Lesson prepared under MHRD project “NME ICT”

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Cambium
Lesson prepared under MHRD project “NME ICT”
Discipline: Botany
Lesson: Vascular Cambium
Lesson Developer:Dr.Pooja Gokhale Sinha and Dr.Ajit Kumar
Kavathekar
Reviewer: Prof. S. C. Bhatla
Institute of Lifelong Learning, University of Delhi
Cambium
Table of Contents
Chapter: VASCULARCAMBIUM
•
Introduction
•
Topic 1 objectives
•
Topic 2 definition
•
Occurrence and distribution
•
Structure of cambial cells
• Fusiform initial cells
• Ray initial cells
• Cambium zone concept
• Types of cambium
o Storied cambium
o Non-storied cambium
Seasonal activity of cambium
Secondary growth
• Dicot stem
• Monocot stem
• Root
Summary
Glossary
Exercise
References/Bibliography/Further Reading
Web links
•
•
•
•
•
•
•
Cambium
Learning Outcomes
After reading this chapter you should be able to
1. Understand what is meant by secondary or lateral meristem.
2. Differentiate between primary and secondary meristem with respect to their position,
function and structure.
3. Understand the concept of cambial zone.
4. Enumerate the products of vascular cambium and relate their structure with function.
5. Understand the heterogeneous character of cambial cells and their contribution
towards the growth of axial as well as lateral systems.
6. Have an insight into the relevance of secondary cambium in wood formation.
7. List and explain the differences between cork cambium and meristem.
Cambium
Introduction
Primary meristem is embryonic tissue zone that confers upon plants a unique property of
open growth. The term meristem is derived from the greek word merizein which means to
divide in recognition of its inherent function. It was first used in 1858 by Karl Wilhelm von
Nägeli in his book titled BeiträgezurWissenschaftlichenBotanik. Meristem cells retain their
property of continual division, wherein one daughter cell differentiates according to the need
of the tissue or organ, and the other further divides to repeat the cycle.Meristematic cells
are the ones which show properties of meristem cells at some specific time e.g. cells that
are involved in wounding are meristematiccells, but not true meristematic cells as they do
not retain this property for life.
In majority of gymnosperms and dicots, a part of the procambium retains its
meristem activity even after the formation of primary vascular bundles. This part of
cambium is responsible for secondary growth and produces secondary xylem centripetally
and secondary phloem centrifugally. The term cambium is applied mainly to two secondary,
lateral meristems. These are (i) Vascular cambium: that produces secondary vascular
tissues, and (ii) Cork cambium: that produces phellem and phelloderm. The current chapter
shall focus on Vascular cambium, its structure, functions and derivatives.
Definition
Based upon their position in a plant, cambium is of two types: Primary or apical and
Secondary or lateral.Unlike the primary meristem that is apical in position and leads
increase in height of a plant, secondary meristem is lateral,increases the girth of a plant and
leads to formation of wood. Secondary growth or increase in girth is mediated by the
activity of the vascular cambium. Vascular cambium is thin layer of meristem cells that
divides periclinallyto produce a radial file of cells- xylem adaxially and phloem abaxially. It
comprises of an internal meristematic tissue that functions as a stem cell niche and is
organized in a tube-like domain encompassing the growth axes. There are two concepts of
cambial development: Uniseriate and Multiseriate. Unseriate concept is endorsed by authors
who emphasize on the developmental and functional aspects of cambium. According to
them each radial file of cells have one initial cell that periclinally divides to two daughter
cells of which one differentiates and other continues to divide. However, morphologically
and anatomically it is difficult to demarcate such a layer. Those having more of structural
Cambium
and morphogenetic orientation describe the cambium as a multilayered file of cells, which
are in a state of flux and are continually dividing and differentiating.
Occurrence and distribution
Vascular cambium is present in angiosperms, gymnopserms and extinct group of
pteridophytes. Evidence from fossil records shows that secondary growth occurred
sometime in the mid Devonian approximately 380 Mya. This led to trees such as
Archaeopterisattain height of upto 30 m. In a plant which shows secondary growth, such
growth takes place in stem as well as roots.Vascular cambium ina stem can be
fascicular(FC) or inter-fascicular (IC) in position.Fascicular cambium originates from
procambium within a vascular bundle, or fascicle particularly in dicot plants. It divides
periclinally to produce secondary xylem towards the inner side and secondary phloem
towards
the
outer
side.This
arrangement
makes
vascular
bundles
of
dicots
open.Interfascicular cambium arises in the region between the vascular bundles or fascicles.
