Transcript document

THE STEM
STRUCTURE & FUNCTIONS
General Terminology
• Annual – a plant that completes its life cycle
(seed, flower, fruit and seed) in one growing
season.
• Ex. Crop plants
• Biennial – a plant that requires two growing
seasons (2 years) to complete its life cycle.
• Ex. Asparagus
– First year is vegetative growth (non-reproductive part)
– Second years growth is reproductive – flowers and
fruits
General Terminology
• Perennial – longer lived vegetative plants,
continue to live after flowering and fruiting
reproduce for many years (seasons).
– Examples
• Trees, shrubs, many flowering plants: impatiens,
periwinkle, dianthus and house plants.
• Stem – the part of the axis of vascular
plants that is above ground, as well as
anatomically similar portions below ground
– rhizomes and corms.
FUNCTIONS
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•
•
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Transport of water & nutrients to the top
Support
Storage
Photosynthesis (sometimes)
Development
• Primary growth
– From the shot apical meristem
primary tissues & increased stem length,
origin of the leaves
• Secondary growth (mostly in woody plants; it is limited in
herbaceous plants
– From the vascular cambium
secondary
tissues & increased diameter
– From the cork cambium
periderm
Development
• Apical meristem at the tip
– From apical meristem you have development
of the leaf primordia. As the shoot continues
to grow the bud primordia develop in the leaf
axil. (These will develop into other shoots
and leaves)
– As the leaf and bud are formed a strand of
xylem and phloem known as the leaf trace,
branches off the main vascular cylinder and
goes into the stem.
TISSUE PATTERNS
• Stele – a central cylinder of vascular tissue
(inside of the cortex)
• Several different arrangements
TISSUE PATTERNS
– Protostele – Most primitive form, consisting of
a solid core of conducting tissues, with the
phloem surrounding the xylem. Found in
many of the extinct vascular plants as well as
in many of the living seedless plants.
xylem
phloem
TISSUE PATTERNS
• Siphonostele – more derived condition,
tubular arrangement with pith in the center
surrounded by the vascular tissue. Found
in most ferns and many gynosperms and
angiosprems.
Pith
Xylem
Phloem
TISSUE PATTERNS
• Eustele – Primary vascular tissues are
arranged in different vascular bundles
around the pith. Vascular bundles are
strands of tissue comprised of xylem and
phloem and procambium.
Vascular
Bundle
Monocot vs. Dicot
• Monocot – Plants that produce one
cotyledon, or seed leaf. (Ex. grasses,
wheat, palms, barley)
• Dicot – Plants that produce two cotyledon.
• Cotyledon – “seed leaf”, the first leaf
formed in a seed.
Monocots
• Outer layer of epidermis, with an underlying
layer of dead sclerenchyma tissues (fibers)
• Vascular bundles scattered throughout the stem,
with a higher concentration area the outer
perimeter. Which also serves to strengthen the
stem.
• The remaining portions of the stem is composed
of parenchyma tissues, or ground tissues.
Cross Section of A Monocot Stem
Stem x.s. of Asparagus
Cross Section of A Monocot Stem
Stem X. S. of Zea mays (corn)
Vascular Bundle
Air Space
Center
Vessels and
tracheids
Xylem
Phloem
Vessels
Seive tube
elements and
companion cells
Outside
Dicots
• Outer epidermis, with the underlying
ground tissues (collenchyma, parenchyma
and sclerenchyma) divided into a cortex
and pith.
• Vascular bundles arranged in a ring
underneath the cortex of the stem. With
ground tissues between the bundles that
are continuous between the pith and
cortex – interfascicular regions.
Cross Section of A Dicot
Stem
Cortex
Interfascicular
regions
Pith
Stem X.S. of Phaseolus vulgaris
Activity of the Vascular Cambium Results in
Addition of Secondary Tissues
Secondary Growth of The Stem
As in roots, the activity of the
vascular cambium in the stem
gives rise to xylem towards
the center and phloem
towards outside. As new cells
are added the diameter of the
stem increases and the cortex
and the epidermis are
crushed and replaced.
The Vascular System of A Plant
From The Root to The Stem
Stem
Root
Secondary Growth of The Stem
• Formation of secondary vascular tissue
– Secondary xylem & phloem
• Periderm
Development
• Secondary growth (mostly in woody plants; it is limited in
herbaceous plants
– From the vascular cambium
secondary
tissues (xylem and phloem & increased
diameter
– From the cork cambium
periderm
Growth and Wood types
Growth rings – growth layers which correspond to
growing seasons.
Early wood – “spring wood”, abundance of rain and
cooler temperature which allow cells to grow larger
and more rapid.
Late wood – “summer wood”, less than optimal growing
conditions, growth slows.
Sapwood – wood found between vascular cambium
and heartwood, functions in the transport of water and
solutes.
Heartwood – wood found in the center of the tree trunk,
usually darker because of resins, oils and gums.
Doesn’t transport water or solutes.
Stem Structures
• Apical or terminal bud (tip), extends the length of the
stem.
• Lateral or axillary buds – develop the shoots or
branches and flowers from the side.
• Bud scale – terminal bud produces terminal bud scales
which protect buds from temperatures, desiccation and
pathogens.
• Leaf scars – scar left on the twig when a leaf falls off.
• Axil – upper angle between a twig or leaf and the stem
from which it grows.
• Axillary Bud – Bud that grows from the axil.
Stem Modifications
• Rhizomes – horizontal, below ground stems,
often near the surface with adventitious roots.
(Irises, and grasses)
• Tuber – fleshy underground stem, typically used
for the storage of starches.
• Bulb – large buds with small conical stems
surrounded by large fleshy leaves.
• Corms – Similar to bulbs, but differ in that they
are composed mainly of stem materials.
Stem Modifications