$A$ green plant turns pale when transferred to prolonged darkness. It represents a phenomenon called

Distinguish between:
$(1)$ Cell enlargement and Cell differentiation
$(2)$ Epigeal germination and Hypogeal germination

The movement of plant organs in response to light is called:

Elongation of internodes is inhibited by

Choose the incorrect pair.

List five main groups of natural plant growth regulators. Write a note on discovery,physiological functions and agricultural/horticultural applications of any one of them.