NCERT Class 11-Biology: Chapter –15 Plant Growth and Development Part 5

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Question 2:

What are the structural characteristics of?

a. Meristematic cells near root tip

b. The cells in the elongation zone of the root

Answer:

Structural characteristics of

(a) Meristematic cells near the root tip:

Large nucleus

Rich protoplasm

Vacuoles are less in number

Cell wall is thin and made up of cellulose

No intercellular spaces

This is an area of cell division.

(b) The cells in the elongation zone of the root

More number of vacuoles

New cellulosic walls are deposited

There is an increase in size

Question 3:

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:

The growth pattern in plants differs from that in animals.

Plants have parts that have the ability to divide all throughout life and grow which is absent in animals i.e. they do not have unlimited growth. Plants have this characteristic due to the presence of meristems at different locations of the plant which is a region of rapid cell division. So new cells keep on getting added while the old ones lose capacity to divide and constitutes the plant body.

No, all the parts of the plant does not grow indefinitely. Only the meristematic tissues have the ability to keep on dividing i.e. the meristems are the parts which grow indefinitely throughout the plant life.

Question 4:

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

The cells of the apices of roots, apices of shoot and cambium has the ability to differentiate and mature so that they can perform specific functions. This is known as differentiation. Example: When a tracheary element is formed, cells lose their protoplasm and develop a strong, elastic, lignocellulosic cell wall that would be responsible to carry water for long distances when conditions are extreme. Usually, the cell changes to a more specialized type during differentiation.

b. De-differentiation: A differentiated cells has the ability to regain its capacity for cell division when placed under certain conditions. This is called dedifferentiation. Example: During formation of meristems from fully differentiated parenchyma cells.

c. Redifferentiation: Redifferentiation is the phenomenon where the dedifferentiated cells lose the ability to divide. Example is formation of phloem and secondary xylem from the vascular bundles.

Question 5:

Auxins are growth hormones capable of promoting cell elongation. They have been used in horticulture to promote growth, flowering and rooting. Write a line to explain the meaning of the following terms related to auxins.

a. auxin precursors

b. anti-auxins

c. synthetic auxins

Answer:

Auxins are plant growth hormone regulators which are generally produced in the growing apices of the stems and roots, from where they are translocated to the regions of their action by vascular bundles.

The meaning of the following terms related to auxins are as follows:

(a) Auxin precursors: Substances that are responsible for production of auxins are known as auxin precursors. For example, the amino acid tryptophan is an auxin precursor, biosynthesis of auxin requires this amino acid.

(b) Anti-auxins: Substances that inhibit auxin action by competing for the same receptor. It does not cause any growth response. An example of anti-auxin is PCIB (p-chloro phenoxyisobutyric acid).

(c) Synthetic auxins: Some chemical compounds that have been synthesized in the laboratories and function as auxins are called synthetic auxins. They are not synthesized naturally by the plant parts. For example: Phenoxyacetic acid.

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