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Summary BOT 2601 LEARNING UNIT 3
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 3TISSUE ORGANISATION IN STEMS, LEAVES AND ROOTS
Textbook reference: chapter 3 pg. 121-159
STEMS:
o 2 main functions: support of organs and the transport of water and dissolved minerals
o Stems have external nodes and internodes and the lateral organs, leaves and auxiliary buds
o Vascular bundles:
In herbaceous or nonwoody shoots, the vascular tissues are found together and are organised into discrete
columns or strands called vascular bundles
In the apical zone of maturation, the primary vascular tissues are differentiated from the procambium
A vascular bundle that has the potential for secondary growth is an open bundle
In monocotyledons all procambial cells within a bundle become primary vascular elements, so the bundle is
termed closed to secondary growth
The vascular system of the stem and leaf become interconnected at the nodal regions. The pattern by which
the stem bundles are connected with the veins of the leaves and branches is varied
A vascular bundle in the stem that is a continuation of a vascular bundle of the leaf base is called a leaf trace
At the level where the leaf trace is directed toward the leaf, a parenchymatous region differentiates and is
called a leaf gap
Each axial bundle and its leaf trace are a sympodium
Types of vascular bundles:
- Collateral vascular bundle: primary phloem and xylem are in the same radius of the stem, and the
phloem is on the side toward the cortex and the xylem is confined to the side toward the pith
- Bicollateral vascular bundle: phloem is positioned on inner and outer sides of the xylem
- Amphivasal vascular bundle: central phloem is surrounded by xylem
- Amphicribral vascular bundle: phloem surrounds the xylem
Different types of collateral vascular bundles occur in monocotyledons
- One type has the xylem elements meeting the phloem along a straight or slightly curved line
- A second type has a conspicuous v-shaped arrangement of xylem with large cells in the arms of the v
- Another type has a single large metaxylem element on each side of the bundle
o Stem structure:
In gymnosperms and dicotyledons the primary vascular bundles are embedded in nonvascular ground tissue
The axial bundles are arranged in a ring near the periphery of the stem – this arrangement is termed eustele
The cells between the vascular bundles and outer edge of the stem are mainly parenchyma and together
they make up the cortex
The cortical parenchyma is green with intercellular spaces
Sclerenchyma, collenchyma and laticifers also make up the cortex
In some stems, photosynthetic parenchyma or sclerenchyma form a subepidermal hypodermis
In other stems, the inner cortex layer is differentiated into a special layer called an endodermis
In monocots, vascular bundles are widely spaced and randomly scattered – this pattern is called an
atactostele
Secondary growth occurs in dicotyledons but is usually absent in monocotyledons
, LEAVES:
o Involved in transport of liquid, gas exchange, light absorption, prevention of water loss and transport of
carbohydrates
o The leaves of seed plants are derived from modified branch systems that have undergone planation and that
have an expanded form by the extension of parenchymatous tissues between the vascular strands
o Leaves are initiated as lateral protuberances from rapidly dividing
subsurface cells on the dome-like stem apex
o The phyllotaxis (arrangement of leaves on the stem) varies
o As a result of elongation of young foliar primordia, young leaves extend
upward beyond the terminal apex
o The early period of growth is followed by divisions in the intercalary
regions to form the major leaf parts
o The mature foliage leaf is composed of a leaf base, a petiole and an
expanded leaf blade/lamina
o Some have small leaflike structures called stipules positioned between
the petiole and stem
o A ligule is at the junction of the sheath and the stem
o Leaves that occur on young plants are called juvenile leaves
o Older leaves are adult leaves
o Lamina structure:
The outermost layers on both surfaces compose the upper
(adaxial) and lower (abaxial) epidermis
The epidermal layers are continuous around the leaf
The outer wall of epidermal cells are thicker than the inner walls
The cuticle covers the surface
Gases are exchanged through stomata in the epidermis
Each stoma consists of a pore and pair of guard cells, and the total stomatal complement is most commonly
restricted to the lower surface of the lamina - hypostomatous
Stomata located on the upper and lower epidermis are amphistomatous
Stomata only found on the upper surface are epistomatous
Guard cells have chloroplasts
All the interior cells between the 2 outer layers compose the mesophyll region of the lamina, except
vascular bundles
The mesophyll is composed of thin-walled parenchyma cells with chloroplasts
The mesophyll is differentiated into 2 regions:
- The columnar, evenly spaced cells toward the upper epidermis, elongated at right angles to the surface
make up the palisade region. This maximises efficiency of photosynthesis at the optimum angles of the
sun’s rays. It can consist of 1 or many layers
- The spongy region is the zone below the palisade that consists of irregularly shaped cells with free
surface area that extends to the lower epidermis
- Leaf blades with palisade parenchyma on one side and spongy parenchyma on the other are termed
bifacial
- Leaves with palisade parenchyma on both sides of the lamina are isobilateral
o Leaf venation:
Leaves are vascularised by a venation system that extends through the petiole in an arrangement that is
specific to each taxon
Within the lamina the vasculature divides into several major venation patterns
Veins are cylindrical bundles of vascular tissue that occur mainly in the median plane of the mesophyll
In dicotyledons, venation patterns consist of one or more branched major veins and a dense network of
smaller minor veins that frequently terminate in freely ending veinlets within the areoles (mesophyll islets
bound by veins of higher order)
If freely ending veinlets are absent, the venation pattern is termed closed
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