Give scientific reasons:
$(1)$ Photoperiodism is essential for flowering in plants.
$(2)$ Abscission (leaf fall,flower fall,fruit fall) increases the vitality of plants.

(N/A) $(1)$ Photoperiodism and flowering are closely linked. For example,in long-day plants,if the dark period is interrupted,or in short-day plants,if the light period is interrupted,the flowering response is inhibited. Furthermore,the pigment phytochrome is responsible for sensing these light periods to trigger flowering.
$(2)$ Plants possess specific abscission layers. The process of shedding leaves,flowers,and fruits is known as abscission. This process helps in maintaining hormonal balance within the plant. After the shedding of old or damaged parts,the plant initiates the growth of new structures. This renewal process increases the overall vitality of the plant. During this phase,hormones like auxins,gibberellins,and cytokinins are involved,where the regulation of auxin levels promotes new growth and enhances plant longevity.