Describe briefly : (a) Arithmetic growth (b) Geometric growth (c) Sigmoid growth curve (d) Absolute and relative growth rates.

(a) Arithmetic growth : In this growth one daughter cell continues to divide after the mitosis while the other cell differentiates to perform specific functions and get matured.

Example : Elongation of root at a constant rate. When a graph is drawn of length against time ; linear curve is obtained.

Mathematically its expression is given below :

Lt = L0+rt

Here, Lt is length at time ‘t’, L0 is length at time 0 and r is the rate per unit time.


(b) Geometric growth : In this type of growth the initial growth is slow and is called the lag phase. After this, the growth is quite rapid and at an exponential rate. This phase is called the log or exponential phase. In this phase, both the daughter cells (formed after mitosis) continue to divide. The last phase marks a slowed down growth. This happens because of limited nutrient supply. This phase is called the stationary phase. The graph of the geometric growth gives a sigmoid curve.

The exponential growth can be mathematically represented as follows:

W1 = W0ert

Here, W1 = final size (weight, height, number etc.), W0 = initial size at the beginning of the period, r = growth rate, t = time of growth and e = base of natural logarithms

(c) Sigmoid growth curve : The S-shaped curve on graph; to show geometric growth is called the sigmoid growth curve. It is S shaped curve which is characteristic feature of living organism in natural environment. It consists of lag phase, log phase or exponential phase and stationary phase.

(d) Absolute and relative growth rates

Absolute growth rate : It is the measurement and comparison of total growth per unit time. When growth is measured in absolute terms, e.g. in terms of length or weight, it is called absolute growth.

Relative growth rate : It is the growth of given system per unit time expressed on common basis eg per unit initial parameter is relative growth rate. When growth is measure in terms of comparative terms; like percentage growth; it is called relative growth.

 

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Both growth and differentiation in higher plants are open. Comment.

Open growth : It is a type of growth where in new cells are always being added to the plant body by the activity of the meristem.

Growth is considered to be open. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. The meristems present in different locations have the capacity to divide and self-perpetuate. The daughter cells may lose the capacity to divide and such cells make up the plant body. ns in which they are formed.

Both growth and differentiation in higher plants are open. Comment.

Diagrammatic representation of locations of root apical meristem, shoot apical meristem and vascular cambium. Arrows exhibit the direction of growth of cells and organ

Differentiation in plants is open, because cells/tissues arising out of the same meristem have different structures at maturity. The location also decides the final
structure of a cell/tissue at maturity. For example, cells positioned away from root apical
meristems differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.

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What do you understand by vernalization ? Describe its significance.


Vernalization ; It is the treatment of low temperature for reducing vegetative phase and hastening the reproductive phase. It was first discovered by Lysenko.

Vernalisation refers specially to the promotion of flowering by a period of low temperature. For example - Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.


Significance of vernalization :

1. The reproductive phase can be quickened.

2. The yield is increased.

3. The plants become more resistant towards diseases.

4. It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity. 

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 Describe the various steps involved in seed germination.

Seed germination : It is the process by which the seed resumes its metabolic activities and grows under favourable conditions.

The following steps are involved in seed germination :

1. Imbibition of water : The seeds imbibe or uptake water and swell. Imbibition takes place against great compressive forces. Due to the imbibition pressure seed coats ruptures.

3. Resumption of metabolic activities : As the protoplast is rehydrated, the metabolic activties resume. The respiration is in the beginning anerobic which later becomes aerobic When the anerobic respiration reaches peak then mitoehondria differentrates in the embryo cell.

4. Mobilisation of reserve food : Activated embryo has long lived RNAs. They induce the production of hormones which synthesise hydrolytic enzymes like amylases, proteases, lipases. The enzymes move into the storage cells and digest the nutrients which are translocated to the tips of embryo axis.

6. Growth of the embryo : On receiving the food the embryo cells metabolise actively. DNA replicates and transcribe RNAs. The cells divide. The radical end enlarges, ruptures the seed coat and passes downward to form primary root which later on forms the root system.The plmule elongates to form the stem, new leaves are formed and cotyledons fall.

Seed germination

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Growth is an important phenomenon of living. Justify the statement with reasons.

Growth is most important phenomenon because of following reasons :

1. Due to growth, size of the organism, increases.

2. The growth results in increase in weight, mass and size of the organism.

3. Growth allows the cells to be differentiated. e.g. In roots, the cells first elongate in the zone of elongation and then in the zone of differentiation they are differentiated into tissues and organs.

4. It is due to growth that radical forms root and plumule forms the stem during germination of the seeds.

5. It is due to growth that buds sprout to form the branches.

8. It is due to the growth that floral buds sprout to form flowers.

9. It is due to the growth of pollen grains and functional megaspore cells that male and female gametophytes are formed respectively.

10. It is due to the growth that pollen tubes grow and siphonogamy occurs.

11. The fruits are formed due to the growth of the ovary.

 

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