The conditions necessary for growth are
i. Water - Water is required for the enlargement of the cells and various metabolic processes. Water also provides the medium for the enzymatic reactions necessary for the growth. Water imparts turgidity which helps in extension growth of the cells.
ii. Oxygen - Oxygen helps in releasing the metabolic energy that is needed for growth activities. It is needed for the respiration of the cells.
iii. Nutrients - are required by plants for the synthesis of protoplasm and act as source of energy.
iv Temperature - Optimum temperature is required the growth
v. Environmental signals - Light and gravity also affect certain phases/stages of growth
Auxins are of two types :
(1) Natural auxins : These are naturally occuring auxins, isolated from plants and are regarded as phytohormones e.g. IAA. NAA etc.
(2) Synthetic auxins : These are synthesized by artificially e.g. IBA 2,4-D etc.