At maturity the fascicular and interfascicular cambium merge to form a complete ring, which
eventually leads tosecondary growth.
Structure of cambial cells
Cambial cells do not have internal cellular organization like other meristematic cells and are
highly vacuolated and have less dense cytoplasm at maturity.Their cell walls posses primary
pit-fields with plasmodesmata. To facilitate periclinal divisions, the radial walls of primary
xylem and phloem are thicker than tangential walls. Even though procambium and cambium
are thought to be two temporally separated stages of meristem, there are certain
demarcations that are used to differentiatebetween them. Unlike cambial cells which are flat
ended and lightly stained, cells of procambium are darkly stained and have gabled ends,
when observed in a radial view.Procambial cells are not differentiated into long fusiform and
isodiametric ray cells. Mature vascular cambium shows a high degree of organization.Also,
some plants do not show secondary growth at all. Thus, based upon the above mentioned
points, it is justified to consider them as separate group altogether.
MATURE PHLOEM
Differentiating
Radially enlarging phloem
phloem
Dividing phloem (phloem mother cells)
Vascular
Vascular
Vascular cambial initial (dividing)
cambial zone
cambium
Cambium
Differentiating
Dividing xylem (xylem mother cells)
xylem
Radially enlarging xylem
Maturing xylem
Mature xylem
Terminology of vascular cambium and its progeny: Meristem, developing and derivative
tissues (Modified after Wilson et al., 1966)
Mature cambial cells are heterogeneous and morphologically can be distinguished into two
forms:
a.)
Fusiform initialcells (FCC)
b.)
Ray initialcells (RCC)
Fusiform Initial Cells: These cells are highly elongated andtapering,and give rise to the
vertical system of a plant.They are prismatic in the centre and wedge-shaped towards the
ends.InTLS(Transverse longitudinal section) they appear flat and prism shaped. The
derivatives
of
fusiform
initials
always
dividepericlinallyand
give
rise
to
the
axial
system.Maturation of fusiform initials show characteristic growth patterns, wherein their
elongation keeps on increasing, reaches a plateau and eventually dies. However in some
species of Pinusplateau has not been reached even after 2200 years.Mean length of fusiform
initial is higher in angiosperms with primitive vessel members than those with advance
vessels. Storied cambia have shorter fusiform initials than non-storied cambia. Dimensions
of products are FCC is directly proportional to the mother FCC.Theirlength may range from
6800 µm in gymnosperm Agathisrobustato 170 µm in Robiniapseudoacacia. Fusiform initials
are the only cell types that divide along their longitudinal axis thereby defying Errera’s law
of cell division.
BOX 1
Some interesting facts: Fusiform cells
Fusiform initial cells are highly elongated cells. In giant redwood tree Sequoia
sempervirens,they reach a maximum length of about 8.7 mm. Interestingly, they
are the only cell type that do not follow the Errera’s law of cell division and divide
in a plane that has maximum of surface area.
Cambium
Giant redwood (Sequoia sempervirens)
http://www.dreamstime.com/stock-photo-sequoia-sempervirens-image5367210
The products formed by division of fusiform initials are:tracheids, vessel members,
xylem fibres, xylem parenchyma, which together forms the secondary xylem.They also
gives rise to components of secondary phloem namely, phloem parenchyma, phloem fibres,
sieve cells,albuminous cells (strasbuger’s cells), sieve tube members along with companion
cells.Number of vessel members is equal to the number of fusiform initials. Every
contagious, adjoining neighboring vessel member has a common perforation plate.
Cambium
Digramatic representation showing fate of Ray and Fusiform initial
Ray initials:Ray cells are isodiametric and upon division lead to formation of the
radial system of secondary growth. They run parallel to the radii which pass through the
centre of the organ.Theyaffect the transport efficiency of water, mineral nutrients and
photoassimilates and ultimatelyplant height.
