Rajasthan Board RBSE Class 11 Biology Chapter 16 Normal Secondary Growth in Stem and Root
RBSE Class 11 Biology Chapter 16 Multiple Choice Objective Questions
Question 1.
Interfascicular cambium is formed from
(a) Pith
(b) Medullary rays
(c) Xylem
(d) Phloem
Question 2.
Cambium ring forms
(a) inside Phloem and outer Xylem
(b) outside phloem and inside Xylem
(c) outer phloem and inside parenchyma
(d) outer xylem and inside parenchyma
Question 3.
Cambium formation starts in dicot root
(a) on upper side of phloem pool
(b) below xylem pool
(c) below xylem pools
(d) uncertain
Answers:
1. b
2. b
3. c
RBSE Class 11 Biology Chapter 16 Very Short Answer Questions
Question 1.
Intrafascicular cambium is example of which kind of meristematic tissue?
Answer:
This fascicular cambium is derived from procambium of shoot apical meristem.
Question 2.
Which different kinds of cells are present in cambium? Name them.
Answer:
The fascicular and interfascicular cambium.
Question 3.
Cork is formed of which kind of cells?
Answer:
Vascular cambium.
Question 4.
Cork cambium is differentiated at which place of stem?
Answer:
The cork cambium (phellogen) produces dead cells or cork (phellem) towards outer side and living cells of secondary cortex (phelloderm) towards inner side. The cork cambium (phellogen) is a secondary lateral meristem which develops from permanent tissues in the region of epidermis, hypodermis, cortex or even in outer layers of phloem.
The cork cambium (phellogen) produces dead cells or cork (phellem) towards outer side and living cells of secondary cortex (phelloderm) towards inner side. These layers of cork, cork cambium and secondary cortex constitute the protective covering called periderm.
Question 5.
Where Complementary tissue are found?
Answer:
Lenticel is usually formed below an old stoma where the cork cambium is more active and cuts a mass of loosely arranged cells, called complementary or filling cells.
Question 6.
Cambium is formed from which tissue in Dicot root?
Answer:
Secondary growth in dicotyledonous roots occurs by the initiation and activity of two secondary meristems-
- Vascular cambium and
- Cork cambium (or phellogen).
Question 7.
Write the names of different layers of periderm.
Answer:
These layers of cork, cork cambium and secondary cortex constitute the protective covering called periderm. The cork cells are highly lignified and check the loss of water from the general surface of stem. Sometimes the layers of cork are interrupted by lenticels.
Question 8.
What are the functions of lenticels?
Answer:
The lenticel is the chief aerating structure occurs as a raised corky spot in the periderm of stem.
Question 9.
How are the lateral roots originate?
Answer:
The development of a lateral root begins with the formation of a meristematic region in the pericycle. As this grows, the new lateral root pushes out through the endodermis and cortex until it ruptures the surface. The structure of the lateral root is similar to that of the main root. Lateral root initiation in dicots will occur opposite the protoxylem poles. Lateral root initiation in monocots will occur between the protoxylem poles.
RBSE Class 11 Biology Chapter 16 Short Answer Questions
Question 1.
Differentiate between primary and secondary growth?
Answer:
- Primary growth of stems is a result of rapidly dividing cells in the apical meristems at the shoot tips.
Subsequent cell elongation also contributes to primary growth. - The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems.
- Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape.
- If the apical bud is removed, then the axillary buds will start forming lateral branches.
- Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
- The growth of the lateral meristem, which includes the vascular cambium and the cork cambium (in woody plants), increases the thickness of the stem during
secondary growth:
Normally it occurs in dicotyledonous stem.
However, a different kind of secondary growth occurs in some monocotyledonous stem (e.g., Dracaena, Aloe, Yucca, Agave, Kingia, San-sieviera, Xanthorrhoea and Lomandra), which is considered as abnormal or anamalous type.
Question 2.
Differentiate between Stellar and extrastellar growth?
