Download The Shoot System: Primary Stem Structure

The Shoot System:
Primary Stem Structure - 1
Shoot System
The shoot system comprises the leaves and stems of plants. Leaves are located
at nodes on the stem; the distance along the stem between nodes is known as an
internode. Shoots develop from shoot meristem, which contains the apical shoot
tip meristem from the epicotyl of the embryo, leaf primordia, and bud primordia,
which are embryonic lateral shoot systems. Flowers, the reproductive organs of
angiosperms, are modified shoots. Shoot meristems are located at the growing
tips of stems, and in buds.
Shoot Meristems
The apical meristem is divided into two regions: tunica and corpus. The tunica
meristem cells divided in a plane the produces additional surface meristem. The
corpus meristem cells under the tunica divide in a plane that adds bulk to the shoot
meristem. Generally there are two tunica layers and one corpus layer.
As a shoot grows, buds are laid down by shoot meristem in the axils of leaf
primordia. The repeating units of leaf and bud primordia are called phytomeres.
These buds are dormant meristems that are activated at some later time in
growth.
Stem tissues are produced from the same three derivative meristems as root
tissues are:
Protoderm is responsible for Epidermis
Ground Meristem differentiates into ground tissues
Procambium produces the vascular tissues
Coleus stem tip, l.s.
Shoot tip, xs
SEM of shoot tip
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The arrangement of derivative meristems is different in the stem from the root
meristem zone. In the shoot meristem, procambium forms a cylinder of cells with
ground meristem to the interior, and to the exterior of the procambium cylinder.
As expected, the protoderm is the outer layer of derivative meristem. Single
central strands of procambium extend out into the leaf primordia from the shoot
meristem. These are called leaf traces, and leave a vascular gap in the stem
tissue at those points.
The positioning of new leaf primordia, and hence branches, is regulated by the
inhibition effect of existing primordia so that leaf and branching patterns generally
spiral along the stem. Growth regulators in the shoot tip and leaf primordia are
responsible for the inhibition.
Stem Functions
• The stem is the axis of the shoot system that provides mechanical support
for and serves as the attachment site for leaves and reproductive
shoots.
• Stems elevate leaves for photosynthesis and position reproductive shoots
for optimal access to pollinators and dispersal agents.
• Stems conduct water and minerals from roots to the leaves and conduct
solutes from leaves to storage and use sites.
• Stems are responsible for the overall growth (height and girth) of the plant
from the primary and secondary shoot meristems
There is much variation in stem types and stem anatomy, although there are some
common features, which we will discuss. Most secondary growth of plant shoot
systems (increase in girth) occurs in stems. (Leaves are generally primary growth
structures.) Our discussion of stems will include both internal anatomy and
external features of primary and secondary growth in stems.
Primary Growth of Stems
There are three basic primary growth patterns in stems:
• Early growth of most herbaceous dicot stems and some woody dicots
• Early growth of some woody dicots
• Monocot stems
Most woody dicots
Herbaceous dicots
Monocots
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While the arrangement of tissues in stems differs from roots, the tissues are the
same, and by now, should be becoming familiar.
Typical Dicot Stem Primary Growth Patterns
Epidermis
• Cuticle for protection
• Some stomata for gas exchange
• Trichomes are common – glands, prickles and hairs
Cortex region
• Few layers of collenchyma for flexible strength
• Parenchyma layers
• Some sclerenchyma may be found in cortex, too
Pith
• The pith is comprised of parenchyma tissue with many intercellular spaces.
• Some plants may have a “hollow” pith. An endodermis layer forms interior
to the vascular tissue in these hollow-stemmed plants.
• Pith rays, extensions of pith between vascular bundles radiate from the pith
to the cortex. Pith rays are difficult to observe in plants that have a
cylinder of vascular tissue in primary growth.
