13.6 Exercise Problems

Q1. Define growth, differentiation, development, dedifferentiation, redifferentlation, determinate growth, meristem and growth rate.

Answer: (a) Growth
It is an irreversible and permanent process, accomplished by an increase in the size of an organ or organ parts or even of an individual cell.
(b) Differentiation
It is a process in which the cells derived from the apical meristem (root and shoot apex) and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions.
(c) Development
It refers to the various changes occurring in an organism during its life cycle – from the germination of seeds to senescence.
(d) De-differentiation
It is the process in which permanent plant cells regain the power to divide under certain conditions.
(e) Re-differentiation
It is the process in which de-differentiated cells become mature again and lose their capacity to divide.
(f) Determinate growth
It refers to limited growth. For example, animals and plant leaves stop growing after having reached maturity.
(g) Meristem
In plants, growth is restricted to specialised regions where active cell divisions take place. Such a region is called meristem. There are three types of meristems – apical meristem, lateral meristem, and intercalary meristem.
(h) Growth rate
It can be defined as the increased growth in plants per unit time.

Q2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

Answer: In plants, growth is said to have taken place when the amount of protoplasm increases. Measuring the growth of protoplasm involves many parameters such as the weight of the fresh tissue sample, the weight of the dry tissue sample, the differences in length, area, volume, and cell number measured during the growth period. Measuring the growth of plants using only one parameter does not provide enough information and hence, is insufficient for demonstrating growth.

Q3. Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates

Answer: (a) Arithmetic growth
In arithmetic growth, one of the daughter cells continues to divide, while the other differentiates into maturity. The elongation of roots at a constant rate is an example of arithmetic growth.

\(
\begin{aligned}
&\text { Mathematically, it is expressed as }\\
&\begin{aligned}
& \mathrm{L}_{\mathrm{t}}=\mathrm{L}_0+\mathrm{rt} \\
& \mathrm{~L}_{\mathrm{t}}=\text { length at time ‘ } \mathrm{t} \text { ‘ } \\
& \mathrm{L}_0=\text { length at time ‘zero’ } \\
& \mathrm{r}=\text { growth rate / elongation per unit time. }
\end{aligned}
\end{aligned}
\)

(b) Geometric growth
Geometric growth is characterised by a slow growth in the initial stages and a rapid growth during the later stages. The daughter cells derived from mitosis retain the ability to divide, but slow down because of a limited nutrient supply.
(c) Sigmoid growth curve
The growth of living organisms in their natural environment is characterised by an S shaped curve called sigmoid growth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase.

Exponential growth can be expressed as:
\(
\mathrm{W}_1=\mathrm{W}_0 \mathrm{e}^{\mathrm{rt}}
\)
where,
\(
\begin{aligned}
& \mathrm{W}_1=\text { Final size } \\
& \mathrm{W}_0=\text { Initial size } \\
& \mathrm{r}=\text { Growth rate } \\
& \mathrm{t}=\text { Time of growth } \\
& \mathrm{e}=\text { Base of natural logarithms }
\end{aligned}
\)
(d) Absolute and relative growth rates
Absolute growth rate refers to the measurement and comparison of total growth per unit time.
Relative growth rate refers to the growth of a particular system per unit time, expressed on a common basis.

Q4. List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Answer: Plant growth regulators are the chemical molecules secreted by plants affecting the physiological attributes of a plant. There are five main plant growth regulators. These are:
(i) Auxins
(ii) Gibberellic acid
(iii) Cytokinins
(iv) Ethylene
(v) Abscisic acid
(i) Auxins

Discovery:
The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary gross bending toward a unilateral source of light.
It was concluded after a series of experiments that some substance produced at the tip of coleoptiles was responsible for the bending. Finally, this substance was extracted as auxins from the tips of coleoptiles in oat seedlings.

Physiological functions:
a. They control plant cell-growth.
b. They cause the phenomenon of apical dominance.
c. They control division in the vascular cambium and xylem differentiation.
d. They induce parthenocarpy and prevent abscission of leaves and fruits.

Horticultural applications:
a. They are used as the rooting hormones in stem cuttings.
b. 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds.
c. They induce parthenocarpy in tomatoes.
d. They promote flowering in pineapple and litchi.

