Like all organisms, plants grow old and die. Their life span may vary from a few years to few days. Even before the death of the whole plant, it is likely that a number of its organs and tissues have died earlier. As the young plant grows old, it undergoes ageing and develops into a mature plant in an orderly fashion. This phenomenon of deteriorative changes with ageing is called senescence.
Senescence may be defined as the period between reproductive maturity and death of a plant or plant part. During senescence the functional capacity decreases, cellular break down and metabolic failures increase.
The branch of botany that studies ageing, senescence and abscission in plants is called as phytogerontology.
In plants such as wheat, rice, gram and mustard the plants die after seed production. This is also observed in monocarpic plants which live for several years but flower only once in their life time as in sago palm and bamboos.
In perennial plants, the tips of the main shoot remain meristematic. It is the progressive senescence of older leaves and lateral organs which may occur at any time of the year. The tips of the main shoot continue to produce new buds and leaves.
In certain herbaceous perennials such as banana and gladiolus, the above the ground part of the shoot dies each year after flowering and fruiting but the underground parts (stem and roots) survive and put out new shoots the next year.
In temperate deciduous trees such as elm and maple, all the leaves senesce and fall at certain seasons of the year like late autumn in October. This is also called as simultaneous senescence. It is controlled by environmental factors.