NCERT Solutions Class 11 Biology Chapter 15

Q.1 Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.

Ans-

Growth- The irreversible permanent increase in the size of an organ or its parts or even of an individual cell brought about by metabolic processes utilizing energy and nutrients over a period of time is called growth.

Differentiation- The process of maturation in which a cell converts into a highly specialized cell by a series of modification in the cell wall and cytoplasm to perform a particular function is called differentiation. For example, to form tracheal cells the cambial cells have to modify their cell wall and also loosen their protoplasm.

Development- The sequential and highly coordinated changes that an organism undergoes throughout their life cycle is called development. For example, in plants development means the changes which take place from seed germination to senescence.

Dedifferentiation- The phenomenon by which a differentiated cell performing specific function regains the capacity of division is termed as dedifferentiation. For example, the formation of meristems – interfascicular cambium and cork cambium from fully differentiated parenchymal cells. It happens only under certain conditions.

Redifferentiation- The process by which dedifferentiated tissues lose the capacity to divide again but mature to perform specific functions is called redifferentiation.

Determinate growth- When the growth of cell/ tissue/organ ceases after a specific size or dimension is attained it is called determinate growth or limited growth.

Meristem- The region in the plant body where actively dividing cells are present is called meristem. They accelerate the growth of plants and these tissues are named according to their location. For example, meristem at the shoot tip is called apical meristem, at the root tip is called root meristem and that in the stem is called lateral meristem.

Growth rate- The increased growth per unit time is termed as growth rate. The growth rate is expressed mathematically in terms of increase in size or number of cell per unit time. If the growth occurs as an increase in total surface area of a cell/ tissue without an increase in the number of cells it is called geometric growth. But when growth is accompanied by an increase in the total number of cells, it is called arithmetic growth.

Q.2 Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

In plants, growth is the symbol of an increase in the quantity of protoplasm. Therefore, measuring the growth of protoplasm can demonstrate the growth of a plant. Like other organisms, plants also grow in various phases of their life cycle. The parameters to measure the growth of protoplasm vary for different parts of the plant; such as the parameters to measure the growth of fruit and seed are very different from each other. Some of the parameters are increased in the height, weight, length, diameter, surface area, volume, and cell number. Measuring growth involves the measurement of the increase of protoplasm in all these parameters. Thus, it is difficult to demonstrate growth throughout the life of a flowering plant using only one parameter.

Q.3 Describe briefly:

(a) Arithmetic growth

(b) Geometric growth

(c) Sigmoid growth curve

(d) Absolute and relative growth rates

Arithmetic growth: When a linear curve is obtained by plotting the growth parameter (for example root length) against time, it is called arithmetic growth. A typical example of arithmetic growth is elongation of root at constant rate. Arithmetic growth is the outcome of mitotic division, where one of the two daughter cells continue to divide while other differentiate and mature to perform specific function.

Mathematically, it is expressed as

L_t = L₀ + rt

Where, L_t = length at the end of experiment

L₀ = Length at the start of experiment

t = Time duration of experiment

r = Growth rate

Geometric growth: Geometric growth results from mitotic division where both the daughter cells retain the capacity to divide. Here, initially the growth is very slow which is called lag phase, but later it becomes very rapid which is called exponential phase. Later on, due to limited nutrient supply, cell division slows down again to attain stationary phase. If growth parameters are plotted against time for geometric growth, a sigmoid curve is obtained.

Sigmoid growth curve: During the early stages of development of plants (say at the time of seed germination), the growth rate is slow, but with time as the cell starts utilizing nutrient, the growth is very rapid and becomes exponential. Later on, when the number of cells increases and the amount of available nutrients become limited, the growth rate becomes stationary. When such growth is plotted against time an S-shaped curve called sigmoid curve is obtained.

Absolute and relative growth rates: Absolute growth rate is the measurement and the comparison of total growth per unit time. The relative growth rate is when the growth is expressed as an increase in specific parameter relative to its initial value per unit time known as relative growth rate.

Q.4 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.

Five main groups of natural plant growth regulators are:

1. Auxin
2. Gibberellin
3. Cytokinins
4. Ethylene
5. Abscisic acid

Discovery of Auxin: The first observation of the presence of auxin in plants comes from the experiment of Charles Darwin and Francis Darwin on canary grass. They observed the bending of coleoptiles of canary grass towards the unilateral light source (phototropism). When the tip of the coleoptiles was cut, no bending was reported but when cut coleoptiles tip was placed over agar block it transmitted some chemical to the agar block. On placing this agar block over the cut coleoptiles, it again showed bending towards the unilateral source of light. Later on, in 1926, Auxin was isolated from the tip of coleoptiles of oat seedlings.

