Short day plants are those plants that flower only when the photoperiod (exposure to duration of light) is below critical period. Tobacco, being a short day plant is unable to show flowering when exposed to light above than the critical period.

(a) Meristematic cells near root tip the cells of this region are called Root Apical Meristems (RAM) and are in active state of division, thus their number increases continuously. The cells of this region are thin walled with dense cytoplasm and large nucleus. They have the ability to divide and self perpetuate.

(b) The Cells in the Elongation Zone of the Root This region is situated just above the meristematic zone. In the cells of this zone, a large number of small vacuoles develop and later these vacuoles fuse to form one large central vacuole filled with cell sap. The cells are elongated in this region.

Growth in plants is of indeterminate type, i.e., plant grow through out their life because of presence of meristematic tissues present in specific parts of plant i.e., apical intercalary and lateral. These tissue has ability to divide continuously and contribute to localised plant growth. Growth in animals They have limited growth period after which further growth of their body stops. All the parts of the plant do not grow indefinitely. The root apex and shoot apex having apical meristematic tissues keep growing, thus contributing to the elongation of plant axis-(height).

(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.