Answer:
S.No | Stelar secondary growth | Extra stelar secondary growth |
1. | Occurs in the stelar region and secondary- growth starts first. | Occurs in the extra stelar region after stelar secondary growth starts. |
2. | Occurs by the activity of vascular and interfascicular cambium. | Occurs by the activity of cork cambium. |
3. | Produces secondary- xylem and secondary- phloem tissue. | Produces cork cells and parenchyma cells. |
4. | Annual ring formation occurs. | No annual ring formation. |
5. | Leads to formation of bark. | Leads to formation of periderm and lenticels. |
6. | Produces more conductive tissues. | Produces protective tissues. |
Question 3.
How does the environment affects the functionality of cambium?
Answer:
Factors controlling cambial activity:
- The effects of temperature on the activity of the cambium are of prime importance. At relatively high temperatures the cambium is active, while at low temperatures dormancy is induced.
- Changes in photoperiod seem to affect the type of wood produced more than the activity of the cambium. Under long day conditions, xylem elements of the early wood type are formed, while under short day conditions the xylem produced is of the late wood type. Photoperiod affects cambial activity and the type of its derivatives through its control of the buds.
- Under high temperature and short day conditions, axillary buds have been found to sprout occasionally. Below these buds the wood produced is of the early type. Inhibitors produced by the leaves and retained in the stems
Question 4.
Differentiate between Autumn wood and spring wood?
Answer:
S.No | Spring wood (Early wood) | Autumn wood (Late wood) |
1. | The portion of xylem formed during favourable spring season is called spring wood. | The band of xylem formed during the unfavourable season of the year is called autumn wood. |
2. | This band of xylem is broad. | This band of xylem is narrow. |
3. | The tracheary elements of xylem are thin walled with wide lumen. | The tracheary elements of xylem are thick walled with narrow lumen. |
4. | Fibres are less is number. | Fibres are abimdant |
5. | Spring wood is comparatively lighter in colour. | Autumn wood is comparatively darker. |
Question 5.
How the bark is formed in plants.
Answer:
The bark includes all the tissues outside the secondary phloem, i.e., pericycle, cortex, periderm and all dead cells external to periderm. Sometimes an addition-al layer of periderm is formed in the region of phloem so that all the tissues lying outside because dead. Under such conditions, all the dead tissues outside the active periderm (i. e., pericvclo, cortex and external periderm) are; considered as bark.
Question 6.
Comment on formation of cambium in roots?
Answer:
Secondary growth in dicotyledonous roots occurs by the initiation and activity of two secondary meristems
- Vascular cambium and
- Cork cambium (or phellogen)
Question 7.
Differentiate between Heart wood / duramen and Sap wood / albernum?
Answer:
Heartwood (Duramen) | Sap wood (Alburnum) |
1. It is the central, dark coloured non-functional part of secondary xylem of an old stem. | 1. It is the outer, light coloured functional part of secondary xylem of an old stem. |
2. Parenchymatous cells of the heart-wood are dead. | 2. Parenchymatous cells of sap wood are living. |
3. The tracheary elements are plugged by tyloses or get filled with resins, gum or tannins. | 3. The tracheary elements are not plugged. |
4. Heart wood is heavier and more durable. | 4. Sap wood is lighter and less durable. |
5. Heart wood is less susceptible to the attack of pathogens and insects. | 5. Sap wood is more susceptible to the attack of pathogen and insects. |
Question 8.
Why the heart wood is used in making wood furniture ? Explain the reasons,
Answer:
Hard and strong in nature.
RBSE Class 11 Biology Chapter 16 Essay Type Questions
Question 1.
How the vascular cambium is formed? With help of well labelled diagram explain stellar secondary growth in Dicot plants?
Answer:
Formation of vascular cambium:
- The dicotyledonous stems are characterized by presence of cambium in between the primary xylem and primary phloem in each vascular bundle. It is called fascicular cambium.
- This fascicular cambium is derived from procambium of shoot apical meristem.
- Secondary growth initiates by the formation of new cambium in the primary medullary rays.
- The parenchymatous cells beside the fascicular cambium become meristematic and develop a new cambium, called interfascicular cambium.
- The fascicular and interfascicular cambium join with each other to form a complete ring of vascular cambium
Secondary Growth in Dicotyledonous Roots: Secondary growth in dicotyledonous roots occurs by the initiation and activity of two secondary meristems
- Vascular cambium.