(ii) Gibberellic acid
Discovery:
Bakane or the “foolish rice seedling” disease was first observed by Japanese farmers. In this disease, rice seedlings appear to grow taller than natural plants, and become slender and pale green. Later, after several experiments, it was found that this condition was caused by the infection from a certain fungus Gibberella fujikuroi. The active substance was isolated and identified as gibberellic acid.

Physiological functions:
a. It causes elongation of internodes.
b. It promotes bolting in rosette plants.
c. It helps in inducing seed germination by breaking seed dormancy and initiating the synthesis of hydrolases enzymes for digesting reserve food.

Horticultural applications:
a. It helps in increasing the sugar content in sugarcane by increasing the length of the internodes.
b. It increases the length of grape stalks.
c. It improves the shape of apple.
d. It delays senescence.
e. It hastens maturity and induces seed production in juvenile conifers.

(iii) Cytokinins
Discovery:
Through their experimental observations, F. Skoog and his co-workers found that the tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut milk, or DNA were added to the culture medium. This led to the discovery of cytokinins. Physiological functions:
a. They promote the growth of lateral branches by inhibiting apical dominance.
b. They help in the production of new leaves, chloroplasts, and adventitious shoots.
c. They help in delaying senescence by promoting nutrient mobilisation.

Horticultural applications:
a. They are used for preventing apical dominance.
b. They are used for delaying senescence in leaves.

(iv) Ethylene
Discovery:
It was observed that unripe bananas ripened faster when stored with ripe bananas. Later, the substance promoting the ripening was found to be ethylene. Physiological functions:
a. It helps in breaking seed and bud dormancy.
b. It promotes rapid internode-elongation in deep-water rice plants.
c. It promotes root-growth and formation of root hairs.
d. It promotes senescence and abscission of leaves and flowers.
e. It hastens the respiration rate in fruits and enhances fruit ripening.

Horticultural applications:
a. It is used to initiate flowering and synchronising the fruit set in pineapples.
b. It induces flowering in mango.
c. Ethephon is used to ripen the fruits in tomatoes and apples and accelerate the abscission of flowers and leaves in cotton, cherry, and walnut.
d. It promotes the number of female flowers in cucumbers.

(v) Abscisic acid
Discovery:
During the mid-1960s, inhibitor-B, abscission II, and dormin were discovered by three independent researchers. These were later on found to be chemically similar and were thereafter called ABA (Abscisic acid).

Physiological functions:
a. It acts as an inhibitor to plant metabolism.
b. It stimulates stomatal closure during water stress.
c. It induces seed dormancy.
d. It induces abscission of leaves, fruits, and flowers.

Horticultural application:
a. It induces seed dormancy in stored seeds.

Q5. Why is abscisic acid also known as stress hormone?

Answer: Abscisic acid is called stress hormones as it induces various responses in plants against stress conditions. It increases the tolerance of plants toward various stresses. It induces the closure of the stomata during water stress. It promotes seed dormancy and ensures seed germination during favourable conditions. It helps seeds withstand desiccation. It also helps in inducing dormancy in plants at the end of the growing season and promotes abscission of leaves, fruits, and flowers.

Q6. ‘Both growth and differentiation in higher plants are open’. Comment.

Answer: Growth and development in higher plants is referred to as being open. This is because various meristems, having the capacity for continuously dividing and producing new cells, are present at different locations in these plant bodies.

Q7. ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.

Answer: The flowering response in short-day plants and long-day plants is dependent on the durations for which these plants are exposed to light. The short-day plant and long day plant can flower at the same place, provided they have been given an adequate photoperiod.

Q8. Which one of the plant growth regulators would you use if you are asked to:
(a) induce rooting in a twig
(b) quickly ripen a fruit
(c) delay leaf senescence
(d) induce growth in axillary buds
(e) ‘bolt’ a rosette plant
(f) induce immediate stomatal closure in leaves.

Answer: (a) Induce rooting in a twig – Auxins
(b) Quickly ripen a fruit – Ethylene
(c) Delay leaf senescence – Cytokinins
(d) Induce growth in axillary buds – Cytokinins
(e) ‘Bolt’ a rosette plant – Gibberellic acid
(f) Induce immediate stomatal closure in leaves – Abscisic acid

Q9. Would a defoliated plant respond to photoperiodic cycle? Why?

Answer: A defoliated plant will not respond to the photoperiodic cycle. It is hypothesised that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Therefore, in the absence of leaves, light perception would not occur, i.e., the plant would not respond to light.

