(1) Juvenile or Vegetative Phase : All the organisms grow to a certain maturity in their life before they start reproducing sexually. The period of growth between their birth upto their reproductive maturity is called the juvenile phase. In plants, the period of growth between seed germination up to initiation of flowering is called vegetative phase. This phase varies from organisms to organisms.
(2) Reproductive Phase : The later part, when the organisms start reproducing sexually is called reproductive phase. In higher plants (seed plants), the flowering phase is called as their reproductive phase. In animals the end of juvenile phase is indicated by appearance of certain morphological and physiological changes prior to reproductive phase. This phase also shows a great variation among different organisms.
(3) Senescence or Old age : The third or last phase of life cycle is known as ‘old age’ or ‘senescence’. During this phase, the end of reproductive phase takes place and ultimately the organism leads towards death. In annuals, biennial plants, vegetative, reproductive and senescent phases can be clearly seen. Whereas in the case of trees which are perennials it is very difficult to clearly define phases. Senescence carries finally to death.
