Shoot Primary Growth - Christian Brothers jmoore25/BotanyPPT/Shoot Primary Growth.pdf ·...
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Shoot Primary Growth
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
Food W t Water
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
R t d itRepeated 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
Number of tunica layers varies from species to species.
Angiosperms have apices consisting of three superimposed layers:three superimposed layers: L1, L2, L3 Outermost = L1 Innermost = L3
Two layered tunica
Initial layer of
Initial layer of corpus is L3.
Central zone Mitotically quiescent
Peripheral zoneMit ti ll ti Mitotically very active
Young shoots tightly packed in buds; protected by bud scales.
Bud scales: highly modified leaves initiated late in the previous growing season.
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 produce only
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
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
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.
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.
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
Stem near completion of primary growth.
Cambia not formed yet!!!
Will become woody.
Sambucus has wide interfascicular regionsgreat portion of vascular cambium develops from interfascicular parenchyma
Undergo little or no secondary growth. Medicago sativa (alfalfa) exhibits some
secondary growth.G th i il t S b Growth similar to Sambucus.
Ranunculus (buttercup): herbaceousness Vascular bundle resembles many monocots VBs 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.
Zea mays Exemplifies stems of
monocots. Vascular bundlesVascular bundles
appear scattered throughout the ground tissue in transverse section.
VBs 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
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)
If each pair is at a right angle to previous pair: Decussate phyllotaxy
Lamiaceae (Coleus) Lamiaceae (Coleus) Three of more leaves at each node
Whorled phyllotaxy Culvers root (Veronicastrum virginicum)
Morphology & Structure of the Leaf
Blade or lamina The expanded portion
Petiole Stalklike portion
Simple leavesSimple 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
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 Ground tissue of the leaf Palisade parenchyma
Spongy parenchyma Spongy parenchyma
Where photosynthesis takes place Chloroplasts
VBs found throughout mesophyll Known as veins in leaves
VenationNetted or reticulate Netted or reticulate
Veins arranged in branching pattern with successively smaller veins branching from larger ones.
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 Grown under high light intensities Shade leaves
Develop under low light intensities
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 leavesSh d l t d t d t hi h li ht Shade leaves not adapted to high light intensities Lower maximum photosynthetic rates
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.