Variability of longevity of multicellular organisms is extremely wide.
There are no one simple correlation between any biological characteristics or attributes of organisms and longevity.
The general tendency shows that most simplest multicellular organisms may have practical immortality and better regeneration abilities than more advanced on the phylogenetic three.
Non-separated or connected plants, one multi-body organisms or connected multi-organisms colony?
Pando or the Trembling Giant (1) is a single living organism with multiple genetically identical trunks sharing the same underground root system. This plant may have a total combined weight up to 6,000 T. Individual trunks or stems are surviving for 100 and more years and die, but their root system is estimated as dozens of thousands of years old. It’s difficult to say if they are dying because of the aging process or any other factors created by the same organism such as a density of clones may lead to a shortage of nutrition or self- poisoning by different accumulated utilized substances, etc. As in the case of fungi, their root system (comparable to mycelium) continues its existing and produces new trunks or stems (comparable to mushroom bodies, but with a much longer life span).
Is this plant an individual single multi-body organism or colony of multiple individual single organisms, connected and conjoined together?
The same question should be asked in other cases of connected plants, fungi, polyps, etc.
The Death of the whole organism or of its parts?
In the case of an organism with multiple connected bodies as some plants with shared root systems and many trunks (“bodies”), we have distinguish death of bodies and death of the whole organism including the sharing underground root system. Death of each or even many “bodies” is not the death of the whole organism, which could live indefinitely. If there are some aging signs in the “bodies,” it is not the aging of whole organism, which is not showing aging. It could be more adequate to compare it to the growth and cut of nails or hair, changing snake skin, “death” of leaves of plants and their replacement next season, etc. We could see it as “death” of some changeable parts, but not the death of the whole organism.
When one or few “bodies” are dead, is it the death of some connected organisms or just dead parts of the organism, which is continuing to live?
Making the question more complicated: in the case of Siamese twins or conjoined twins, if they will be defined as one organism with additional parts of two or more tweens’ bodies, having only one head with one brain? In case of human or other animals, we intuitively will define it as one organism, but if conjoined twins have two heads and two brains with some additional body parts or without them, would we define them as two conjoined organisms? If additional parts would be surgically removed – will it be death of the organism or only of its parts? In the case of one body and two heads with two brains we intuitively will define them as two organisms with conjoined bodies. In the case of two heads and two brains and only one body, it certainly would be very hard to define for humans. In the case of human beings we have to add to biological definitions also psychological self-determination as an individual single organism; the ability of self-recognition as an individual single organism. In the case of multi-body plants, we could define it as one single organism and death of one or few connected parts as removing of additional parts (or bodies), from the same single organism, but not as a death of the whole organism, claiming “immortality” for this organism (which is a very semantic definition in such cases). Alternatively, if we would define multi-body plants as a few individual single organisms connected between them, we should define the death of any of them as death of this specific organism, claiming its “mortality.” To distinguish between “mortality” and “immortality” of such multi-bodies organisms would be tested on the presence or absence of aging correlated to an accepted definition: a) in reference to the definition of “connected individual organisms,” if each part or body of multi-body plants has its own aging and natural death according to its chronological creation, it would be the case of mortal organisms and vice versa; b) in reference to the definition of “one organism with many connected bodies,” when an individual body has both aging and death, but the whole organism has neither overwhelming aging nor natural death, it would be the case of immortality.
Pando or the Trembling Giant
Western North American Naturalist 68(4):493-497. 2008
2. Pando, located about 1 mile southwest of Fish Lake on State Highway 25, is believed to be the largest organism ever found at nearly 13 million pounds. The clone spreads over 106 acres, consisting of over 40,000 individual trees.
4. Grant and his CU-Boulder colleagues calculated the Utah aspen clone weighed roughly 13 million pounds, or 6,500 tons, which would make it the most massive organism in the world. – See more at: http://www.colorado.edu/news/releases/2014/09/19/fall-aspen-tree-colors-setting-prime-time-says-cu-boulder-prof#sthash.emTfLtdr.dpuf