Ageing in plants.

Ageing in green plants differs in some fundamental ways from the process in animals. The seasonal cycle and persistence of a plant is governed by a combination of the determinate or indeterminate status of meristems (growth centres) and the cell death and disposal strategies employed by plants to generate well-adapted anatomies and morphologies. The degree of perenniality depends on the balance between exploratory growth and the wave of tissue death that succeeds it, and extremes of longevity can arise by relatively minor changes in the quantitative relationship between growth and death. The senescence and elimination of organs and tissues are related to the internal reallocation of resources but are programmed phases in the integrated development of the whole plant and do not represent a kind of ageing by stress or starvation. Meristems of long-lived plants accumulate genetic damage but selection mechanisms exist within the organism to control genetic load, and even to exploit somatic mutations that confer adaptive benefits. It is concluded that most plants do not age in the strict gerontological sense and that extremely long-lived forms like trees and clonal creeping perennials are sustained by selection and correction at the level of semi-autonomous cell lineages.