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why do plants wilt when deprived of water

If you think about tall trees over a 100 feet tall в water must be delivered to leaves at the top of the tree all of the time. If not, those leaves will wilt and die. When water escapes through the leaves into the air through transpiration, it provides a pulling force. This pulling force pulls water up and out a plant through tiny little tubes called the xylem. These tubes (xylem) can be thought of as pipes inside the plant that deliver water to all parts of the plant. They are highly effective at stacking up water molecules into long chains and pulling them upward and outward to the leaves of the plant. Xylem exists in all parts of the plant в roots, stems, leaves and everywhere in between.

As water is pulled up the plant through the xylem, the water molecules are all tied together like a long chain (capillary forces). The pulling force created by transpiration pulls these long chains of water upward and outward to the leaves. These chains of water create turgidity (meaning the plant is rigid, strong and upright; essentially the opposite of wilting). Plants do not have bones to keep them upright в they rely on this turgidity to keep them upright and strong. When the soil of a plant runs too low of available water, the water chains in the xylem become thinner and thinner due to less water.

Effectively, the plant is losing water faster than it is absorbing it. When this happens, the plant loses its turgidity and begins to wilt.
Just like humans and many other organisms, plants are mostly made of water. If you are dehydrated, your metabolic processes tend to become slow, and eventually you will die. The biggest reason plants wilt, however is that their skeletal structure is made up of really tiny tubes, called xylem and phloem, that run throughout the plant that also carry water up the plant and nutrients down through the plant after photosynthesis. The pressure caused by carrying this water up the plant is called turgor pressure.

Basically, the way the water is moved through the plant is through tiny, generally microscopic pores, called stomata, on the leaves that open and close based on environmental conditions. When the stomata are open, a low pressure is created within the xylem tubes, causing water to be absorbed at the roots and to be brought up by the plants- the same principle as drinking through a straw. When a plant is stressed due to a lack of water, these stomata close, making it difficult for the plant to take water up through the plant, leading to loss of turgidity, and eventually to wilting.

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