How do plants transport water?
Plants might look static, but they’re hydraulic engineers that are always at work
Water is vital to plants. From growth and photosynthesis to flowering and keeping their leaves in shape, they need it for everything. To make sure they’re getting enough of the stuff, plants have evolved an efficient water transportation system.
A plant’s roots take in water and minerals from the soil as they move from the damp ground into the dry plant through a permeable layer. The longer they grow, the more surface area they have for absorption, and, amazingly, they can even grow in the direction of the wettest patches – a process called hydrotropism.
While we have a heart to pump blood around our body, plants have to rely on physics to get water from the ground to their leaves. Xylem tubes, made out of dead cells, are strong tubes running the whole length of the plant. As openings on the leaves (known as stomata) open to allow carbon dioxide in, water evaporates through transpiration. More water molecules are drawn up from further down the xylem tube to replace it and to balance out the difference in pressure, sticking to the molecules ahead of them and producing an effect similar to sucking on a drinking straw. On sunny and windy days, water evaporates from the leaves at a higher rate, so more is pulled up from the roots to counteract this.
From the ground up
With this effective system, the tallest tree can get water to its leaves
Living without water
When there’s no rain and the soil is drying up, plants have to adapt to survive. By keeping their stomata closed during the hottest part of the day they can reduce transpiration, but with no carbon dioxide entering the leaves they can’t photosynthesise to produce sugars. When temperatures drop at night, stomata can be opened to let carbon dioxide in while losing as little water as possible.
Some plants are used to coping with water scarcity. These species grow extremely long roots, and their leaves are either fleshy and covered in a waxy layer or reduced to spines to minimise water loss through evaporation. When rain does arrive, some species can store the water in tubers or bulbs under the ground for later use.
This article was originally published in How It Works issue 116, written by Victoria Williams
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