Transformation of ray initials into fusiform initials and vice versa: Fusiform and ray
initial cells may interconvert into the other. Formation of ray initial from fusiform initial may
be brought about by lateral partitioning, direct partitioning and septation. Reverse pattern
of transformation of rayinitial to fusiform initial is although present,but uncommon.
Cambium
Different ways of differentiation of meristem cell
http://www.resnet.wm.edu
Cambium zone concept
During rigorous meristem growth, the rate of cell division is higher than the rate of
cell differentiation, leading to formation of a zone of cambium called the cambial zone. The
cambial zone comprises of single layer of true cambial cells, their immediate derivatives and
undifferentiated or partially differentiated xylem and phloem cells.Continued differentiation
of the cambial derivatives into xylem and phloem does not affect the existence of the
cambial zone.There have been successful attempts to distinguish between xylem and
phloem mother cells in cambial zone. Xylem mothercells are somewhat more elongated than
phloem mother cells.In an active cambium, cambial zone is wide, and in dormant state it is
only around single layer thick.Cells of proper cambial zone are mitotically active but their
mitotic rate decreases as one moves towards the periphery of the zone. Morphogenetically
cambium zone is single layered and morphologically it may be few to several layers thick.
Cambium
There is however, some variability in the type of derivatives produced by the cambial zone
that may be influenced by factors such as season. In Caryasp.,the production of xylem per
growing season is around three to five times higher than that of phloem. Generally,
production of phloem derivatives predominates in the early phase of a season and that of
xylemderivativesin the later phase. Thus whether the cambium has any border and are
those borders true or real, is still not a completely answered question.
Types of cambium
Depending upon the number and arrangement of fusiform initials in tangential view,
cambium can be either storied or non-storied.
Storied or stratified cambium:Fusiform initials are arranged in horizontal rows in such a
way that their ends are approximately at the same level. Their length ranges between 140
to 520 µm. They are arranged uniformly in tiers. It is found in species such asTamarix and
Robinia. Storied arrangement is considered to be present in phylogenetically advanced
genera. Usually short fusiform initials have storied arrangement. They are not found in
gymnosperms.
Storied cambium showing regularly arranged fusiform initial at same level.
Cambium
(http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/410Labs/LabsHTML99/StemSecGrow/Image332.gif)
Non-storied or non- stratified cambium:Fusiform initials partially overlap and their ends
do not lie at the same level. The initials are longer and their length varies between 320 to
2300 µm. They are found in primitive groups of plants andarephylogenetically primitive.
Non-storied fusiform initial showing fusiform initial cells arranged in different levels
http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/410Labs/LabsHTML99/StemSecGrow/LABVCXR99.html
Seasonal activity
Activity of cambium is under the influence of several environmental factors such as
season.Trees growing in the tropics show more or less uniformity in the size or dimension of
their
vessel
elements.
Whereas
those
growing
in
temperate
climates
show
clear
morphological variationin summer and winter wood. In summer, xylem vessels are wider as
compared to winter.There is a difference in thebehavior of cambium in situations such as
drought or floods. Plants as a system are highly sensitive and well aware of their stationary
character and prepare to face new seasons / situations well in advance. An interesting
example may be the emergence of new foliage on the onset of summer when the plant shall
need more and higher conductance.Likewise, reduced or no vessels in severe winters may
Cambium
be seen as a preparation for autumn. Characteristic annual rings seen in wood of trees
species is due to seasonal activity.These complex physiological processes proceed at a rate
which depends on several factors, acting at various levels: growth regulators, resource
availability and environmental factors. Several hormonal signals and more recently, further
regulatory molecules, have been shown to be involved in the induction and maintenance of
cambium and the formation of secondary vascular tissues. The control of xylem cell
patterning is of particular interest, because it determines the diameter of xylem vessels,
which is central to the efficiency of water and nutrient transport from roots to leaves
through the stem and may strongly influence growth inthe height of a tree. Increasing
scientific evidence has proved the role of other hormones in cambial cell activities and the
study of hormonal signals and their crosstalk in cambial cells may foster our understanding
of the dynamics of xylogenesis and the mechanism of vessel size control along the stem.
Some interesting facts
The same cambial cell produces xylem centripetally and phloem centrifugally.
