How plants move water
by Paula Ogilvie:
Drink Up! Imagine being rooted in place, unable to move around, yet requiring food, water, and shelter to survive. With this dilemma plants have developed ways to successfully survive. Like animals, plants are mostly made of water, which is needed for all living processes including photosynthesis. When you water plants, you water the ground at the roots. But how does the water get from the roots to the leaves?
Plants have long hollow tubes called xylem cells that are connected from root tips to leaf tips. Roots take up a solution of water and dissolved minerals, which moves up the tubes with gravity-defying action as each water molecule sticks to the next. As water is used by the plant or lost to the environment through transpiration from the leaves, the pull upward on the next water molecule keeps the process going. And this pull is strong! Water goes all the way up to the tips of the tallest trees; consider redwoods over 350 feet tall. It’s called capillary movement; no energy is used, it’s all passive movement. (You can also see the attraction between water molecules on a pond with strider bugs. The surface tension between water molecules allows them to slide across the top of the water.)
At night, when transpiration is very low, water moves into the root hairs thru osmosis - the movement of water molecules from a lower concentration solution (of dissolved minerals in soil) to a higher concentration solution (in the root hairs) - and pressure builds. The root pressure forces water up from the root into and through the xylem as more water and minerals are “pulled” into the root from the soil. This force results in tiny droplets forming on the ends of leaves or grass early in the morning - moisture that comes from within the plant. It isn’t the same as dew, which is moisture condensed from the air.
Here is something interesting. You’ve probably heard about tree rings that tell a tree’s age. Each spring trees produce new water-carrying xylem cells located just inside the bark. When a tree trunk is cut you can see the rings, each one representing a year’s growth. The closer the rings the drier the years; those further apart show wetter years. By counting the rings you can determine both the tree’s age and the weather for each year. The dead xylem cells are also called – wood!
Since Colorado is naturally dry - and getting drier – our plants don’t want to lose much water. Here are a few conservation tricks that plants employ. Many tropical plants have paper-thin leaves, while those from dry climates are often thicker and sometimes have a waxy coating. This coating or waxy “cuticle” is usually found on the upper side of the leaf only. It reflects light and helps reduce evaporation. The leaf’s underside lacks the coating and has tiny, microscopic pores where plants take in carbon dioxide for photosynthesis. Many plants close these pores during the day to prevent water loss and open them in the cool of the evening.
In the grocery store we see green flowered carnations and very odd colored blue orchids. The color is on the inside. How are white flowers easily changed to these unnatural colors?
Try this demonstration project for kids—of all ages:
You need 3 clear glasses, 3 stalks of celery, and 3 food color dyes—darker colors like red, blue and green. At the bottom of each glass add 1 dye color and a bit of water—you want strong color. Trim the base of a celery stalk and put it in the glass. Repeat for each color. Place in a window so you can watch the leaves pull up the water and the dye. You may be able to see streaks of dye going up the stalk. If you cut across the stalk—you might see the tube ends. Once completed, it is ok to eat this experiment.
Colorado gardeners are often told to look for plants from Mediterranean or from other dry climates. What characteristics do many of these plants have?
First, a soft grey to white leaf color. A pale color helps reflect and reduce the amount of sunlight hitting the leaf, reducing moisture loss. Many have fuzzy leaves with tiny hairs that provide shade by reflecting sunlight and slowing drying wind. The thorns on cactus plants are actually for shade, in addition to preventing grazing animals from eating their succulent, water filled stems.
Thicker leaves, stems, and roots are other ways that plants hold onto water. Yet many tropical orchids have thick leaves and fat grey roots. That’s because they grow high up on tree branches where the air is moist but windy enough to dry them out. So a plant’s leaves will actually tell you where it lives and give you clues on how much to water.
If you forget to water them, your plants will slowly go limp as they use up water for daily metabolism. The plant cells are losing the water that keeps them rigid. Since they lack bones, the water stored in their cells is what keeps the plant upright. Hopefully, they will revive once you give them a drink.
Plants also need air (oxygen) at their roots, which will rot without it. Waterlogged soil becomes a habitat for fungi and bacteria that attack plant roots. The lack of oxygen also interferes with water and nutrient uptake and photosynthesis. Overwatered plants develop symptoms that resemble drought stress. They become unable to absorb water through damaged roots while they continue to lose moisture through their leaves. It’s quite possible to overwater gardens in a dry climate like Denver’s and it’s the most common reason for the demise of houseplants.
Paula Ogilvie lives and gardens in Denver. She teaches botany at Community College of Denver and presents workshops for student science and gardening clubs.