainwater and irrigation water falling on a plant cleans the leaves and stems. Without this service, the plant would have a hard time carrying out photosynthesis. Similar to solar panels (which were designed from a plant's ability to photosynthesize), the plant needs its surfaces to be clean in order to function optimally.

Air Humidifier: Humid air around a plant provides two helpful functions. One is to help prevent the plant's sensitive leaves from drying out. The other is to carry scent to foraging insects and birds. Scent carries much further in moist air, which is why roses smell stronger right after it rains. Because plants produce some of their own humidity through transpiration, they are often used as small humidifiers indoors.

Extrafloral Nectary Supply: Extrafloral nectaries do the same thing as nectaries in flowers, but serve a different purpose. In this case, nectaries are positioned on leaves, stems, and sometimes on the outsides of flowers to attract predatory insects that will eat the plant's enemy bugs. Again, water is used to produce the nectar in these outdoor nectaries.

Soil Softening: Water softens and cools hard soil. Softer soil makes it easier for plants to push their roots down and out further, giving them greater access to nutrients and providing a more stable base to hold them up. This is why soil should be soaked before planting baby plants. In turn, roots provide underground tunnels through which water can flow down into the aquafir when it rains. Water stored in the aquafir (underground) becomes future food for trees and larger bushes.

Seed Penetration: The last main service water provides to plants exterior to itself is to soften and penetrate the protective covering of its seeds. This starts the germination process. Many plants produce seeds that can last years and even decades under very dry conditions. When water finally arrives it soaks the seed, softens its walls, and the plant sprouts, thereby perpetuating its species.

Growing Plants with Hydroponics

Hydroponics is a way of growing plants in water without soil, to which nutrients have been added. The Hanging Gardens of Babylon is an example of a well known hydroponic garden. Hydroponic plants grow faster and yield more than their "ensoiled" brethren. Nutrients are delivered several times a day, so the plant doesn't have to search for them, and the energy it saves can be used for plant growth. Because the plant is more vigorous, there are also fewer problems with bugs, fungus, and disease. Watch the video below to learn how cities can use hydroponics to feed their residents.


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    • Simone Smith profile image

      Simone Haruko Smith 5 years ago from San Francisco

      I have never read such a thorough look at all the roles water plays in a plant's life! I usually only hear about the internal chemical process. What a great overview this is!

    • watergeek profile image

      watergeek 5 years ago from Pasadena CA

      Thanks everyone. The more I write about water, the more I see how integral it is to life on earth - we couldn't have life without it. Hydroponics is almost a physical depiction of it's power, since plants are grown with just water alone. You've heard that the ocean is a primordial soup that we all came from? Water filled with nutrients. That's what hydroponics is. So cool.

    • leahlefler profile image

      Leah Lefler 5 years ago from Western New York

      Hydroponics is so cool! I once went on a ride at Epcot (years and years ago) and they had a giant display of growing plants using only water. It was really fascinating! Also, I have been wanting to use the words xylem and phloem in conversation for SO long, and now have a chance to do so! Great hub, watergeek!

    • Joy M profile image

      Joy M 5 years ago from Sumner, Washington

      Interesting hub - particularly the bit about hydroponics. I live in the suburbs so I have space to garden, but the maximizing of space and the health of the plants looked good. I'll have to read more about it. Thanks for the info.

    • whonunuwho profile image

      whonunuwho 5 years ago from United States

      An interesting hub and well done. Few people realize the need for clean water and abundant green plants to renew and continue the basic life processes involved in photosynthesis and the mutual relationship of water and plants, as well as the oxygen that is produced on the planet by green things. Deforestation and poisoning waters with dangerous chemicals as well as heavy air pollution, create an unhealthy environment for all living things. Thanks for your insightful hub.