Helianthus stem, xs
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Vascular Tissue
Pattern 1:
Ring of discrete vascular bundles with pith rays
• Vascular bundles may have a fiber bundle cap (which is called a phloem
fiber cap because it is adjacent to the phloem) or a sheath of
sclerenchyma surrounding the entire vascular bundle. Individual
bundles are separated from each other by pith rays, parenchyma
tissue that extends from the interior pith region of the stem to the
cortex.
• Primary Phloem is found toward the exterior of the vascular bundle
(rarely may have phloem on the inside of the xylem, too. Cucurbita
stem vascular tissue is an example of this)
• Primary Xylem is found toward the interior of the vascular bundle
• When there is secondary growth, there will be a layer of procambium
retained between the primary xylem and primary phloem. Such vascular
bundles are said to be open vascular bundles, because they can
proceed to secondary growth.
• Dicots that lack secondary growth have closed vascular bundles,
and no procambium remains between the primary xylem and primary
phloem.
Dicot open vascular bundle
Closed vascular bundle
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Pattern 2:
Cylinder of Vascular Tissue
Some dicots that have secondary growth, particularly woody plants, may
have a complete cylinder of vascular tissue in primary growth separating the
cortex from the pith. However, within the cylinder, whose interior and
exterior “borders” are typically formed by a layer of sclerenchyma cells, are
radiating rows of ground parenchyma that separate alternating xylem and
phloem vascular bundles”. It can be difficult to distinguish the bundles
because there are no bundle sheaths or fiber caps.
Primary stem in Tilia
There is a reasonable amount of variation in secondary growth in dicots.
Woody plants have extensive secondary growth. Most herbaceous dicots
have open bundles and exhibit some secondary growth, although much of the
secondary growth will be for support more than for conduction. The
transition in stem growth pattern from primary to secondary is discussed
with secondary growth.
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Monocot Stem Variations
Most monocots are reasonably small, herbaceous plants. Most monocots have no
secondary growth, even perennial monocots. There are some notable exceptions,
however, such as the palms. The common monocot families are the lily, grass and
orchid families.
Some distinctive monocot stem features:
• Monocot vascular bundles are closed. No procambium remains so generally
monocots have no cambium (no increase in girth)
• There may be 2 or 3 layers of sclerenchyma beneath the epidermis layer for
strength and support of the stem structure. Some parenchyma cells may
also develop thickened walls in monocot stems as they mature.
• Vascular bundles are "scattered" in appearance throughout the ground
parenchyma, so there is no distinction between cortex and pith. Vascular
bundles have an anastomosing, or weaving pattern in longitudinal section.
The parenchyma cells between vascular bundles are just referred to as
ground tissue. This is the third vascular bundle pattern common in primary
growth of stems.
• Most monocot vascular bundles contain two large xylem vessels toward the
interior of the bundle, and some smaller vessels between them. The very
first vessels formed usually collapse from stretching leaving an air space.
As a result the vascular bundle often takes on the appearance of a "clown
face". Phloem is always located toward the epidermis layer, and contains no
fibers. A sclerenchyma bundle sheath surrounds each vascular bundle.
Monocot stem, xs
Monocot vascular bundle
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Special Variations in some Monocots
Hollow Stems
A central pith cavity develops from cells of the ground tissue that are destroyed
during early growth. This gives an appearance of a ring of vascular bundles, similar
to dicot patterns.
Intercalary Meristems
Many grasses have meristem layers at the bases of nodes, which provide for nonapical growth and enlargement of cells throughout the plant stems. (Which is why
we have to mow lawns).
Thickened Meristems
Some monocots achieve great dimensions without secondary growth. Although we
will discuss this more when we discuss secondary growth and woody plants, one
way in which a plant can have a larger girth is to produce a meristem that
proliferates laterally just below the apical meristem. This thickened meristem is so
large that the true apical meristem with its leaf primordia appears sunken into this
area. The thickened meristem is procambium, which produces huge numbers of
vascular bundles within the stem. This region of proliferating procambium is known
as the meristematic cap.