Q10. What would be expected to happen if:
(a) \(\mathrm{GA}_3\) is applied to rice seedlings
(b) dividing cells stop differentiating
(c) a rotten fruit gets mixed with unripe fruits
(d) you forget to add cytokinin to the culture medium.

Answer: (a) If \(\mathrm{GA}_3\) is applied to rice seedlings, then the rice seedlings will exhibit internode elongation and increase in height.
(b) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cell is called callus.
(c) If a rotten fruit gets mixed with unripe fruits, then the ethylene produced from the rotten fruits will hasten the ripening of the unripe fruits.
(d) If you forget to add cytokinin to the culture medium, then cell division, growth, and differentiation will not be observed.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Fill in the places with appropriate word/ words.
a. A phase of growth which is maximum and fastest is __________.
b. Apical dominance as expressed in dicotyledonous plants is due to the presence of more ________ in the apical bud than in the
lateral ones.
c. In addition to auxin, a ________ must be supplied to culture medium to obtain a good callus in plant tissue culture.
d. ________ of a vegetative plants are the sites of photoperiodic perception.

Answer: (a)exponential phase
(b) auxin
(c) cytokinin
(d) Leaves

Q2. Plant growth substances (PGS) have innumerable practical applications.
Name the PGS you should use to
a. Increase yield of sugar cane.
b. Promote lateral shoot growth.
c. Cause sprouting of potato tuber.
d. Inhibit seed germination. 

Answer:  (a) Gibberellin
(b) Cytokinin
(c) Ethylene
(d) Abscisic acid

Q3. A primary root grows from 5 cm to 19 cm in a week. Calculate the growth rate and relative growth rate over the period.

Answer: Growth is dependent on three factors -initial size \(\left(W_0\right)\), rate of growth \((r)\) and time interval (+) for which the rate of growth is retained.
\(
\begin{aligned}
& W_1=W_0 e^{r t} \\
& W_1=\text { final size, } \\
& W_0=\text { Initial size, } \\
& r=\text { growth rate, } \\
& t=\text { time, } \\
& e=\text { base of natural logarithm. }
\end{aligned}
\)
\(
\begin{aligned}
& 19=5 \times(2.7)^{r \times 7} \\
& \frac{19}{5}=(2.7)^{r \times 7} \\
& 3.8=(2.7)^{r \times 7} \\
& \log 3.8=r \times 7 \times \log (2.7) \\
& 0.5798=r \times 7 \times 0.4314 \\
& \frac{0.5798}{7 \times 0.4314}=r=0.1907
\end{aligned}
\)
\(
\begin{aligned}
&\begin{aligned}
& \text { Relative growth rate }=\frac{\text { Growth in given time period }}{\text { Measurement at start of time period }} \\
& \qquad=\frac{19}{5}=3.8 \mathrm{~cm}
\end{aligned}\\
&\text { Thus absolute growth rate is } 0.1907 \text { while relative growth rate is } 3.8 \mathrm{~cm} \text {. }
\end{aligned}
\)

Q4. Gibberellins were first discovered in Japan when rice plants were suffering from bakane (the foolish seedling disease) caused by a fungus Gibberella fujikuroi.
a. Give two functions of this phytohormone.
b. Which property of Gibberellin caused foolish seedling disease in rice?

Answer: (a) Gibberellins are weakly acidic growth hormones having gibbane ring structure which cause cell elongation of intact plants in general and increased internodal length of genetically dwarf plants in particular. Functions of gibberellins are as follows:
(i) Bolting : Gibberellin induce subapical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants.
(ii)Seed germination : During seed germination, especially of cereals, gibberellins stimulate the production of some messenger RNAs and hydrolytic enzymes like amylases, lipases, ribonucleases and proteases; which solubilise the reserve food of the seed. This food is then transferred to embryo axis for its growth.
(b) Gibberellins help in cell growth of stem, leaves and other aerial parts. Therefore, they increase the size of stem, leaves, flowers and fruits. This property of gibberellin lead to abnormal increase in stem length of rice plants causing foolish seedling disease or bakane disease.

Q5. Gibberellins promote the formation of ______ flowers on genetically _____ plants in Cannabis whereas ethylene promotes formation of _______ flowers on genetically ______ plants.

Answer: Gibberellins promote the formation of male flowers on genetically female plants of Cannabis whereas ethylene promotes formation of female flowers on genetically male plants.

