According to the given information:
a. Elongation of cells: Auxin
b. Growth of stem: Gibberellins
c. Promotion of cell division: Cytokinins
d. Falling of senescent leaves: Abscisic acid
Explanation:
Auxin:
This hormone is primarily responsible for cell elongation, which is crucial for plant growth, particularly in the stem and root tips.
Gibberellins:
These hormones promote stem growth by stimulating cell division and elongation. They also play a role in seed germination and other developmental processes.
Cytokinins:
These hormones primarily stimulate cell division, which is essential for growth and development in various parts of the plant, including roots, shoots, and leaves.
Abscisic acid (ABA):
This hormone is associated with senescence (aging and falling of leaves), as well as other stress responses like drought. It also plays a role in regulating seed dormancy.