(a) Differentiation is permanent in size, structure, composition and function of cells, tissue or organs. For example the meristematic tissues in plants gives rise to new cells which mature and get differentiated into special tissue or an organ of the plant, e.g., cells, distal to root apical meristem form root cap, cell of the periphery form epiblema, followed by cortex, endodermis, pericycle and vascular.

(b) De-differentiation is the process of despecialisation of differentiated cells so that they again become differentiated and able to divide. e.g., in dicot stem, the cortical cells get de-differentiated and become meristematic to form cambium (interfascicular cambium, and fascicular cambiums).

(c) Re-differentiation The cambium cells thus formed, again re-differentiate to form secondary cortex cells secondary xylem and phloem elements and phelloderm in case of secondary growth of woody dicot plants.

Auxin is an important plant phytohormone required for the growth and development of every plant in its life cycle.

(a) Auxin Precursors are raw materials required to initials synthesis of auxins. IAA is synthesised from tryptophan, adenine compounds and derivatives of carotenoids.

(b) Anti-Auxins are compounds that inhibit action of auxins, e.g., p-chlorophenoxy isobutyric acid (PCIB), TIBA.

(c) Synthetic Auxin These are synthesised not by plants but artificially by man, e.g., 2-4 D (weedicide), NAA, dicamba and IBA, (Indole Butyric Acid) which are natural as well as synthetic auxin.

Hormone	Positive Effects	Negative Effects
Ethylene	$\bullet$ Promotes horizontal/transverse growth. $\bullet$ Makes stem positively geotropic. $\bullet$ Promotes apical dominance. $\bullet$ Breaks seed and bud dormancy. $\bullet$ Enhances fruits ripening and root initiation.	$\bullet$ Inhibits longitudinal growth. $\bullet$ Decreases sensitivity to gravity. i.e., root become apogeotropic $\bullet$ Enhances senescence of leaves and flowers, fruits. $\bullet$ Prolongs dormancy of lateral buds. $\bullet$ Mediates formation of ABA under water stressed conditions. $\bullet$
Abscissic Acid	$\bullet$ ABA is used as anti transpirant, by reducesing water requirement and during irrigation (by partial closure of stomata). $\bullet$ Also, induces flowering and root initiation in some plants. $\bullet$ Also, induces flowering and root initiation in some plants. $\bullet$ Used in prolonging dormancy of buds and seeds. $\bullet$ Increases resistance of plants, to cold and other types of stresses thus also known as stress hormone.	$\bullet$ Promotes abscission of flower and fruits. $\bullet$ Stimulates senescence of leaves by stopping protein and RNA synthesis. $\bullet$ Promotes bud and seed dormancy. Retards cambial activity by stopping mitosis in vascular strands and cambium. $\bullet$ Act as antagonist to gibberellins and counteracts the effect of other growth hormones (IAA and cytokinin). $\bullet$ Inhibits seed germination and growth of embryos in them.

Phytohormones are synthesised by plant cells, itself and individually. These are auxin, GA, ABA ethylene and cytokinin. There is not a separate system of their translocation in plants. So, their effects on plants are also inter mixed. e.g., many effects of auxins GA shows the same function. Similarly, ethylene and ABA support each other for many roles in plants. Secondly, effects of phytohormone in vitro and in vivo will also be different.

The phenomenon by which apical bud suppress the growth of lateral buds is known as apical dominance. It is caused due to release of plant hormone auxin from apical duds. It can be overcome either by the spray of cytokinin, that promotes the growth of lateral buds or by trimming of bushes which remove apical buds and allow lateral buds to grow.