Q6. Classify the following plants into Long-Day Plants (LDP), Short Day Plants (SDP) and Day Neutral Plants (DNP) Xanthium, Henbane (Hyoscyamus niger), Spinach, Rice, Strawberry, Bryophyllum, Sunflower, Tomato, Maize.

Answer: Long day plants are henbane (Hyoscyamus niger), spinach. Short day plants are Xanthium, rice, strawberry, Bryophyllum. (It is a short day plant but requires long photoperiods for floral initiation and short photoperiods for blossoming). Day neutral plants are sunflower, maize, tomato.

Q7. A farmer grows cucumber plants in his field. He wants to increase the number of female flowers in them. Which plant growth regulator can be applied to achieve this?

Answer: Ethylene, is a plant growth regulator that has a feminising effect on sex expression. Ethylene promotes formation of female flowers in monoecious plants like cucumber.

Q8. Where are the following hormones synthesized in plants
a. IAA
b. Gibberellins
c. Cytokinins

Answer: (a) Shoot apices, leaf primordia and developing seeds.
(b) Apical shoot buds, root tips and developing seeds.
(c) Roots, endosperm region of seeds, growing embryos, developing shoot buds, etc.

Q9. In botanical gardens and tea gardens, gardeners trim the plants regularly so that they remain bushy. Does this practice have any scientific explanation?

Answer: In botanical gardens and tea gardens, gardeners trim the plants regularly so that they remain bushy. Does this practice have any scientific explanation?

Q10. Light plays an important role in the life of all organisms. Name any three physiological processes in plants which are affected by light.

Answer: Physiological processes in plants that are affected by light are:
(i) Photosynthesis : It is the process of manufacturing of food from inorganic substances in presence of sunlight.
(ii)Transpiration: It is loss of water in vapour form, from the stomata. In majority of plants, stomata open in light.
(iii)Photoperiodism : Duration of light hours affect the growth and development of plants, especially flowering.

Q11. In the figure of Sigmoid growth curve given below, label segments 1, 2 and 3.

Answer: The sigmoid growth curve has three prominent phases:

• Initial lag phase: Slower development due to fewer cells introduced into a set of conditions.
• Rapid exponential phase: Fast development due to abundant resources and minimal environmental resistance.
• Gradual stationary phase: Onset of unfavorable conditions like gradual exhaustion of nutrients cause decline in growth.

Q12. Growth is one of the characteristic of all living organisms? Do unicellular organisms also grow? If so, what are the parameters? 

Answer: Growth is one of the major characteristic to distinguish living beings from non-living things. It is defined as permanent or irreversible increase in dry weight, size, mass volume of cell, organ and organism. In unicellular organisms, growth is synchronous with reproduction. A single cell synthesises protoplasm, increase in size and – divides into two daughter cells.

Q13. The rice seedlings infected with fungus Gibberlla fujikuroi is called foolish seedlings? What was the reason behind it? 

Answer: The rice seedlings infected with fungus Gibberella fujikuroi are called foolish seedlings because infected plants grow excessively taller than rest of the non infected rice plants in the field, fall over and be unharvestable.

SHORT ANSWER TYPE QUESTIONS

Q1. Nicotiana tabacum, a Short Day Plant, when exposed to more than critical period of light fails to flower. Explain.

Answer: Nicotiana tabacum is a short day plant, i. e., it requires a continuous critical dark period, which may be exceeded. If the plant is exposed to even a flash of light before achieving critical dark period, flowering is prevented. These plants require photoperiods below a critical length and they do not flower under long day conditions, i.e., when exposed to more than critical period of light.

Q2. What are the structural characteristics of
a. Meristematic cells near root tip
b. The cells in the elongation zone of the root

Answer: (a) Meristematic zone is the growing point of root and is subterminal. It is made up of compactly or closely arranged small thin walled isodiametric and meristematic cells which have dense protoplasm. They undergo repeated divisions and produce new cells for the root cap and basal region of the root. Therefore, it is essential for the growth of root.
(b) The cells is elongation zone are newly formed cells which lose the power of division. They elongate rapidly and increase the length of root. The cells of this region can absorb, water and minerals from the soil.

Q3. Does the growth pattern in plants differ from that in animals? Do all the parts of plant grow indefinitely? If not, name the regions of plant, which can grow indefinitely.

