Q4. Describe briefly: (a) arithmetic growth, (b) geometric growth, (c) the sigmoid growth curve, (d) absolute and relative growth rates. (NCERT Exercise 3)
(a) After each mitotic division only one daughter cell keeps dividing, while the other differentiates and matures; for example, a root elongating at a constant rate, with Lt = L0 + rt, a straight line against time. (b) Both daughter cells keep dividing; growth is slow at first, then exponential, W1 = W0ert, and slows as nutrients run short. (c) Plotting geometric growth against time gives an S-curve with lag, log (exponential) and stationary phases, typical of cells, tissues and organs of a plant. (d) Absolute growth rate is total growth per unit time; relative growth rate is growth per unit time on a common basis, such as per unit initial size (NCERT pages 170–171).
Q5. Define growth, determinate growth, meristem and growth rate. (From NCERT Exercise 1)
Growth: an irreversible permanent increase in size of an organ or its parts or even of an individual cell. Meristem: a tissue whose cells can divide and self-perpetuate, giving the plant its capacity for unlimited growth. Growth rate: the increased growth per unit time (NCERT pages 167 and 169). Determinate growth: limited growth, as in leaves, flowers and fruits, which have limited dimensions (reasoned from NCERT page 166; NCERT uses the term on page 172).
Q6. Assertion: in geometric growth, the final size depends on the initial size. Reason: in W1 = W0ert, W1 is proportional to W0. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT page 170).
Q7. What is the efficiency index?
The relative growth rate r in W1 = W0ert, a measure of the plant’s ability to produce new plant material (NCERT page 170).
Q8. Why is water essential for plant growth? Give two reasons.
Cells grow in size by cell enlargement, which needs water, and turgidity helps extension growth; water is also the medium for the enzymatic activities of growth (NCERT page 171).