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Summary BOT 2601 LEARNING UNIT 4
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 4ORIGIN AND STRUCTURE OF THE SECONDARY PLANT BODY
https://www.youtube.com/watch?v=TcVxFXltBPw
Textbook reference: chapter 4 pg. 161-202
SECONDARY PLANT GROWTH:
o Growth derived from lateral meristems is called secondary growth
o Secondary growth is composed of secondary tissues, which form the secondary plant body
o The vascular cambium is a bifacial meristem, which adds to the girth of the stem and root by the production of
secondary xylem and secondary phloem
o This only occurs in gymnosperms and dicotyledons
o Cambium can be stimulated by levels of auxin
o Phloem cells are produced first, followed by xylem
VASCULAR CAMBIUM FORMATION:
o Develops in the shoot and root apex, a short distance behind the apical meristem
o The cambium that forms within the vascular bundles is called the fascicular cambium
o Later in the maturing process, interfascicular cambium is initiated
o Cambium differentiates in an acropetal direction and is continuous with the cambium of branches
o In stems with separate vascular bundles and those with a continuous cylinder of primary vascular tissue, the
activity of the fascicular and interfascicular cambia together produce an uninterrupted cylinder of secondary
vascular tissue
o Because the vascular cambium persists for the life of the plant, it continues to gain circumference after
secondary growth begins
o In some herbaceous dicotyledons the interfascicular cambium does not form a woody cylinder. Instead more
bundles are formed in the middle of the medullary rays
o The formation of the interfascicular vascular cambium is an example of cell differentiation in which cells of one
morphological type are transformed first into cells of another morphological type and then maintained in that
state
o Induction is the ability of one tissue to influence the development of another tissue
o Inductive effects are the responses or changes produced within a group of cells/tissues, when they are exposed
to the influence of neighbouring cells
STRUCTURE AND FUNCTION OF THE VASCULAR CAMBIUM:
o At maturity the cambium normally forms a continuous cylinder around the stem and root and extends into the
leaf petioles
o Cambial initials form new derivatives on either side where xylem differentiates towards the inside and phloem
towards the outside
o New derivatives are added by additive periclinal (multiplicative) divisions
o The derivatives undergo radial expansion and tip growth
o The entire meristematic region is referred to as a cambial zone, composed of 3 regions:
- A region of expanding derivatives prior to initiation of secondary wall thickenings
- A zone of active assimilation of secondary wall materials
- Zone of mature tracheary elements
o Most vascular cambia possess 2 categories of initials:
- Fusiform initials:
Large, elongated, spindle-shaped elements with tapering, overlapping ends.
They give rise to an axial system of radially aligned, vertically elongate tracheary elements, fibres and
sieve elements.
Have a rich collection of organelles and are highly vacuolated.
During actively growing months these cells contain a large elongated vacuole and their walls have
primary pit fields.
They form unstratified and stratified/storied cambium.
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