bark, wood, roots, & leaves
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forest canopy
Above: The branches and leaves of taller trees form the forest canopy, home to a range of completely arboreal animals. Below right: Far beneath the canopy, fern leaves grow on the forest floor. Photos by Maya Walters.
The stomata* in the leaves, however, also let water vapor escape from the leaves. While this does work to cool leaves in the hot sun, plants in dry areas have fewer stomata or keep them closed for most of the day. This slows this rate of water loss (called transpiration) and prevents dehydration. In most trees, the fact that much more liquid is being moved upwards from the roots to the leaves than in the opposite direction is obvious when you consider that cells within the entire outer trunk carry nutrients upwards, but only the thin layer of bark carries them downwards.
When a plant appears to wilt during a dry season, it is because there isn't enough water in each of its cells to keep them rigid, and they have "deflated".
How does the water get up to the leaves to begin with? Of course it is brought up from the tree roots through long chains of cells in the tree trunk, but what is the force that makes it rise? This question has puzzled people for a long time. The most popular explanation now is that since water tends to "stick" to itself, any water that is evaporated at the top of the tree (through the leaves) is immediately replaced by more water being sucked up through the roots, keeping the tree's "plumbing" full of liquid at all times.
fern leaves

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