Plants are classified according to their life cycles into annual, biennial and perennial. An annual plant is a plant that completes its life cycle in one growing season, from seed to overall senescence. These plants go over different stages that can be defined as their growth cycle. The first stage in this developmental process is called germination. Germination is the process of activation and growth of a previously dormant embryo inside the seed.
The second stage of the cycle is called juvenility, in which there is a considerable growth rate in the plant, promoted by hormones such as auxins and giberellic acid, which are present in the apical meristem. These hormones are in charge of stimulating cell elongation. During this stage, the plants are not capable of reproduction.
After proceeding through a transitional phase, the annual plants reach maturity, at which point, the plants are able to reproduce through flowering, but are not necessarily in bloom yet they are capable of producing such organs.
The last stage of the cycle in annual plants is called senescence, during which the plant naturally deteriorates and dies, ultimately completing its life cycle. The sunflower (Helianthus annuus), is a great example of an annual plant, completing its cycle from seed to senescence in one growing season.
A biennial plant completes its life cycle in two growing seasons. Biennial plants go through the same cycle as annuals, but this cycle is interrupted by a period of dormancy. Dormancy can be defined as the failure of plant’s growth during unfavorable conditions such as: cold, drought, lack of light, due to physiological factors. An excellent example of a biennial is the common foxglove (Digitalis purpurea). This plant goes through the phases of germination and juvenility during its first growing season, followed by a period of dormancy caused by the unfavorable conditions for growth and development of the plant over the winter. During this dormancy period, the foliage produced in the first growing season dies, stimulated by the action of abscisic acid and ethylene, upon when the roots become dormant and the plant is able to mediate stress. In the second spring, due to the accumulation of cold days and the action of auxins and giberellic acid, new foliage of common foxglove quickly grows back along with a tall flower spike that will produce blossoms.
Perennial plants are those which can develop for over two growing seasons. They distinguish themselves from other plants in their ability to suspend and resume growth recurrently in response to environmental, and often seasonal conditions. Garden mums (Chrysanthemum sp.) exemplify the typical life stages of a perennial. During their first growing season they go through the germination and juvenility stages promoted by the auxins and giberellic acid. The developmental process to reach maturity is interrupted by a dormancy period promoted