What are the signals involved in governing this differentiation? Can a xylem cell
made to be transformed to a phloem cell by altering theses signals? If yes! Then
these observations may change the process of wood formation and wood
formation can be modified to form special type of wood catering to individual
needs! In their classical studies on factors governing xylem and phloem
differentiation,
Wetmore
and
Rier
(1963)
demonstrated
that
the
relative
concentration of auxin and simple sugars play a critical role in inducing xylem and
phloem tissue.They concluded thatlow concentration of sugars favorsxylem
differentiation and high sugar concentration favors phloem differentiation.
Secondary growth in dicot stem
Secondary growth results from activity of vascular cambium, primarily leads to increase in
girth of a plant. It mainly takes place in main shoot, root, lateral branches and sometimes in
leaves too. It is prominently observed in gymnosperms and woody as well as herbaceous
dicots. Onset of secondary growth is marked by increased activity of fascicular cambium in
the beginning. This isfollowed by increased meristem activity of interfascicular cambium.
Radial polarity in cells of interfascicular cambium is pre-determined and not dependent on
the activity of fascicular cambium.
Cambium
In common type of secondary growth, the fascicular and interfascicular cambium
merges to form a complete ring and produces secondary xylem centripetally and secondary
phloem centrifugally.The ring formation continues in different cycles of secondary growth
the each cylinder have their axial and radial systems in turn derived from fusiform and ray
initials, respectively.Formation of secondary xylem and secondary phloem (which shall be
discussed in details in subsequent chapters) often puts a lot of pressure on the primary
xylem and phloem. In a way secondary xylem and phloem intrude the space between
primary xylem and phloem and lead to significant alteration in the internal structure of a
stem.The pith shrinks in size as its space is almost taken over by the secondary xylem,
which also takes over the primary xylem structurally and functionally.Likewise, the primary
phloem is pushed towards outside and is eventually crushed due to increasing internal
pressure, it also becomes redundant in function also. Sometimes indicots, primary
phloemdevelops into fibers.
T.S of stem showing secondary growth (secondary xylem and secondary phloem).
(http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/410Labs/LabsHTML99/StemSecGrow/LABVCXR99.html)
Cambium
Secondary growth in monocots
Monocotyledonous plants do not have the typical secondary growth as dicots. However
some monocots such as palms, show a specialized type of secondary growth wherein, they
develop a thick stem by higher rate of cell division and enlargement by parenchymatous
cells of the ground tissue (not clear). Such secondary growth is called diffuse secondary
growth. Another mode of secondary growth is exhibited by some herbaceous and woody
members of liliflorae such as Agave, Aloe, Yucca, Dracaenawhich is mediated by a
specialized cambium,arisingin parenchymatous tissue outside the vascular bundle. It
produces secondary vascular bundles and parenchyma towards inside and parenchyma
towards outside.
Diagrammatic representation of secondary growth in monocots
http://plant-bio.wikispaces.com/Stem+anatomy
Secondary growth in roots
As in dicot stem, secondary growth in roots is mediated by the activity of vascular
cambium.It is a characteristic feature of gymnosperms and occurs in varying degrees in
some dicots.In roots,the cambial layer occurs asa stripe of undifferentiated cells lying in
between primary xylem and primary phloem. These strips may be two in number in diarch
roots and three in number in triarch roots. As secondarygrowthprogresses, some cells of
pericycle located outside the xylem ridges become active and start functioning as cambium.
This leads to formation of a complete cambium encircling the xylem core. Such an
Cambium
arrangement gives diarch root an oval structure and triarch root a triangular look. Cambium
situated on the inner face of phloem becomes active first. It produces xylem and phloem
cells by periclinal divisions. Anticlinal divisions lead to an increase in the circumference.
T.S. dicot root showing secondary growth
https://learning.uonbi.ac.ke/courses/SBT204/scormPackages/path_2/74_consequence_of_s
econdary_thickening_to_the_cambium.html
Summary
Cambium
Primary meristem is embryonic tissue zone that confers upon plants a unique property of
open growth.It comprises of meristem cells that retain their property of continual division.