Answer: Growth in plants is different from growth in animals because animals show definite or limited growth and stop growing after attaining maturity whereas in plants, some parts show definite or limited growth, i.e., stop growing after attaining maturity, e.g., leaves, flowers and fruits, whereas some parts show indefinite growth, e.g., stems and their branches etc. In other words, in plants growth continues throughout life as they have meristematic areas where cell division occurs regularly whereas in animals growth occurs to a certain age after which cells divide only to replace worn out and lost cells.

Q4. Explain in 2-3 lines each of the following terms with the help of examples taken from different plant tissues
a. Differentiation
b. De-differentiation
c. Redifferentiation

Answer: (a) Differentiation : During growth, meristematic cell divides by mitotic division to form daughter cells. The cells from root and shoot apical meristem, cambium or other meristems tend to differentiate into mature cells to perform specific functions. This act leading to maturation is known as differentiation. E.g., cell tends to loose their protoplasm, in order to form tracheary element. These cells also develop a very strong, elastic, lignocellulosic secondary cell wall in order to transport water to long distance even under extreme conditions.
(b) Dedifferentiation : The living differentiated cells also show another interesting phenomenon during which they regain the capacity to divide mitotically under certain conditions. The dedifferentiated cell can act as meristem, e.g., formation of interfasicular cambium and cork cambium from fully differentiated parenchyma cells.
(c) Redifferentiation : The products of dedifferentiated cells or tissue when lose the capacity to divide but mature to perform specific functions, it is known as redifferentiation, e.g., secondary cortex and cork.

Q5. Auxins are growth hormones capable of promoting cell elongation. They have been used in horticulture to promote growth, flowering and rooting. Explain the meaning of the following terms related to auxins.
a. auxin precursors
b. anti-auxins
c. synthetic auxins

Answer: (a) Auxin precursors : These are the raw materials used in synthesis of auxin. E.g., tryptophan is precursor for indole-3-acetic acid (IAA).
(b) Anti-auxins : There are the compounds which inhibit the action of auxin. E.g., TIBA (2, 3, 5 triiodobenzoic acid) acts as anti-auxin by blocking the transport of auxin.
(c) Synthetic auxin : Auxins which are manufactured synthetically and do not occur naturally in plants. E.g., 2 : 4 D, NAA etc.

Q6. The role of ethylene and abscissic acid is both positive and negative. Justify the statement. 

Answer: Ethylene and abscisic acid have both positive and negative effects. Ethylene is a gaseous hormone which promotes apical dominance, breaks dormancy of seeds, buds, helps in root initiation and aids in ripening of climacteric fruits. These are some positive roles of ethylene. Contrary to this, ethylene also hastens the senescence of leaves and flowers and causes abscission of leaves, flowers and fruits. Abscisic acid promotes flowering in short day plants, induces parthenocarpic development in rose and rooting of stem-cutting (e.g., bean). On the other hand, it also induces dormancy of seeds and buds, promotes abscission of flowers and fruits and stimulates leaf senescence. Hence, ethylene and abscisic acid have both positive and negative roles.

Q7. While experimentation, why do you think it is difficult to assign any affect seen to any single hormone?

Answer: Regulatory links between various plant hormone metabolism include both synergistic and antagonistic interactions and have been described in a number of plant tissues. It is evident that the balance between synergistic and antagonistic relationships is the dominating principle of integral hormone action in plants. A single hormone can regulate an amazingly diverse array of cellular and developmental processes while at the same time multiple hormones often influence a single process. When used in vitro, plant hormones display a very broad and complex action spectrum which implies that exogenous application of plant hormone does pot ensure the same effect in the plant as in the culture medium. This is because, different cells, tissues respond differently to same hormone and different processes are sensitive to different levels of hormones, even in the same cell. Thus, it is difficult to assign any effect seen while experimentation to any single hormone.

Q8. What is the mechanism underlying the phenomenon by which the terminal/apical bud suppresses the growth of lateral buds? Suggest measures to overcome this phenomenon.

Answer: Apical dominance is the phenomenon by which presence of apical bud does not allow the nearby lateral buds to grow. When the apical bud is removed, the lateral buds sprout. This produces dense bushy growth. The phenomenon is widely used in tea plucking and hedge making. Apical bud inhibits the growth of lateral buds by releasing auxins. Apical dominance caused due to auxin can be counteracted by application of cytokinin. Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. Thus they act antagonistically to auxin.

Q9. In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant hormones formed? How are the hormones translocated to the site of activity?

