Test: Plant Growth and Development- Growth and Rate of Growth (July 8) - NEET MCQ

Growth is defined as a permanent or irreversible increase in dry weight, size, mass or volume of a cell organ or organism. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability of the plants is due to the presence of meristems at curtain locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to divide and such cells make up the plant body. This form of growth wherein new als are always being added to the plant body by the activity of the meristem is called the open form of growth.

ncrease in growth per unit time is called as growth rate. The growth rate may be arithmetic or geometrical. Arithmetic Growth is a type of growth in which the rate of growth is constant and increase in growth occurs in arithmetic progression-- 2, 4, 6, 8, 10,12. Meristematic cells at the growing point divide in such a fashion that one daughter cell remains meristematic while the other grows and differentiates. the process continues. Mathematically, arithmetic growth is expressed as
L_t =L₀ +rt.
where L_t = length after time t, L₀   = length at the beginning, and r = growth rate. On plotting growth against time, a linear graph is obtained.

An embryo shows geometrical growth initially but later it passes into arithematic phase.

Absolute growth rate (AGR): Increase in total growth of two organs or organisms is measured and comparison of total growth per unit time is called absolute growth. Absolute growth rate is the total growth per unit time. Relative growth rate (RGR) : It is growth per unit time per unit initial growth.
RGR= Growth in given time period / Measurement at start of time period
AG for leaf A = 10cm² = 5cm² = 5cm²
RGR for leaf A = 5/5 ​× 100 = 100%
AG for leaf B = 55cm² − 50cm² = 5cm²
RGR for leaf B = 5/50 ​× 100 = 10%
Though the absolute growth is same for both the leaves (A and B), relative rate of growth is more in leaf A because of its initial small size.

The meristem which is consumed in the formation of an organ is called determinate meristem. The meristem which continues its activity throughout life of the plant is called indeterminate meristem. Root apical meristem, shoot apical meristem, intercalary meristem (e.g., grass) and lateral meristems are all Indeterminate meristems. Plant growth is generally indeterminate, i.e., plants retain the capacity for unlimited growth throughout their life, whereas it is determinate in the meristem which is consumed in the formation of an organ.

Lateral meristem occurs on the sides and takes part in increasing girth of the plant. Intra fascicular cambium is the primary lateral meristem which lies in vascular bundles of dicot and gymnosperm stems in between phloem and xylem. Examples of  secondary lateral meristems are vascular cambium of the root, inter fascicular cambium of stem,cork cambium etc. that  take part in the secondary growth.

The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions. This process leading to maturation of cells is termed as differentiation. During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm e.g., to form a tracheary element the cells would lose their protoplasm, they also develop strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension.

The living differentiated cells, that have lost the capacity to divide can regain the power of division under certain conditions, this phenomenon is termed as de-differentiation. E.g. formation of meristems-interfascicular combium (formed form outer layer of cortex) from fully differentiated parencyma cells. while doing so, such meristems/tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get redifferentiated. E.g., cells of secondary xylem, secondary phloem, periderm. Thus redifferentiation can be defined as maturation or differentiation of dedifferentiated tissues.

Different structures develop in different phases of growth as well as in response to environment. The ability to change under the influence of internal or external stimuli is called plasticity.

Auxin is a plant hormone. Its synthesis occurs in young meristematic tissues like root and shoot tips. It is synthesized from tryptophan amino acid. It acts as plant growth promoter. It is involved in various physiological processes of plants. It stimulates apical dominance and suppresses the growth of lateral buds. It also plays important role in phototropism.