The physiological function of auxins: Shoot and root apices are the sites of auxin production in plants from where they are transported to their site of action. The auxins isolated from plants are indole-3-acetic acid (IAA) and indole butyric acid (IBA) while, NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are artificially synthesised auxins. The major physiological roles played by auxins are as follows:

• Promote apical dominance in plants
• Prevent premature falling of fruits and leaves
• Promotes abscission of older mature leaves and fruits
• Helps in xylem differentiation
• Promotes cell division

Agricultural/ horticultural application of auxins are:

• Used for root induction in cuttings when the plant is propagated through stem cutting
• Synthetic auxin 2,4-D is used as herbicides that selectively kills the dicotyledonous weeds without harming monocot plants
• Used for the development of seedless fruits as they promote parthenocarpy in tomatoes
• Sprayed on plants as they promote flowering for example in pineapples

Discovery of Gibberellins: Japanese farmers reported that few seedlings in rice field grow taller than others and never bears seeds; they called it “bakane” or foolish seedling disease. These seedlings were infected by a fungal pathogen, Gibberalla fujikuroi. Later on, E.Kurosawa, showed the reappearance of symptoms when the sterile filtrate was applied to uninfected plants and the active substance was later identified as gibberellic acid.

The physiological functions of Gibberellins:

• Help in breaking seed dormancy by activating the group of enzymes, hydrolyses, in the seeds which in turn utilises the stored nutrient.
• Determine the length of internodes
• Promote bolting in rosette leaves
• Delay senescence.

Agricultural/ horticultural application of Gibberellins:

• Delay fruit senescence, thus fruit remains on the tree for an extended period.
• Spraying Gibberellins increase the length of grapes stalks.
• In apple, it leads to elongation and improves the shape of fruits
• In Sugarcane crop, spraying of it increases the length of internode thus increasing yield.

Discovery of Cytokinins: Cytokinins were discovered by F. Skoog and his co-workers during tissue culture experiment of tobacco stem. They observed that the callus (undifferentiated mass of tissue) differentiate into plant only when it is supplemented with auxin along with coconut milk (or extract from vascular tissue, yeast extract or DNA). Skoog and Miller, latter were able to purify this substance, crystallized it and identified it as a cytokinesis promoting substance. They called it kinetin.

The physiological functions of Cytokinins-

• Synthesised in the region of rapid cell growth and promote cytokinesis
• Promote the formation of new leaves
• Enhance chloroplast formation in leaves
• Promote lateral shoot growth
• Delay leaf senescence by enhancing nutrient metabolism

Agricultural/ horticultural application of Cytokinins:

• Delaying senescence help in long-lasting flower which holds economic importance
• Differentiation of callus by application of cytokinins has great use in plant tissue culture, thus helps in cloning purpose
• Prevent apical dominance

Discovery of Ethylene: This is a gaseous hormone produced in large amount by ripening fruits. It was discovered by the observation that when ripen orange is kept with banana, it result in hastened ripening of bananas.

Physiological functions of ethylene:

• Shows the antagonistic effect of dormancy and break seed and bud dormancy
• Shows triple response in plants and stimulates shoot and root growth and differentiation
• Enhances leaf and fruit abscission
• Induction of femaleness in dioecious flowers
• Stimulates flower opening
• Enhances flower and leaf senescence
• Hastens fruit ripening

Agricultural/ horticultural application of Ethylene:

• Used to hasten fruit ripening
• Initiate flowering and synchronises fruit set in pineapples
• Used as an inducer of female flower in cucumbers thus increasing the fruit yield.

Q. 5 What do you understand by photoperiodism and vernalisation? Describe their significance.

Photoperiodism: The flowering in certain plants depends not only on the combination of light and dark exposures but also their relative duration. This response is called photoperiodism. It is the ability of the plant to detect and respond to the duration of light (length of day and night). Based on the flowering response of plants toward the length of light condition, they are divided into three classes:

Long Day Plants: These plants flower when the length of day (light duration) exceeds a critical duration, thus, in turn, they need a shorter dark period (night length).

Short Day Plants: For flowering, these plants need day length (light condition) less than a critical duration, thus, in turn, they need a long dark period (night) exceeding a critical duration.

Day-Neutral Plants: Plants in this group do not show any correlation between flowering and duration of light exposure.