Answer: In plants, auxins are synthesised in different regions E.g., auxin is synthesised in shoot apices, leaf primordia and developing seeds and is transported to stem and root from cell to cell by diffusion. Transport of gibberellins synthesised in shoot buds, root tips and developing seeds occurs through simple diffusion and through conducting channels. Cytokinin synthesised in roots move upwardly through xylem. Abscisic acid produced in many parts of plants, mainly inside the chloroplasts of green cells is transported through diffusion and transport channel.

Q10. Many discoveries in science have been accidental. This is true for plant hormones also. Can you justify this statement by giving an example? Also what term is used for such accidental findings?

Answer: Discovery of plant hormone gibberellin was accidental. It happened by chance in rice fields. A few plants of rice were observed, which were thin, pale green, unusually taller than the normal ones. Scientific investigation revealed that these were infected with a fungus, known as Gibberella fujikuroi. This fungus releases a plant hormone gibberellic acid in excess concentrations which makes the plant to grow unusually tall. This compound was later known as gibberellin, a hormone present in plants and fungi. The term used for accidental finding is ‘serendipity’ which means ‘fortunate happenstance’, or ‘pleasant surprise’. This term was coined by Horace Walpole in 1754.

Q11. To get a carpet like grass lawns are mowed regularly. Is there any scientific explanation for this?

Answer: Regular mowing (cutting at apex) of lawn grass removes the apical portion of the plant which causes the lateral branches to grow faster. As the apical buds inhibits growth of lateral buds by releasing auxin, (phenomenon called apical dominance). Because of mowing of the grass, it becomes bushy and growth is faster. So,-to maintain evenness of the grass (carpet-like) in the lawn, this practice is followed regularly to overcome the problem of apical dominance.

Q12. In a slide showing different types of cells can you identify which type of the cell may be meristematic and the one which is incapable of dividing and how?

Answer: The meristematic cells can be identified on the basis of the following characteristics:
(i) Cells have thin cellulose walls and dense cytoplasm with large nucleus.
(ii) Plasmodesmatal connections are more numerous among meristematic cells.
(iii)ell division, i.e., mitosis and its various stages are distinctly visible.
(iv)Chromosomes of cells replicate and divide into two homologous chromatids.
All these features contribute to open ended growth where structure is never complete in meristematic regions.
On the other hand, cells incapable of dividing show features such as:
(i) Cells have large vacuoles.
(ii)Attain particular shape, size and thickening.
(iii)Undergo structural and physiological differentiation.

Q13. A rubber band stretches and reverts back to its original position. Bubble gum stretches, but it would not return to its original position. Is there any difference between the two processes? Discuss it with respect to plant growth (Hint: Elasticity (reversible) Plasticity (irreversible)) 

Answer: In an experiment with oat coleoptile, hormone auxin stimulates bending of shoot tip towards the source of light. This is called plasticity and is irreversible. Plasticity is the ability of plant to acclimatise with the prevailing environmental conditions. Attaching weight to coleoptile segment causes it to bend, stretching it. After the weight is removed, the shoot returns to its original position. This is called elasticity and is a reversible phenomenon.

Q14. Label the diagram
a. This is which part of a dicotyledonous plant?
b. If we remove part 1 from the plant, what will happen?

Answer: (a) The diagram shows shoot apex of a dicotyledonous plant.

(b) Part 1 is showing inner cellular mass of shoot apex known as corpus. Cells of corpus are large and divide in different planes. Cells derived from corpus form procambium and ground meristem.

Q15. Both animals and plants grow. Why do we say that growth and differentiation in plants is open and not so in animals? Does this statement hold true for sponges also?

Answer: Plant growth is different from animal growth as growth in plants is unlimited and indefinite. Root and shoot tips in the plants are open ended, i.e., always growing and forming new organs to replace the older and senescent ones due to presence of meristem cells which are capable to grow and divide. Thus, the plant growth continues throughout the life. On the contrary, animal growth is limited as growth/stops as soon as they mature. Sponges are those animals which show cellular level of organisation. These animals posses totipotent cells which are capable of giving rise to all other cells in sponges. A small part detached from a sponge can regenerate into a whole new sponge. However, growth in sponges cannot be called open or indefinite as they cannot grow beyond a certain size. Practically, they do not show open ended growth.

Q16. Define parthenocarpy. Name the plant hormone used to induce parthenocarpy.

Answer: Define parthenocarpy. Name the plant hormone used to induce parthenocarpy.

Q17. While eating watermelons, all of us wish it was seedless. As a plant physiologist can you suggest any method by which this can be achieved.

