What tissues have dead cells

In plants, a special tissue, the xylem, ensures the water conduction from the soil to the leaves. It consists of dead cells that are connected to one another by tiny pores called pits. There the pressure gradient and line resistance determine the water flow. In the case of drought stress, the water threads tear and gas bubbles are formed. The result: a dangerous increase in resistance that reinforces itself. All of these are physical processes. And so the xylem was previously thought to be a physical enclave beyond biological control. Botanists around Michele Holbrook from Harvard University in Cambridge (Massachusetts) are now shaking doctrine. In flow experiments on branches of 27 plant species, they found that the resistance of the xylem is reduced to 40 percent when potassium ions are added to the water. The potassium apparently attacks the membranes of the pits, changing their swelling state and pore size. In this way, plants could counteract the loss of power during drought stress. A lively exchange of potassium between the dead xylem and living neighboring tissues shows that they actually make use of this possibility. (Sciencexpress January 25, 2001)

From: Spectrum of Science 4/2001, page 27
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This article is included in Spectrum of Science 4/2001