- Cork cambium (or phellogen).
Question 2.
Write short note on following:
Answer:
(i) Annual rings:
The vascular cambium remams active throughout the life of plant, but its activity gets affected by seasonal variations. The cambial activity varies greatly in those parts of the world where the changes of climate are muchpronounced.
In autumn, when the plants do not require active translocation of nutrients and ascent of sap. the xylem elements remain compact and develop thick wall with narrow lumen. The band of xylem, produced in this season, is called autumn wood or summer wood.
On the other hand, the plants need active translocation of nutrients and ascent of sap in spring season. The cambium grows much actively in this season and produces broad, thin-walled xylem elc-mcnts wide lumen. This band of xylem is called spring wood.
Thus, the two bands of secondary xylem, i. e., autumn wood and spring wood are produced in one year. These two bands make an annual ring. The annual rings of woody angiosperms are much distinct and one can easily determine the age of plant by counting the annual rings (It is called dendrochronolgy).
(ii) Lenticles:
The lenticel is the chief aerating structure occurs as a raised corky spot in the periderm of stem. Lenticel is usually formed below an old stoma where the cork cambium is more active and cuts a mass of loosely arranged cells, called complementary or filling cells. The complementary cells possess numerous intercellular spaces. These cells are large, colourless and lack cellular contents. The phellogen, in the region of lenticel, also has intercellular spaces.
(iii) Differentiate between Heart wood / duramen and Sap wood / albernum:
Heartwood (Duramen) | Sap wood (Alburnum) |
1. It is the central, dark coloured non-functional part of secondary xylem of an old stem. | 1. It is the outer, light coloured functional part of secondary xylem of an old stem. |
2. Parenchymatous cells of the heart-wood are dead. | 2. Parenchymatous cells of sap wood are living. |
3. The tracheary elements are plugged by tyloses or get filled with resins, gum or tannins. | 3. The tracheary elements are not plugged. |
4. Heart wood is heavier and more durable. | 4. Sap wood is lighter and less durable. |
5. Heart wood is less susceptible to the attack of pathogens and insects. | 5. Sap wood is more susceptible to the attack of pathogen and insects. |
(iv) lateral root:
The development of a lateral root begins with the formation of a meristematic region in the pericycle. As this grows, the new lateral root pushes out through the endodermis and cortex until it ruptures the surface. The structure of the lateral root is similar to that of the main root. Lateral root initiation in dicots will occur opposite the protoxylem poles.Lateral root initiation in monocots will occur between the protoxylem poles.
Question 3.
Explain extrastelar growth in stem and root with help of well labelled diagram?
Answer:
Formation of Annual Rings (or growth rings):
The cambial activity varies greatly in those parts of the world where the changes of climate are much pronounced. In autumn, when the plants do not require active translocation of nutrients and ascent of sap, the xylem elements remain compact and develop thick wall with narrow lumen. The band of xylem, produced in this season, is called autumn wood or summer wood.
On the other hand, the plants need active translocation of nutrients and ascent of sap in spring season. The cambium grows much actively in this season and produces broad, thin-walled xylem elements with wide lumen. This band of xylem is called spring wood. Thus, the two bands of secondary xylem, i.e., autumn wood and spring wood are produced in one year. These two bands make an annual ring.
The annual rings of woody angiosperms are much distinctand one can easily determine the age of plant by counting the annual rings (It is called dendrochronolgy). In most of the woody stems, the central older part of xylem becomes non-functional. The conduction of water is mainly carried by outer younger part of xylem. The older xylem tracheids and vessels become non-functional because
- they get filled with resins, gummy or tannin-like substances produced by adjacent living parenchymatous cells
- due to tyloses.
The tyloses are bladder like ingrowths of thin- walled parenchymatous cells into xylem vessels and tracheids through pits. These are formed as a result of compression of thin-walled living cells during enormous secondary growth. These factors block the passage of xylem elements so that they become non-fimctional and consequently the parenchymatous cells also die.
This central portion of non-functional xylem is called heart-wood. It is generally dark in colour and stores various kinds of products. Some heart wood of commercial importance contain different kinds of pigments, such as, haematoxylin (Hematoxylon campechianum). brasilin (Caesal-pinia sappan) and santalin (Pterocarpus santalinus). The outer young and functional part of xylem is called sap wood.The conversion of sap wood into heart wood is a progressive phenomenon which continues throughout the life of plant.
