The sequence of steps in the history of an organism is called

Assertion: Senescence is the time when age-associated defects are manifested.
Reason: Certain genes may be undergoing sequential switching on and off during one's life.

The ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures is called:

Senescence in plants leads into . . . . . . of cells.

Leaves fall from the branches on account of

$A$: Development is the emergence of a multicellular organism from a single group of cells.
$R$: Development involves growth,differentiation,and morphogenesis.