You’ve probably noticed that plant tissues are generally moist inside. Leaf tissues
contain lots of water—in the form of both liquid and vapor. Remember that layer of
spongy cells in the mesophyll? The spaces between those cells is where all sorts of gas
exchanges take place, as oxygen and carbon dioxide gases are formed and consumed. Those
spaces also contain water vapor that has evaporated from the moist surfaces of the cells. 
During photosynthesis, the stomata must open to take in
carbon dioxide. But when the stomata are open, water vapor from the area around the spongy
cells escapes into the surrounding air. The evaporation and release of water vapor cools
the leaf surfaces, in much the same way that the evaporation of perspiration cools our
bodies. (Think of how much cooler and more humid it is in a thick, lush forest. All those
plants are releasing water vapor as they photosynthesize.) However, this evaporation also
drives some other very important functions.
Did you ever wonder how plants actually take up water?
Since they don’t have "muscles," how do plant roots pull water into their
tissues and transport it all the way up to the treetops? You guessed it: the engine
driving the uptake of water is transpiration. As water evaporates and escapes through the
stomata, it creates a negative pressure—something like a vacuum—within the xylem
cells. The xylem cells respond by drawing in more water.
Water Movement
From soil, through plant, into atmosphere
The following analogy isn’t perfect, but it may help
you imagine the process. If you suck on a straw in a glass full of water, you pull water
up the straw. By sucking on the straw, you are creating a negative pressure at that end,
and the water moves up the straw. Now imagine the xylem cells in a stem as a system of
conduits, and you can see how negative pressure at one end would draw water through the
conduits. What creates the negative pressure? The surface tension of water evaporating
from the spongy mesophyll cells in the plant’s leaves creates the pulling force.
Why is transpiration so important? Besides cooling the
plant, transpiration drives its "circulatory system." Imagine a vein at the tip
of a leaf. Through the intricate network of vascular tissues, this leaf is connected to
the roots—similar to the way the capillaries in your fingertips are connected to your
heart. In humans, the force driving circulation is the heartbeat; in plants, it’s
transpiration. Water vapor escaping the leaf surface creates a tension that draws more
water up to replace it. Through the network of veins, this process occurs in every leaf
throughout the entire plant.
Just what is it that the plant needs to circulate? For one
thing, nutrients. Water drawn from the soil usually contains a variety of dissolved
nutrients. As the roots take in water, they also take in these nutrients. The process of
transpiration drives the circulation of these nutrients throughout the plant. This network
also provides pathways for transporting manufactured carbohydrates out of the leaves and
into storage.
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Thirsty?
Just how much water do plants lose by transpiration? On a sunny summer afternoon,
an average-size maple tree loses more than fifty gallons per hour by
transpiration!
It's a long way
up! Imagine yourself in a forest,
looking up into the treetops. Consider for a moment that the negative pressure generated
by transpiration is pulling water up through the xylem all the way to the very highest
branches. The water may travel 300 feet or more, from root to treetop. And remember, this
is happening against the force of gravity! |
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