Vascular
cambium
is
present
in
angiosperms,
gymnopserms
and
one
group
of
pteridophytes. The term cambium is appliedprimarily to two secondaryand lateral meristems
which areVascular cambium and Cork cambium.Vascular cambium comprises of thin layer of
meristem cells that divides periclinally to produce a radial file of cells- xylem adaxially and
phloem abaxially. A zone of meristematic tissue functions as a stem cell niche and is
organized in a tube-like domain encompassing the growth axes and is called as a cambium
zone. There are two concepts of cambial development: Uniseriate and Multiseriate.Mature
cambial cells are heterogeneous and morphologically can be distinguished into two
forms:Fusiform initial cells (FCC) and Ray initial cells (RCC).Fusiform initials always divide
periclinally and give rise to the axial system of the plant, and ray cells upon division lead to
theformation of the radial system of secondary growth. Depending upon the number and
arrangement of fusiform initials in tangential view, cambium can be either storied or nonstoried. Onset of secondary growth in typical dicots is marked by increased activity of
fascicular
cambium
in
the
beginning,
followed
by
increased
meristem
activity
of
interfascicular cambium. Monocots such as Palms, show a specialized type of secondary
growth wherein, they develop thick stem by higher rate of cell division and enlargement by
parenchymatous cells of the ground tissue. Such secondary growth is called diffuse
secondary growth. In roots, the cambial layer occurs a stripe of undifferentiated cells lying
in between primary xylem and primary phloem and cambial cells lead to formation of a
complete cambium encircling the xylem core giving rise to either adiarchor a triarch root.
Glossary
Anticlinal divisions: Cell division in which plane of the division is at right angle to the
surface of the plant body.
Cambium zone: A zone of meristematic cellswhichfunctions as a stem cell niche producing
its derivatives on either sides.
Errera’s law:Errera’s law of cell division states that a cell divides such that the plane of
division has minimum surface area.
Fascicular vascular cambium: Vascular cambium present in vascular bundle, present in
the primary body of the plant.
Cambium
Fusiform initial: highly elongated, has tapering ends, and give rise to the vertical system
of a plant.
Interfascicular vascular cambium: Vascular cambium present in between two adjacent
vascular bundles, it develops at the initiation of secondary growth.
Lateral meristem: Secondary meristem that lead to lateral growth (increased girth) of
plant body.
Non-storied/Non-stratified cambium: Arrangement of fusiform initials in horizontal rows
such that their ends are at different levels.
Periclinal division: Cell division that is parallel to the surface of the plant body.
Ray initial: Ray cells are isodiametric and upon division lead to formation of the radial
system of secondary growth.
Secondary growth: Growth in the plant body due to activity of vascular cambium.
Storied/Stratified cambium: Arrangement of fusiform initials in horizontal rows such that
their ends are approximately at the same level.
Vascular cambium: A meristem responsible for secondary growth.
Exercises
Answer the following questions
Q1. Define secondary meristem. How is it different from primary meristem?
Q2. Enumerate the key characters of cambial cells. How do these features confer plasticity
upon them?
Q3. Comment on heterogeneous character of cambial cells and their respective contribution
in plant growth?
Q4. What is a cambium zone?
Q5. Differentiate between:
•
Ray Initial Cell and Fusiform Initial Cell
•
Vascular cambium and Cork cambium
Cambium
•
Storied and non-storied cambium
Q6. Enumerate the events involved in secondary growth in wood of dicot stem.
Q7. How does secondary growth differ in monocot and dicot stem?
Q8. Write a short note on distribution of vascular cambium.
Q9. Give an account of secondary growth in roots.
Q10. What is the difference between fascicular and inter-fascicular cambium.
References/Suggested readings
Weblinks
http://bio1152.nicerweb.com/Locked/media/ch35/vascular-cambium.html
http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/410Labs/LabsHTML99/StemSecGrow/LABVCXR99.html
http://plant-bio.wikispaces.com/Stem+anatomy
https://learning.uonbi.ac.ke/courses/SBT204/scormPackages/path_2/74_consequence_of_s
econdary_thickening_to_the_cambium.html
http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/410Labs/LabsHTML99/StemSecGrow/Image332.gif
http://www.resnet.wm.edu/~mcmath/bio205/

Lesson prepared under MHRD project “NME ICT”

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