Significance: Photoperiodism is a very important phenomenon in the life cycle of a plant as it affects the flowering of the plant. The plant does not flower if it does not receive certain day and light conditions and thereby is not able to complete the life-cycle. Understanding the phenomenon of photoperiodism is highly helpful in horticulture (flowering industry) for cultivating and obtaining flowers throughout the year. This is also an important feature in agriculture. Farmers choose the crop in a given area depending upon the photoperiodism response of the crop.

Vernalisation: The process of initiation of flowering or acquisition of a plants ability to flower in spring by exposure to prolonged cold or low-temperature conditions is called vernalisation. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn. In such plants, low temperatures control the flower either in a quantitative or qualitative manner. Several cereal plants such as wheat, barley and rye have two varieties (spring and winter varieties) depending on their requirement for low temperature for flowering and grain filling. A similar phenomenon is observed in biennials herbs such as sugar beet and cabbage, which show vegetative growth in first season and flower and die in the second season following low-temperature exposure. In perennial plants, a period of cold is needed first to induce dormancy and then later, after a certain time frame, plants flower.

Significance: The process of vernalisation ensures that the plant has fully developed vegetative phase and is ready for flowering.

Q.6 Why is abscisic acid also known as stress hormone?

Abscisic acid (ABA) is one of the plant growth regulators which helps in increasing the tolerance of plants to withstand stress conditions such as:

• Water scarcity or high temperature: ABA stimulates the closure of stomata to control the water loss. This makes plant tolerant of such conditions.
• It favours seed dormancy and inhibits seed germination so that the seeds can withstand desiccation and other environmental conditions unfavourable for growth.

Thus, due to its role in stress tolerance ABA is called the stress hormone.

Q.7 ‘Both growth and differentiation in higher plants are open’. Comment.

Both growth and differentiation in higher plants are open. A plant continues to grow throughout its life by adding new shoot, branches, leaves, etc. Growth of a plant is brought about by the meristems located at different locations in the plant. The apical meristem results in the growth of root and shoot apices while lateral meristem increases the girth of the plant. The cells of these meristems have the capacity to divide and form specialized cells that make the plant’s body, while they also self-perpetuate. This type of growth where new cells are always being added to the plant body by the activity of the meristem is called open growth.

The cells derived from root and shoot meristems differentiate and mature to perform specific functions. This process is known as differentiation. In plants, differentiated cells undergo dedifferentitation under certain conditions wherein the cells which had lost the capacity to divide regain the capacity to divide again. During this process, meristems/tissues divide and produce cells that once again lose the capacity to divide but mature to perform a specific function. This process is known as redifferentiation. Thus the differentiation process is also open – cells/tissues arising out of the same meristem differ in the structure at maturity.

Q.8 ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.

The flowering in some plant takes place only when they get light exposure exceeding a critical photoperiod or dark period less than a critical duration; such plants are called long day plants. Similarly, some plants need the less day length or photoperiod and a long dark period exceeding certain critical duration are called short day plants. Both short day plant and long day plant can flower simultaneously in the same place if grown with adequate photoperiods by artificial means. For example, if both long day plant and short day plants are grown under long day condition (say during summer when day are longer) but the short day plants are shifted to dark after a critical photoperiod, then both long day plant and short day plant will flower simultaneously.

Q.9 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.

(a) Auxin
(b) Ethylene
(c) Cytokinins
(d) Cytokinins
(e) Gibberellic acid
(f) Abscisic acid

Q.10 Would a defoliated plant respond to photoperiodic cycle? Why?

A plant where all the leaves are removed is called defoliated plant. A defoliated plant will not respond to photoperiodic cycle. This is because before flowering takes place, the shoot apices have to get modified into flowering apices. Flowering in plants depends on the specific duration of light and dark (photoperiod) which is perceived by leaves. According to the hypothesis, the hormonal substance necessary for flowering is synthesised in leaves in response to specific photoperiod and is transported to shoot apices to induce the formation of flowering apices. In a defoliated plant this hormonal substance is absent due to which they do not respond to photoperiodic cycle.

Q.11 What would be expected to happen if:

(a) GA3 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.

Ans-

(a) Application of GA3 to rice seedlings results in an increase in the length between two nodes thus increasing the inter-nodal axis which makes the plant tall.

(b) If dividing cells stop differentiating, they form a callus. This may be due to the absence of cytokinin in the system.

(c) If a rotten fruit gets mixed with unripe fruits, it will produce ethylene and enhance the rate of ripening of unripe fruits.

(d) If we forget to add cytokinin to the culture medium, the cell will stop differentiating and form a callus.