Answer: Seedless watermelons are actually obtained through cross hybridisation between a diploid and a tetraploid parent. These are triploid, i.e., have three sets of chromosomes. This genome makes watermelon plant sterile and they do not produce seeds. Triploid genome is obtained by mating diploid male with tetraploid female. The traditional melons are made tetraploid by treating them with colchicine. Triploid plants are grown alongside with diploid lines to produce pollen to carry out pollination, resulting in seedless watermelon.

Q18. A gardener finds some broad-leaved dicot weeds growing in his lawns. What can be done to get rid of the weeds efficiently?

Answer: Weeds can be eradicated by using weedicides. Weedicides are chemicals which kill weeds growing in the fields. Application of 2 : 4-D and 2:4: 5-T removes broad leaved weeds in cereal crops and lawns because they do not affect mature monocotyledons while dalapon (2-2 dichloropropionic acid) kills grasses in broad leaved crops. Weedicides should be used very carefully and only occasionally as they have wide spectrum and long lasting action.

Q19. On germination a seed first produces shoot with leaves, flowers appear later,
a. Why do you think this happens?
b. How is this advantageous to the plant?

Answer: (a) On germination, a seed first produces shoots with leaves i.e., a plant enters its vegetative growth phase. The leaves receive light stimulus and manufacture food for the whole plant by the process of photosynthesis and the plant grows vegetatively. After achieving some vegetative growth the critical photoperiod regulates flowering in a plant. Photoperiodic, stimulus is picked up by phytochrome in fully developed leaves and photoperiodic induction occurs when the plant has achieved certain minimum vegetative growth,, e.g., 8 leaves in Xanthium strumarium. Leaves receiving the stimulus produce a chemical which induces flowering that has been named as florigen.
(b) Reproductive phase follows vegetative growth phase in plants. This is an adaptation or rather necessity as vegetative growth period prepares the plant to bear reproductive structures like flower, fruits and seeds. Moreover, reproductive or gametophytic phase in flowering plants is totally dependent on vegetative phase for nourishment as it does not bear any structure to carryout photosynthesis or absorb mineral nutrition directly from soil.

Q20. Fill in the blanks:
a. Maximum growth is observed in ________ phase.
b. Apical dominance is due to ____________
c. ___________ hormone initiate rooting
d. Pigment involved in Photoperception in flowering plants is ______

Answer: (a) exponential
(b) auxin
(c) Auxins
(d) phytochrome

LONG ANSWER TYPE QUESTIONS

Q1. Some varieties of wheat are known as spring wheat while others are called winter wheat. Former variety is sown, and planted in spring and is harvested by the end of the same season. However, winter varieties, if planted in spring, fail to flower or produce mature grains within a span of a flowering season. Explain, why?

Answer: Many plants do not come to flower before they experience a low temperature. These plants remain vegetative during the warm season. When they receive low temperature during winter, they grow further and then bear flowers and fruits. Requirement of low temperature prevents precocious reproductive development in autumn. It allows the plant to reach vegetative maturity before reproduction can occur. The condition occurs in winter varieties of some annual food plants (e.g., wheat, barley, rye), some biennial (e.g., cabbage, sugar beet, carrot) and perennial plants (e.g., Chrysanthemum). The annual winter plants also possess spring varieties. The spring varieties are planted in spring. They come to flower and bear fruits prior to end of growing season. If the winter varieties are sown similarly, they fail to flower and produce fruits before the end of growing season. They are planted in autumn, form seedlings in which form they pass winter. The seedlings resume growth in spring. They bear flowers and fruits in summer.

Q2. It is known that some varieties of wheat are sown in autumn but are harvested around next mid summer.
a. What could be the probable reason for this?
b. What term is used for this promotion of flowering under low temperature?
c. Which plant hormone can replace the cold treatment?

Answer: It is known that some varieties of wheat are sown in autumn but are harvested around next mid summer.
a. What could be the probable reason for this?
b. What term is used for this promotion of flowering under low temperature?
c. Which plant hormone can replace the cold treatment?

Q3. Name a hormone which
a. is gaseous in nature
b. is responsible for phototropism
c. induces femaleness in flowers of cucumber
d. is used for killing weeds (dicots)
e. induces flowering in long day plants

Answer: (a) Ethylene
(b) Auxin
(c) Ethylene
(d) Auxin
(e) Gibberellin