Angiosperms reproduce both by vegetative as well as by sexual methods. Sexual reproduction occurs by fusion of male and female gametes produced in the flower. Thus, flower represents the reproductive unit of a flowering plant.
Angiosperms can be classified as annuals, biennials and perennials depending upon the time they take to complete the life cycle including flowering, fruiting, and death.
(a) Annuals: The plants which complete their life cycle including flowering to seed formation within one season are called annuals. For example, pea.
(b) Biennials: Plants which complete their life cycle in two seasons are called biennials. In the first season these plants remain in the vegetative state, and in the second season, they produce flowers, fruits, and seeds and then die. For example, radish.
(c) Perennials: Plants which live for several years are termed perennials. Their vegetative stage may last from one to a few years after which they produce flowers, fruits, and seeds every year, For example, mango, peepal, and neem.
(d) Monocarpic: All the annuals, all the biennials and, some perennial plants that reproduce only once in their life-time and then die, are called Monocarpic. For example, bamboo, agave, all the annuals and all the biennials.
(e) Polycarpic: Plants which flower and fruit many times in their life cycle and live for several years, are called polycarpic. For example, many perennial fruit bearing trees like mango, guava, apple and pear.
As the seed germinates a new plantlet emerges from it. The young plant grows vigorously and continues to grow till it attains a definite shape and size with its vegetative parts (roots, stem, leaves) well developed. This phase of the life cycle represents the young or the juvenile phase.
Then, at a certain point of time on completion of vegetative growth the plant switches over to its reproductive phase or adult phase and vegetative shoot apex transforms into a reproductive or floral apex and starts bearing flowers. This transition from vegetative to the flowering stage may take several years in trees but only a few weeks or days in annuals.
Flowering in a plant is affected by temperature (vernalisation) and light (photoperiodism).
Vernalisation: Low temperature treatment which stimulates early flower formation in some plants is called vernalisation.
Photoperiodism: It is the biological response, in growth and flowering, to the duration of light and dark period received by a plant in a specific sequence.
A typical flower bears four whorls born on a thalamus or stalk. These whorls from outside are:
The two outermost whorls are known as non essential or accessory whorls as they aid in reproduction but do not directly take part in the process. The other two whorls, Androecium (male reproductive organ) and Gynoecium (female reproductive organ), are known as the essential whorls as their absence from flowers will lead to failure of sexual reproduction.