Lecture 7 - Secondary Xylem | Plant Anatomy - Biotechnology Engineering (BT) PDF Download

 Page 1


 
Secondary Xylem 
 
 
Institute of Life Long Learning, University of Delhi 
 
0 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
 
Discipline: Botany 
Paper: Plant Anatomy 
 
National Coordinator: Prof. S.C. Bhatla 
 
Lesson: Secondary Xylem 
Lesson Developer: Dr Seema Talwar 
Department/College: Shivaji College 
 
 
Lesson Reviewer: Dr Basudha Sharma 
Department/College: MM (PG) College,Modinagar, Uttar Pradesh 
 
 
Language Editor: Dr Sonal Bhatnagar 
Department/College: Hindu College 
 
 
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Page 2


 
Secondary Xylem 
 
 
Institute of Life Long Learning, University of Delhi 
 
0 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
 
Discipline: Botany 
Paper: Plant Anatomy 
 
National Coordinator: Prof. S.C. Bhatla 
 
Lesson: Secondary Xylem 
Lesson Developer: Dr Seema Talwar 
Department/College: Shivaji College 
 
 
Lesson Reviewer: Dr Basudha Sharma 
Department/College: MM (PG) College,Modinagar, Uttar Pradesh 
 
 
Language Editor: Dr Sonal Bhatnagar 
Department/College: Hindu College 
 
 
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
0 
 
Table of Contents  
Chapter: Secondary Xylem 
     
? Introduction  
? Secondary xylem in Gymnosperms  
? Axially oriented elements in Gymnosperms 
? Radial system in gymnosperms 
? Resin ducts 
? Secondary xylem in Dicotyledons 
Axially oriented elements in dicotyledons 
? Tracheary elements 
? Fibres 
? Wood parenchyma 
? Axial parenchyma 
? Ray parenchyma 
? Tyloses 
? Gum Ducts 
    Radially oriented elements in dicotyledons 
? Sapwood and heartwood 
? Arrangement of vessels in dicots 
? Early and late wood 
? Reaction wood 
? Dendrochronology 
? Summary  
? Exercise/ Practice 
? Glossary 
? References 
Page 3


 
Secondary Xylem 
 
 
Institute of Life Long Learning, University of Delhi 
 
0 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
 
Discipline: Botany 
Paper: Plant Anatomy 
 
National Coordinator: Prof. S.C. Bhatla 
 
Lesson: Secondary Xylem 
Lesson Developer: Dr Seema Talwar 
Department/College: Shivaji College 
 
 
Lesson Reviewer: Dr Basudha Sharma 
Department/College: MM (PG) College,Modinagar, Uttar Pradesh 
 
 
Language Editor: Dr Sonal Bhatnagar 
Department/College: Hindu College 
 
 
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
0 
 
Table of Contents  
Chapter: Secondary Xylem 
     
? Introduction  
? Secondary xylem in Gymnosperms  
? Axially oriented elements in Gymnosperms 
? Radial system in gymnosperms 
? Resin ducts 
? Secondary xylem in Dicotyledons 
Axially oriented elements in dicotyledons 
? Tracheary elements 
? Fibres 
? Wood parenchyma 
? Axial parenchyma 
? Ray parenchyma 
? Tyloses 
? Gum Ducts 
    Radially oriented elements in dicotyledons 
? Sapwood and heartwood 
? Arrangement of vessels in dicots 
? Early and late wood 
? Reaction wood 
? Dendrochronology 
? Summary  
? Exercise/ Practice 
? Glossary 
? References 
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
1 
Introduction 
A large amount of secondary xylem is produced by the activity of vascular cambium. This is 
known as secondary wood. Vascular cambium produces secondary xylem towards the center 
of the stem and root. It also produces secondary phloem but the amount of phloem is 
usually less than the amount of xylem produced. The secondary xylem forms bulk of 
vascular tissue in woody plants. In woody plants (gymnosperms and dicotyledons) 
secondary xylem is more persistent than secondary phloem. As the tree matures, more 
secondary xylem is produced, which enables the plant to transport additional water required 
and also supports the increasing width of the stem. The elements of secondary xylem are 
tracheids, vessels, fibres, xylem parenchyma cells, xylem rays and sometimes secretory 
cells. Usually it is difficult to distinguish between the primary and secondary xylem, but still 
there are some features which differentiate them from each other- 
 Primary xylem Secondary xylem 
1. It is formed by activity of procambium 
of apical meristem and includes 
protoxylem and metaxylem.  
It is produced from lateral meristem 
 
