BIOLOGY M.C.Q (1 Marks)

(b) Herbicide $-2,4-D($ Auxin $)$

(c) Bolting agent $- GA _{3}$ (Gibberellin) [Helps in internode elongation]

(d) Stress hormone - Abscisic acid (ABA)

$(i)$ Apical meristem

$(ii)$ Intercalary meristem

$(ii)$ Lateral meristem

$(i)$ Meristematic phase

$(ii)$ Elongation phase

$(iii)$ Maturation phase

Water, oxygen, nutrients and other factors are very essential elements for growth

$(i)$ The plant cells grow in size by cell enlargement, which in turn requires water. Turgidity of cells help in extension growth. Thus, plant growth and further development are intimately linked to the water status of the plant

$(ii)$ Water also provides the medium for enzymatic reaction

$(iii)$ Oxygen helps in releasing metabolic energy essential for growth activities

$(iv)$ Nutrients are required by plants for the synthesis of protoplasm and all as sources of energy

Arithmetic Growth Rate The expression of arithmetic growth is exemplified by roots (or organ) elongating at constant rate. On plotting the length of an organ against time, a linear curve is obtained. Mathematically it is expressed as

Constant linear growth, a plot of length L against time

$L_{t}=L_{0}+r t$

$L_{t}=$ Length of time ' $t$ '

$L_{0}=$ Length of time to

$r=$ Growth rate or elongation per unit time

Geometrical Growth In most system the initial growth is slow (lag phase), and it increases there after at a exponential rate (log or exponential phase). Both the progeny cells following mitotic cell division retains the ability to divide and continue to do so. However due to the limited nutrient supply, the growth slows down leading to stationary phase. If we plot the parameter of growth against time, a typical sigmoid curve is obtained.

It has following stages

$1.$ During lag phase, organism adapt themselves to growth conditions. It is the period where the individual organism are maturing and not yet able to divide. During the lag phases of the bacterial growth cycle, synthesis of $RNA$, enzyme and other molecules occurs

$2.$ The log phase (sometimes called the logarithmic phase or the exponential phase) is a period characterised by cell doubling. The number of new organism appering per unit time is proportional to the present population.

$3.$ The stationary phase is often due to a growth-limiting factor such as the depletion of an essential nutrient, and/or the formation of an inhibitory product such as an organic acid. Stationary phase results from a situation in which growth rate and death rate are equal

$4.$ Death phase, organism run out of nutrients and die

$(i)$ rich in protoplasm

$(ii)$ conspicuous nuclei

$(iii)$ cell wall are primary in nature, thin and cellulosic with abundant plasmodesmata connection

$(i)$ cell enlargement

$(ii)$ new cell wall deposition

$(iii)$ increased vacuolation

Plant follows different pathways in response to environment or phases of life to form different kind of structures. This ability is called plasticity, $e . g$., heterophylly in cotton, coriander and larkspur