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Leaf To Root, Come In, Please
| Article
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11193 |
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Section : |
NATURAL SCIENCE
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| Issue
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10 / 1993 |
2,860 Words |
| Author
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Eric Davies
Eric Davies is on the faculty of the School of Biological Sciences at the University of Nebraska. He recently received a major grant from the National Science Foundation for studying the role of electrical signals in the expression of genes. |
Imagine an oak tree in Oshkosh, Nebraska, or, better yet, imagine being an oak tree in Oshkosh, Nebraska. It is midsummer, and it is 108 in the shade (but there isn't any--you're it!). There has been no rain for four weeks, and you are suffering from drought. You can't crawl under a tree, burrow into the ground, put up a parasol, or go buy a drink. All you can do is cut down on your water loss and try to survive until things get better--and for what? For when it will be 20° below zero, winds howling in from the Rockies, no rain for weeks--just snow and more snow. There's no flying south like the birds, and there's no central heating. Despite all this, plants, and trees especially, are the longest living organisms in the world, some being 3,000 and perhaps 4,000 years old.
The oak tree is a marvel of bio-organic engineering, with its acorn sprouting roots that grow downward (how do they know?) and a stem that grows upward (how does it know?) and bears leaves, flowers, and fruit. The leaves use the energy of sunlight to manufacture sugars in the process called photosynthesis, and these sugars travel to the rest of the plant through a transport system called the phloem. Roots use energy derived from the sugars made by the leaves to take up nutrients, which, along with copious quantities of water, are transported upward to the rest of the plant in a distribution system called the xylem. Together, the phloem and xylem constitute the tree's vascular system.
Oak trees live in an environment that can be either dependable and useful (gravity, day length) or variable and damaging (hail, insect attack, floods, or scorching heat). Unlike animals, oak trees cannot escape their environment, and so they must be highly sensitive and responsive to it. Although not all parts of the oak are exposed to the same stimuli, different parts of the tree respond in a coordinated fashion. This raises the possibility that there is some kind of communication between various regions of the tree.
For years, researchers have concentrated their efforts on chemical (hormonal) signals in plants. A growing body of research evidence shows, however, that plants also rely on electrical and hydraulic signals. In fact, it is in huge plants such as an oak tree that the need for signals other than hormones becomes more obvious. For instance, for all the leaves of an oak to respond to a chemical signal coming from the roots, enormous amounts of the chemical would need to be synthesized and transported. In contrast, both electrical and hydraulic signals consume no chemicals in their propagation.
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