Different Phases in Sexual Reproduction

Different phases in sexual reproduction in Plants:

1. Vegetative Phase: All plants have to reach a certain stage of growth and maturity in their life before they can reproduce sexually. The time period required between birth and sexual maturity is called the vegetative phase in plants. This phase is of variable durations in different organisms. The end marks the beginning of the reproductive phase.
   
   
2. Reproductive Phase/ Maturity Phase: The reproductive stage is the stage of the life cycle of a plant that has reached reproductive maturity and is capable of producing gametes. This stage is characterized by changes in certain hormones. The beginning of the reproductive phase can be seen easily in the higher plants when they come to flower. The reproductive phase is also of variable duration in different organisms.
   
   
3. Senescent Phases: It is the end of the reproductive phase. The end of the reproductive phase can be considered as one of the parameters of senescence which ultimately leads to death.

• Annual and biennial plants show vegetative, reproductive and senescent phases, but in the perennial species, it is very difficult to clearly define these phases. 
• A few plants exhibit unusual flowering phenomena; some of them such as bamboo species flower only once in their lifetime, generally after 50-100 years, produce a large number of fruits and die. 
• Another plant, Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. As many of you would know, this plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches.

Different phases in sexual reproduction in animals:

1. Juvenile phase: All animals have to reach a certain stage of growth and maturity in their life before they can reproduce sexually. That period of growth is called the juvenile phase in animals. This phase is of variable durations in different organisms. In animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour.
   
   
2. Reproductive Phase/ Maturity Phase: The reproductive phase is also of variable duration in different animals. The reproductive stage is the stage of the life cycle of an animal that has reached reproductive maturity and is capable of producing gametes. This stage is characterized by changes in certain hormones. The females of placental mammals exhibit cyclical changes in the activities of ovaries, accessory ducts and hormones during the reproductive phase.
   a) Oestrus cycle: The cyclic changes in non-primates like cows, sheep, rats, deers, dogs, etc. is known as the oestrus cycle. 
   b) Menstrual cycle: The cyclic changes in primates like monkeys, apes and humans.
   
   
   
   Based on breeding season, mammals are of 2 types: 
   (i) Seasonal breeders: Many mammals, especially those living in natural, wild conditions exhibit such cycles only during favourable seasons in their reproductive phase and are therefore called seasonal breeders. Example - frogs, lizards, most birds, deers etc.,
   (ii) Continuous breeders: Many other mammals are reproductively active throughout their reproductive phase and hence are called continuous breeders. Example - honey bees, poultry, rabbits etc.,
   
   
3. Senescent Phase: It is the end of the reproductive phase. The end of the reproductive phase can be considered as one of the parameters of senescence or old age. Old age ultimately leads to death. During this phase, concomitant changes (like declining metabolism, etc.) occur in the body.

In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and certain environmental factors regulates the reproductive processes and the associated behavioural expressions of organisms.