10/6/2011
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 – WtWater
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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 • Leaf primordia – Develop into leaves • Bud primordia – Develop into lateral shoots
Protects apical meristem
RtditRepeated units
• Shoot apical meristem – Tunica-corpus organization • Tunica consists of the outermost layer or layers of cells that divide anticlinally – Perpendicular to the surface of meristem • Corpus consists of a body of cells that lie beneath tunica layers. – Initials of corpus occur beneath the tunica and add cells to the corpus by dividing periclinally • Parallel with the apical surface
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• Number of tunica layers varies from species to species. • Angiosperms have apices consisting of three superimposed layers: – L1, L2, L3 – Outermost = L1 – Innermost = L3
Peripheral zone
Central Zone
Coleus blumei
Two layered tunica
Initial layer of corpus is L3.
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• Central zone – Mitotically quiescent • Peripheral zone – Mitoti ca lly very ac tive
http://www.trainingreference.co.uk/free_pictures/gallery_3/horse_chestnut.jpg
Young shoots tightly packed in buds; protected by bud scales.
Bud scales: highly modified leaves initiated late in the previous growing season.
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Buds open to reveal oldest rudimentary leaves.
Internodal elongation has separated the nodes from one another.
Terminal bud is a mixed bud, containing both leaves and flowers.
Lateral buds ppyroduce only leaves.
http://farm2.static.flickr.com/1142/1470553921_3 c4422524c_o.jpg
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• 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 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 meristem. • 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 tissue.
• In others, the primary vascular tissues develop as a cylinder of discrete strands, or bundles, separated from one another by ground tissue.
• In stems of most monocots and some herbaceous eudicots, the arrangement is more complex. Vascular bundles appear scattered throughout the ground tissue.
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• Tilia americana – Vascular cylinder is composed of vascular bundles that are separated from one another byyp inconspicuous re gggions of ground parenchyma call interfascicular regions.
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• Sambucus canadensis – The interfascicular regions pith rays are relatively wide, hence the procambial strands and ppyrimary vascular bundles form a s ystem of discrete strands around the pith. Very similar to Tilia
Young stem, showing protoderm, ground meristem, and three discrete procambial strands.
Mature sieve element
Primary tissues farther along in development
9 10/6/2011
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. – GthiiltGrowth similar to SbSambucus.
Very discrete
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• Ranunculus (buttercup): herbaceousness – Vascular bundle resembles many monocots – VB’s retain no procambium after primary vascular tissues mature • Hence, bundles never develop a vascular cambium • Lose potential for future growth
Bundles are closed; All procambial cells mature precluding secondary growth.
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• Zea mays – Exemplifies stems of monocots. – Vascular 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 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) • Three of more leaves at each node – Whorled phyllotaxy – Culver’s root (Veronicastrum virginicum)
Helical
Whorled
Opposite
Helical
Morphology & Structure of the Leaf
• Blade or lamina – The expanded portion • Petiole – Stalklike portion • Simple leaves – Blades not divided into distinct parts • May be deeply lobed • Compound leaves – Divided into leaflets – each with own petiole –Two types • Pinnately & Palmately • Pinnately – leaflets arise from either side of an axis rachis
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• Distinguishing leaflets from leaves. • 1. buds are found in the axils of leaves – simple and compound – but not in leaflets.
• 2. leaves extend from the stem in various planes, whereas leaflets of a given leaf all lie in the same plane.
Mesophyll
• Mesophyll – Ground tissue of the leaf – Palisade parenchyma – Spongy parenchyma
• Where photosynthesis takes place – Chloroplasts •Thylakoids
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• VB’s found throughout mesophyll – Known as veins in leaves • Venation – Netted or reticulate • Veins arranged in branching pattern with successively smaller veins branching from larger ones. • Angiosperms – Parallel • Veins that extend along the long axis of the leaf • Monocots
Sun vs. Shade leaves
• Light can have substantial effects on leaf development. – Sun leaves • Grown under high light intensities – Shade leaves • Develop under low light intensities
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• Quercus rubra sun vs shade leaves
Sinuses very shallow Petioles rarely reddish
• Sun leaves thicker • Internal surface area of the mesophyll is higher in sun leaves • ShdlShade leaves no tdt adap tdthihlihtted to high light intensities – Lower maximum photosynthetic rates
Sun Leaf
Shade Leaf
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Leaf Abscission
• Separation of leaf from stem – Occurs near base of petiole – Creates an abscission zone
• After separation occurs (i.e., leaf falls) the protective layer is recognized as a leaf scar on the stem.
Overview
• Shoot apical meristem produces leaf primordia, bud primordia, and stem primary tissues. • Three basic types of organization exist in the primary structure of stems. • Leaves and stems are closely elated physically and developmentally. • Leaf structure variation is related to habitat. • Leaves exhibit determinate growth, stems exhibit indeterminate growth. • Separation of a leaf from a branch by abscission is a complex process. • A flower is a determinate stem tip bearing modified leaves. • Stems may serve food-storage or water-storage functions.
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