Test: Growth and Development (NCERT) - 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.

Development is the sequence of events that occur in the life history of a plant which includes seed germination, growth, differentiation, maturation, flowering, seed formation and senescence. The process of development is considered as the sum of growth and differentiation controlled by several intrinsic factors (including genetic and hormonal) and extrinsic factors (including light, temperature, water, oxygen, nutrients, etc).

Increase 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.

Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm. Growth is measured by a variety of parameters such as  (i) increase in length, e.g, stem,root, pollen tube (ii) increase in volume eg., fruits. (iii) Increase in area, eg.leaves (iv) increase in diameter, eg, tree trunks, fruits (v) Increase in fresh or dry weight. One single maize root apical meristem can given to more than 17,500 new cells per hour, wheres as cell in watermelon may increase in size by upto 3, 50,000 the former, growth is expressed as increase the cell number latter expresses growth as increase in the size of cell.

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.

The Lateral meristem e.g. vascular cambium and  cork cambium(in dicotyledons and gymnosperms) are the meristems that cause increase in girth of the organs in which they are active, This is known as secondary growth of the plant.

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.

A meristematic tissue is a group of similar and immature cells that are in continuous state of division. These tissues are found in growing regions of plants. The chief characteristics of these tissues are:
(i) The cells may be round, oval, polygonal or rectangular.
(ii) They have thin cellulosic cell walls.
(iii) They do not have intercellular spaces.
(iv) They have dense cytoplasm with prominent nuclei.
(v) Vacuoles are absent or many small vacuoles may be present. The cells are in active state of metabolism.

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 portion of pith taken as an explant comprises of parenchymatous cells (i.e., simple permanent tissue which have lost the capcity to divide). When such cells are cultured on solid culture media, the parenchymatous cells of pith become meristematic and start dividing resulting in a mass of undifferentiated cells called callus. this is an example of dedifferentiation. These cells may be made to divide in different manners in order to produce different organs so that the meristematic cells of callus may become permanent tissues by the process of redifferentiation. for exmaple, treatment of certain hormones may lead to the differentitation of shoot buds or roots.

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.

(A) Auxin - (IV) Promotes flowering in pineapple: Auxins are a class of plant hormones that influence cell growth by altering cell elongation. However, the role of promoting flowering in pineapple is actually played by Ethylene. Therefore, there's an error in this match.

(B) Gibberellin - (III) Increase in the length of grape stalks: Gibberellins are plant hormones that stimulate stem elongation, seed germination, and flowering. They are known to increase the length of grape stalks.

(C) Cytokinin - (II) Overcoming apical dominance: Cytokinins are plant hormones that promote cell division and delay leaf senescence. They also play a role in overcoming apical dominance, encouraging the growth of lateral buds.

(D) Ethylene - (I) Promotes female flower formation in cucumber: Ethylene is a gaseous plant hormone which regulates a wide range of biological processes. It is known to influence sex determination in certain plants, promoting female flower formation in cucumber.

Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.