How Water Travels Through a Plant
When we need to “Water” ourselves we take a drink towards the top of our stem, our mouth.
This water, sooner or later is then is eliminated, as if by gravity below the point of entry.
Plants, on the other hand, take a drink at the very bottom of their structure, their roots, and then, defying gravity, eliminate the excess at the very top of their structure, their leaves.
How is this journey accomplished?
Pathway of Water Through a Plant
- Water enters the plant through the root hairs.
- It is then conducted upward in the stem via the xylem.
- Water exits the plant through the stomata located on the leaves.
Osmosis is the process used for the water to enter the root hairs.
Cohesion-tension theory is believed to be the method that water is conducted upward via the xylem. Think of adjacent drops of water, which when their exterior barriers are broken, move & merge into one larger drop.
Transpiration is the process of water evaporating from the leaf.
Roots have microscopic root hairs to take up more water
Roots
At the root of it all…are the roots.
Roots are designed in different ways to anchor plants.
Their main purpose, however, is to gather from the soil the water & nutrients needed to sustain & grow the plant.
To do this the root is designed in a point called the apical meristem.
This apical meristem pushes through the soil in search of nourishment.
In order to increase the surface for absorption the root has microscopic root hairs that increase the surface several hundred times. By osmosis, these root hairs bring water into the root.
Osmosis is the process that allows water to pass through cell epidermal walls. Water molecules attempt to balance the amount of water pressure on either side of the wall. Once the water enters the root hairs the equalizing mechanism is shut down & the water can’t escape. It then moves from the root hairs to the roots via a process called turgor.
Stem
Water moves through the plant from roots to leaves through the xylem. Osmosis moves the water from the ground to the root systems & cell turgor moves the water through the xylem.
Cell turgor is what keeps the firmness in plants. With low turgor you get wilting. You want to keep all cells filled with water or the cells begin to die.
This is why you cut flowers under water to use in your flower arrangement.
To protect the cells from exploding with too much water as it moves from the roots to the xylem, the cell walls protect themselves by pushing the water out into the hollow tubular cells in the root’s center using a gentle pumping action called root pressure. This brings the water into the xylem, which now conducts the water upwards from the roots to the leaves.
Transpiration
Rhododendron leaves in winter will curl up to reduce the number of stomates on exposed surfaces to reduce the transpiration rate of water. In my garden the rhodi leaves curl when the temperature moves into the 20’s
Transpiration occurs in the leaves by way of the stomates usually located on the underside of the leaves. Water is vaporized through the stomates & is replaced by liquid water that has been delivered by the continuous flow upwards through the xylem system of roots, stems and leaves.
This transpirational pull is felt throughout the entire length of the plant.
In most plants about 98% of the water is lost through the transpired water vapor from under the leaves. A 48-ft tall Silver Maple is thought to transpire up to 58 gallons of water an hour.
You can perceive this on a hot summer day when sitting under a large shade tree & enjoying the cooling effects of the transpired water.
Stomates
Plants protect themselves from too much loss of moisture by closing the stomates in the underside of the leaves. Generally, stomates are open during the day & close in the night-time. In the winter when the ground freezes you can see the curled leaves of large leaved rhododendrons as they reduce the available surface of exposed stomates.
In preparation for winter and in times of drought, leaves will fall from the trees in a process called abscission further reducing the stomatic surfaces used for transpiration.
Cold Hardening
In winter, the deciduous trees have lost their leaves and water movement halts. If the water freezes in the cells, it ruptures the cell membranes. You can see this quickly in Colocasia AKA Elephant Ears after the first cold snap.
One way that winter-hardy plants survive is by a process of cold hardening, which uses sugars that function somewhat like antifreeze.
The pathways for water distribution are also modeled for nutrient distribution in the plants but that is worth another entire post.