Formation of Periderm:
As the secondary growth continues for several years, the stem constantly grows in thickness due to increase in secondary vascular tissues. This increase may result in cracking and breaking of outer tissues. To avoid such a breaking of external tissues, the plants develop a new cambium ring, called cork cambium (phellogen) in the outer region. The cork cambium (phellogen) is a secondary lateral meristem which develops from permanent tissues in the region of epidermis, hypodermis, cortex or even in outer layers of phloem.
The cork cambium (phellogen) produces dead cells or cork (phellem) towards outer side and living cells of secondary cortex (phelloderm) towards inner side. These layers of cork, cork cambium and secondary cortex constitute the protective covering called periderm.The cork cells are highly lignified and check the loss of water from the general surface of stem.Sometimes the layers of cork are interrupted by lenticels.
Lenticles:
The lenticel is the chief aerating structure occurs as a raised corky spot in the periderm of stem. Lenticel is usually formed below an old stoma where the cork cambium is more active and cuts a mass of loosely arranged cells, called complementary or filling cells. The complementary cells possess numerous intercellular spaces. These cells are large, colourless and lack cellular contents. The phellogen, in the region of lenticel, also has intercellular spaces.
Question 4.
Explain secondary growth in dicot root with help of well labelled diagram?
Answer:
Secondary growth in dicotyledonous roots occurs by the initiation and activity of two secondary meristems.
- Vascular cambium.
- Cork cambium (or phellogen)
(a) Initiation and activity of vascular cambium
The process of secondary growth in dicotyledonous roots begins with the initiation of vascular cambium strips. These cambial strips develop from the parenchymatous cells present along the inner edges of primary phloem strands.The number of cambial strips depends on the number of phloem or xylem strands.
For example, if the root is diarch it develops two cambial strips, if it is triarch it develops three cambial strips and so on. The cells of cambium strips (vascular cambium) divide repeatedly to produce new cells both towards inner as well as outer side. The cells produced towards innerside (centripetally) differentiate into secondary xylem elements and those produced towards outerside (centrifugally) differentiate into secondary phloem.
Subsequently, the cells of pericvcle lying towards outer side of protoxylem divide by tangential divisions. The inner derivatives of these cells become meristematic and function as ray initials (cambium). The cambia derived from pericycle join with the cambial strips to form a complete ring of cambium. The cambium derived from pericycle (i.e., ray initials) produce wide vascular rays. The cells of rays are thin-walled parenchymatous. In addition, multiseriate vascular rays are also produced from vascular cambium. These rays run radially between secondary xylem and secondary’ phloem.
Soon, the secondary vascular tissue assumes the shape of a continuous cylinder interrupted at places where wide Vascular rays connect primary xylem to the cortex through pericycle and endodermis. The primary xylem remains in its original position but the primary phloem is pushed towards outerside and crushed.
The secondary growth, in most of the perennials, shrubs and woody climbers, is affected by seasonal variations. It leads to the formation of larger and thin-walled xylem elements in the spring and smaller and thick-walled xylem elements in the autumn season. The two kinds of concentric layers of secondary’ xylem are called spring wood and autumn wood. Both of them make an annual ring. In this way, the wood of roots also shows presence of annual rings but it is not as distinct as in stem.
(b) Initiation and activity of cork cambium:
Addition of more and more tissues of secondary xylem and secondary phloem in the central part of root exerts pressure towards periphery. The tissues outside this zone of active growth get crushed and are sloughed off. To avoid injury’ of secondary phloem, the roots develop a new cambium ring in the pericycle, called cork cambium (phellogen).
Actually the cork cambium originates as soon as the secondary vascular tissues begin to form. Cork cambium develops by the periclinal divisions in the cells of pericycle. The cells of cork cambium divide to form cells of cork (phellem) towards outer side and secondary cortex (phelloderm) towards inner side. It results in the formation of outer protective covering consisting of multilayered Cork, cork cambium and multilayered secondary cortex. These three layers make the periderm.
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