2. Tracheary elements are long and are 
comparatively less thick walled 
Tracheary elements are short and are 
comparatively more thick walled  
3. It is present in all organs of plant 
including root, stem & leaves 
It is restricted to stem and root of 
perennial dicots and gymnosperms 
4 Annual rings are absent Annual rings are present 
5. Fibres are few or absent Fibres are generally abundant 
6. Tyloses are usually absent Tyloses are present 
 
The secondary xylem has two systems of elements which differ in the orientation of the 
plant. The vertical or longitudinal system is also known as axial system consists of vertical 
files of tracheary elements, fibres and wood parenchyma. The horizontal or transverse also 
known radial system consists of xylem rays or wood rays. The structure of secondary xylem 
is simpler and more homogenous in gymnosperms as compared to the angiosperms due to 
Page 4


 
Secondary Xylem 
 
 
Institute of Life Long Learning, University of Delhi 
 
0 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
 
Discipline: Botany 
Paper: Plant Anatomy 
 
National Coordinator: Prof. S.C. Bhatla 
 
Lesson: Secondary Xylem 
Lesson Developer: Dr Seema Talwar 
Department/College: Shivaji College 
 
 
Lesson Reviewer: Dr Basudha Sharma 
Department/College: MM (PG) College,Modinagar, Uttar Pradesh 
 
 
Language Editor: Dr Sonal Bhatnagar 
Department/College: Hindu College 
 
 
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
0 
 
Table of Contents  
Chapter: Secondary Xylem 
     
? Introduction  
? Secondary xylem in Gymnosperms  
? Axially oriented elements in Gymnosperms 
? Radial system in gymnosperms 
? Resin ducts 
? Secondary xylem in Dicotyledons 
Axially oriented elements in dicotyledons 
? Tracheary elements 
? Fibres 
? Wood parenchyma 
? Axial parenchyma 
? Ray parenchyma 
? Tyloses 
? Gum Ducts 
    Radially oriented elements in dicotyledons 
? Sapwood and heartwood 
? Arrangement of vessels in dicots 
? Early and late wood 
? Reaction wood 
? Dendrochronology 
? Summary  
? Exercise/ Practice 
? Glossary 
? References 
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
1 
Introduction 
A large amount of secondary xylem is produced by the activity of vascular cambium. This is 
known as secondary wood. Vascular cambium produces secondary xylem towards the center 
of the stem and root. It also produces secondary phloem but the amount of phloem is 
usually less than the amount of xylem produced. The secondary xylem forms bulk of 
vascular tissue in woody plants. In woody plants (gymnosperms and dicotyledons) 
secondary xylem is more persistent than secondary phloem. As the tree matures, more 
secondary xylem is produced, which enables the plant to transport additional water required 
and also supports the increasing width of the stem. The elements of secondary xylem are 
tracheids, vessels, fibres, xylem parenchyma cells, xylem rays and sometimes secretory 
cells. Usually it is difficult to distinguish between the primary and secondary xylem, but still 
there are some features which differentiate them from each other- 
 Primary xylem Secondary xylem 
1. It is formed by activity of procambium 
of apical meristem and includes 
protoxylem and metaxylem.  
It is produced from lateral meristem 
 
2. Tracheary elements are long and are 
comparatively less thick walled 
Tracheary elements are short and are 
comparatively more thick walled  
3. It is present in all organs of plant 
including root, stem & leaves 
It is restricted to stem and root of 
perennial dicots and gymnosperms 
4 Annual rings are absent Annual rings are present 
5. Fibres are few or absent Fibres are generally abundant 
6. Tyloses are usually absent Tyloses are present 
 
The secondary xylem has two systems of elements which differ in the orientation of the 
plant. The vertical or longitudinal system is also known as axial system consists of vertical 
files of tracheary elements, fibres and wood parenchyma. The horizontal or transverse also 
known radial system consists of xylem rays or wood rays. The structure of secondary xylem 
is simpler and more homogenous in gymnosperms as compared to the angiosperms due to 
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
2 
the absence of vessels (except in Gnetales) and presence of small amount of axial 
parenchyma. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure: Cross section of 2 year old Tilia stem 
Source: http://www.deanza.edu/faculty/mccauley/6a-labs-plants-05.htm;  
Differences between the secondary xylem of gymnosperms and dicotyledons 
Characters Gymnosperms Dicotyledons 
Vessels Wood lacks vessels                               They have vessels 
Axial 
parenchyma 
it is present in less amount Axial parenchyma is present 
Wood Wood is known as soft wood                 Wood is known as hard wood                 
Torus Bordered pits with tori is a common 
occurrence in Ginkgo, Gnetales and in 
Tori is not the common occurrence   
Page 5


 
Secondary Xylem 
 
 
Institute of Life Long Learning, University of Delhi 
 
0 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
 
Discipline: Botany 
Paper: Plant Anatomy 
 
National Coordinator: Prof. S.C. Bhatla 
 
Lesson: Secondary Xylem 
Lesson Developer: Dr Seema Talwar 
Department/College: Shivaji College 
 
