|MadSci Network: General Biology|
The reason it is still alive is that it grows very slowly and lives in (what was once) a very stable environment. I am surprised that you did not ask how its age is known. This is an interesting answer.
The Creosote-bush grows in a circle. The initial plant is a bush which spreads by getting wider. As the bush gets wider the periphery encounters new soil and thrives while the center depletes the old soil and dies out. Eventually, it forms a circle of (what appear to be) separate bushes.
If the rate at which the diameter of the circle increases can be estimated, then it is possible to determine the approximate age of a particular specimen (which presumably grew from a single seed) by measuring the diameter of the circle. The Mojave Desert specimen in question was estimated to be about 6000 years old, which makes it twice as old as the oldest California Redwood trees (Sequoia) and at least 1000 years older than the oldest Bristle-cone pine (Pinus aristata, found in Arizona, New Mexico and Colorado). The ages of both the Bristle Cone pines and the Redwood trees is determined by counting the concentric growth rings of the wood, each of which represents a single year.
In a related story, the largest living thing is, apparently, a fungus. Funguses grow in the soil in a manner analogous to the Creosote-bush. They start from a single cell (spore) and spread outward. They die off in the middle but survive around the periphery as is evidenced by the external manifestation of their presence, the mushroom. Mushrooms are often seen growing in circles or "Fairy Rings" as they are called. The mushrooms are reproductive structures which develop periodically from an underground mass of cells which form the treads of the mycelium, which is the name for the body of the fungus.
Well, somewhere in Michigan a giant Fairy Ring (many miles in diameter, I forget exactly how many) was found. I guess it was identified by aerial photography and later verified on the ground by DNA tests to be part of the same clone (continuous line of cells of one individual). A rough estimate of the mass of living matter (again, I don't have the figures on hand) puts it way above anything else in terms of sheer bulk. No word, as far as I know, on the age of the fungus.
If you want to see a picture of Creosote-bush, go here
For more information and other pictures of Creosote-bush go here
For information about plants in general go here.
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