What is the movement of a plant towards gravity called?

$(a)$ Define hormone. Write four characteristics of hormones in humans.
$(b)$ Name the disorder caused by the following situations:
$(i)$ Under secretion of growth hormone
$(ii)$ Over secretion of growth hormone
$(iii)$ Under secretion of insulin
$(iv)$ Deficiency of iodine

What is the correct direction of flow of electrical impulses?

Dwarfism results due to :

Which part of the nervous system controls reflex arcs?

List the names of hormones secreted by the following endocrine glands and mention their functions:
$(i)$ Thyroid gland $(ii)$ Pituitary gland $(iii)$ Adrenal gland.