 
Lesson Reviewer: Dr Basudha Sharma 
Department/College: MM (PG) College,Modinagar, Uttar Pradesh 
 
 
Language Editor: Dr Sonal Bhatnagar 
Department/College: Hindu College 
 
 
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
0 
 
Table of Contents  
Chapter: Secondary Xylem 
     
? Introduction  
? Secondary xylem in Gymnosperms  
? Axially oriented elements in Gymnosperms 
? Radial system in gymnosperms 
? Resin ducts 
? Secondary xylem in Dicotyledons 
Axially oriented elements in dicotyledons 
? Tracheary elements 
? Fibres 
? Wood parenchyma 
? Axial parenchyma 
? Ray parenchyma 
? Tyloses 
? Gum Ducts 
    Radially oriented elements in dicotyledons 
? Sapwood and heartwood 
? Arrangement of vessels in dicots 
? Early and late wood 
? Reaction wood 
? Dendrochronology 
? Summary  
? Exercise/ Practice 
? Glossary 
? References 
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
1 
Introduction 
A large amount of secondary xylem is produced by the activity of vascular cambium. This is 
known as secondary wood. Vascular cambium produces secondary xylem towards the center 
of the stem and root. It also produces secondary phloem but the amount of phloem is 
usually less than the amount of xylem produced. The secondary xylem forms bulk of 
vascular tissue in woody plants. In woody plants (gymnosperms and dicotyledons) 
secondary xylem is more persistent than secondary phloem. As the tree matures, more 
secondary xylem is produced, which enables the plant to transport additional water required 
and also supports the increasing width of the stem. The elements of secondary xylem are 
tracheids, vessels, fibres, xylem parenchyma cells, xylem rays and sometimes secretory 
cells. Usually it is difficult to distinguish between the primary and secondary xylem, but still 
there are some features which differentiate them from each other- 
 Primary xylem Secondary xylem 
1. It is formed by activity of procambium 
of apical meristem and includes 
protoxylem and metaxylem.  
It is produced from lateral meristem 
 
2. Tracheary elements are long and are 
comparatively less thick walled 
Tracheary elements are short and are 
comparatively more thick walled  
3. It is present in all organs of plant 
including root, stem & leaves 
It is restricted to stem and root of 
perennial dicots and gymnosperms 
4 Annual rings are absent Annual rings are present 
5. Fibres are few or absent Fibres are generally abundant 
6. Tyloses are usually absent Tyloses are present 
 
The secondary xylem has two systems of elements which differ in the orientation of the 
plant. The vertical or longitudinal system is also known as axial system consists of vertical 
files of tracheary elements, fibres and wood parenchyma. The horizontal or transverse also 
known radial system consists of xylem rays or wood rays. The structure of secondary xylem 
is simpler and more homogenous in gymnosperms as compared to the angiosperms due to 
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
2 
the absence of vessels (except in Gnetales) and presence of small amount of axial 
parenchyma. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure: Cross section of 2 year old Tilia stem 
Source: http://www.deanza.edu/faculty/mccauley/6a-labs-plants-05.htm;  
Differences between the secondary xylem of gymnosperms and dicotyledons 
Characters Gymnosperms Dicotyledons 
Vessels Wood lacks vessels                               They have vessels 
Axial 
parenchyma 
it is present in less amount Axial parenchyma is present 
Wood Wood is known as soft wood                 Wood is known as hard wood                 
Torus Bordered pits with tori is a common 
occurrence in Ginkgo, Gnetales and in 
Tori is not the common occurrence   
Secondary Xylem 
Institute of Life Long Learning, University of Delhi 
 
3 
Coniferales                                  
Resin ducts Resin ducts are the common feature of 
all Coniferales                             
Resin ducts are not the common feature 
in diots 
Rays Rays are mostly uniseriate Rays are mostly multiseriate 
Fibres Libriform fibres are not present            Libriform and fibre-tracheids both are 
present 
Crassulae and 
trabeculae 
Crassulae and trabeculae are another 
important feature 
These type of thickenings are not 
present. 
Fenestriform 
pits 
pits are present (window like pits) These pits are not present 
Example with 
diagram 
  
Figure:  
 
 
Source: 
Wood of a Gymnosperm (Thuja 
occidentalis); vessels, fibres and axial 
parenchyma are absent 
http://nickrentlab.siu.edu/PLB400/imag
es/Figure9_1.jpg 
Wood of a Dicotyledon (Liriodendron 
tulipifera); vessels, fibres and axial 
parenchyma are present 
http://nickrentlab.siu.edu/PLB400/image
s/Figure9_4.jpg