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Distinguish between short day plants and long day plants.


Short day plants : 

1. These are plants which require less exposure to light for a period less than the critical duration.

2. If night is interrupted by light then there is no flowering.

3. These need light of short wave lengths whose value is below critical point.

4. These produce flowering in winter.

7. If days are interrupted by darkness then no effect.

Long day plants :

1. These are plants which require the exposure to light for a period exceeding a well defined critical duration.

2. If night is interrupted by light then no such effect on flowering.

3. These need light whose value is above critical point.

5. These produce flowers in summer.

6. If days are interrupted by darkness then no flowers.

 

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Explain different phases of growth.


Plant growth has following three phases

(i) Meristematic Phase - This phase is characterised by constantly dividing cells. The cells are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections. 

(ii) Elongation Phase - The cells of this phase elongate. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.

(iii) Maturation Phase - The cells in this phase stop dividing and undergo differentiation and maturation.The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.


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Explain experimentally the phases of growth.

Apparatus : Few pea seeds, raw sand, marking pencil, blotting paper, petridish, scale, etc.

Procedure :

1. A few pea seeds are taken and placed on moist sand for 24-28 hours. The radicles are seen to come out of the seeds after some time. 

2. The seedling with a radicle measuring more than 3cm in leghth is chosen. 

3. The radicle is divided and marked into 15 equal parts from the tip to top.

4. The seedling is placed on a moist blotting paper and kept in a petridish undisturbed for 3-4 days.

Explain experimentally the phases of growth.

The figure shows.

A. Youngroot marked into 1 mm zones.

B. The same after 24 hours.

Observation : After 3-4 days it is observed that the size of the radicle increases. The distance between the marks near the tip remains almost the same. But in the middle, distance between the marks increased and at the upper end again the distance remained the same.

Result : The distance between marks in upper and lower regions remains the same. Thus, no growth has taken place. But the middle part where the growth has taken place is called phase of elongation.

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The role of enthylene is both positive and negative. Justify the statement.

Positive role of ethylene are as follows:

1. Breaking of dormancy : It breaks dormancy of storage organs.

3. Root initiation : In low concentration ethylene helps in root initiation, growth of lateral roots and root hairs.

4. Fruit ripening : It aids in ripening of fruits. C Ethylene is used to induce artificial ripening of these fruits e.g. Citrus, Apple, Mango, Banana etc.

5. Flowering : It stimulates flowering in pineapple and related plants though in other cases the gaseous hormone causes fading of flowers.

Negative role of Ethylene :

1. Growth : It inhibits longitudinal growth and stimulates transverse growth.

2. Senescence : It hastens the senescence of leaves and flowers.

3. Abscission : Abscission of various parts is stimulated by ethylene which induces the formation of hydrolases.

4. Apical dominance : Ethylene promotes apical domiance and prolongs dormancy of lateral buds.

 

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Write a note on Growth curve.

Growth curve is the graphical representation of growth against time, thus it is also called sigmoid curve.

A typical S-shaped grand period of growth curve

It has three phases :

(1) Lag phase: It is in the beginning of the growth and represents the time in which minimum growth occurs. The growth is very slow in this phase. 

(2) Log/Exponential phase : It represents the phase of maximum growth. If log phase persists for longer time then growth is linear.  Both the progeny cells following mitotic cell division retain the ability to divide and continue to do so.

(3) Stationary phase : It is the phase in which no growth occurs. The limited nutrient supply, the growth slows down leading to a stationary phase.

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