Poets, writers and artists turn to autumn leaves, ripening crops, ephemeral flowers and so forth as melancholy symbols of mortality, senility, ageing and death.

Such allusions strike a familiar chord in any receptive person - but can be unhelpful when it comes to understanding the biology of senescence.

A question often asked is - do plants age?  In other words, does a thousand year old tree show any of the same biological symptoms as an ageing human?  There isn't a simple answer.

Plants are of gerontological interest because of the enormous range of lifespan in the botanical world, from ephemerals that exist for a few weeks to some of the oldest living individuals on the planet

In many species the whole plant dies following flowering and fruiting, behaviour that seems to be similar in principle to suicidal reproduction in animals like Pacific salmon, mayflies and some octopi and squids.

Senescence is critical for plant longevity and the interplay of sex and death.

The life-expectancy outcome for a plant is the product of the modular, fractal design of the plant body and the quantitative relationship between the making and senescence of structural modules.

Senescence is undoubtedly programmed; the machinery of senescence works within cells; and it is usually followed by death.

Naturally enough, therefore, plant senescence is classified as an example of programmed cell death (PCD), a subject of fundamental biological (and medical) significance.

Ludwig Wittgenstein asserted that "death is not an event in life".  The Reverend Sydney Smith realised that "death must be distinguished from dying, with which it is often confused".  The same confusion besets senescence and dying.

As these pages make clear, I prefer to think of senescence as a developmental rather than a degenerative (pathological or PCD) process which defines the phase of the life of a green organ that follows morphological maturity and may end (but not inevitably) in death (but is functionally distinct from it).

It looks to me as if PCD is what happens to a leaf or other plant part after senescence has been completed.

We can see clues inside the senescing and post-senescent cell, where the relationship between the plastid and another important organelle, the vacuole, is played out in response to genetic and environmental prompting.

The vacuole sustains viability during senescence through protecting the cell from damage by opportunist pest and diseases, excess light and accumulations of toxic by-products of recycling.

But once senescence is over, the vacuole becomes a kind of suicide pill, rapidly killing and cauterising the cell that contains it.

Now there's a romantic image, don't you agree?  Straight out of Sonnet 73 (one of Shakespeare's most sombre musings on senescence) - "...Consumed with that which it was nourished by..."

It just goes to show that deeper scientific understanding doesn't have to destroy the poetry of natural processes.
 

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