10/6/2011 1 Shoot Primary Growth Ch 25 Outline • Origin & growth of primary stem tissues • Primary structure of stem • Relation between vascular tissues of the stem & leaf • Morphology and structure of the leaf • Grass leaves • Development of the leaf • Leaf abscission • Transition between the Vascular Systems of the Root & Shoot • Development of the flower • Stem and Leaf modifications Primary Shoot functions • Support • Conduction – Food Wt – Water
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10/6/2011
1
Shoot Primary Growth
Ch 25
Outline• Origin & growth of primary stem tissues• Primary structure of stem• Relation between vascular tissues of the stem & leaf• Morphology and structure of the leaf• Grass leaves• Development of the leaf• Leaf abscission• Transition between the Vascular Systems of the Root
& Shoot• Development of the flower• Stem and Leaf modifications
Primary Shoot functions
• Support• Conduction
– Food W t– Water
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Origin and Growth of the Primary Tissues of the Stem
• Apical meristem adds cells to the primary plant body– Also, repetitively produces leaf primordia and
bud primordia– Results in repeated units called phytomeres
• Primary stem tissues go through periods of growth similar to the root– Stem cannot be divided along its axis into regions of
cell division, elongation, and maturation like roots.• Shoot gives rise to leaf primordia in such rapidShoot gives rise to leaf primordia in such rapid
succession that nodes & internodes can not be distinguished at first.
• Thus, increase in length of stem occurs by intrenodal elongation:– This may occur simultaneously over several
internodes.
• Apical meristem of shoot gives rise to same primary meristems found in root:– Protoderm, procambium, and ground
meristemmeristem.• These meristems develop into the mature
tissues of the primary plant body:– Epidermis, primary vascular tissues, and
ground tissue.
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Primary structure of the stem
• In seed plants other than monocots, the vascular system of the internode appears as more or less continuous cylinder within the ground tissuethe ground tissue.
• In others, the primary vascular tissues develop as a cylinder of discrete strands, or bundles, separated from one another by ground tissueground tissue.
• In stems of most monocots and some herbaceous eudicots, the arrangement is more complex. Vascular bundles appear scattered throughout the ground tissuescattered throughout the ground tissue.
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• Tilia americana– Vascular cylinder is composed of vascular
bundles that are separated from one another by inconspicuous regions of ground y p g gparenchyma call interfascicular regions.
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• Sambucus canadensis– The interfascicular regions pith rays are
relatively wide, hence the procambial strands and primary vascular bundles form a system p y yof discrete strands around the pith. Very similar to Tilia
Young stem, showing protoderm, ground meristem, and three discrete procambialstrands.
Mature sieve element
Primary tissues farther along in development
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Stem near completion of primary growth.
Cambia not formed yet!!!
Will become woody.
Sambucus has wide interfascicular regions—great portion of vascular cambium develops from interfascicular parenchyma
Herbaceous stems
• Undergo little or no secondary growth.• Medicago sativa (alfalfa) exhibits some
secondary growth.G th i il t S b– Growth similar to Sambucus.
Very discrete
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• Ranunculus (buttercup): herbaceousness– Vascular bundle resembles many monocots– VB’s retain no procambium after primary
vascular tissues maturevascular tissues mature• Hence, bundles never develop a vascular
cambium • Lose potential for future growth
Bundles are closed; All procambial cells mature precluding secondaryprecluding secondary growth.
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• Zea mays – Exemplifies stems of
monocots. – Vascular bundlesVascular bundles
appear scattered throughout the ground tissue in transverse section.
– VB’s are closed
Mature vascular bundle surrounded by a sheath of thick-walled sclerenchyma cells.
Relation between Vascular Tissues and Stem and Leaf
• Extensions from the vascular system in the stem toward the leaves – Leaf traces
• The wide interfascicular regions or gaps of ground tissue in the vascular cylinder located above the level where leaf traces diverge toward the leaveswhere leaf traces diverge toward the leaves– Leaf trace gaps
• Leaf trace extends from its connection with a bundle in the stem (stem bundle) to the level which it enters the leaf.
• Buds commonly develop in the axils of leaves, and their vascular system is connected with the main stem by branch traces
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Leaf arrangement• Phyllotaxy
– Arrangement of leaves on the stem• Most common helical or spiral.
– Quercus & Morus• Other plants have a single leaf at each node
– i.e, grasses –phyllotaxy is distichous• When leaves are formed in pairs at each
node – Phyllotaxy = opposite (Acer & Lonicera)
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• If each pair is at a right angle to previous pair:– Decussate phyllotaxy
Lamiaceae (Coleus)– Lamiaceae (Coleus)• Three of more leaves at each node