Transport of Water
The transport of water and mineral nutrients occurs in the xylem of all plant organs, but it occurs over the greatest distances in the stems of tall trees. During the day, water is constantly evaporating from the plant, mainly through leaf stomata. This water loss is called transpiration (TRAN-spuh-RAY-shuhn). The large amount of water lost from the plant is a result of the plant's need to obtain carbon dioxide from the air through the stomata. Exactly how do huge trees, like redwoods, move water and mineral nutrients up their 100 m tall trunks?
According to the cohesion-tension theory, water is pulled up the stem xylem by the strong attraction of water molecules to each other, a property of water called cohesion. The movement also depends on the rigid xylem walls and the strong attraction of water molecules to the xylem wall, which is called adhesion. The thin, continuous columns of water extend from the leaves through the stems and into the roots. As water evaporates from the leaf, the water column is subject to great tension. However, the water column does not break because of cohesion, and it does not pull away from the xylem walls because of adhesion. The only other possibility is for it to be pulled upward. The pull at the top of the tree extends all the way to the bottom of the column. As water is pulled up the xylem, more water enters the roots from the soil.
With abundant parenchyma cells in the cortex, plant stems are adapted for storage in most species. In some species, storage is a major function. Cactus stems are specialized for storing water. The roots of a cactus are found close to the soil surface, enabling them to absorb rainwater quickly and transport it to the cactus's fleshy stem. The edible white potato is an underground stem that is specialized for storing starch. The "eyes" of white potatoes are buds that have the ability to develop into new shoots.
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