Lesson 7 bio101 (c)Dr. Evangelista

39
Apical meristems

Transcript of Lesson 7 bio101 (c)Dr. Evangelista

Page 1: Lesson 7 bio101 (c)Dr.  Evangelista

Apical meristems

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Definitions

Apical meristem •  Synonyms root apex, shoot apex •  Promeristem – includes the initials •  Protomeristem/ metrameristem –

initials and most recent derivatives

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• a cell is an initial because of its position in the meristem

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Experimental evidence regarding existence of initials

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Apical meristem organization

Apical cell – origin of all tissues of the plant body

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Histogen theory • developed by Hanstein (1868, 1870) •  the main body of the plant arises from

the histogens – Dermatogen -the precursor of the

epidermis – Periblem -gives rise to the cortex – Plerome- constitutes the inner mass

of the axis.

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Histogen theory

• Assumes that the destinies of the different regions of the plant body are determined by the discrete origin of these regions in the apical meristem.

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Tunica- corpus organization • Tunica layer shows anticlinal divisions • Corpus –divides in various planes • Exhibited by angiosperms

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Cyto-histologic zonations • Distal axial zone –terminating the axis

• Proximal axial zone (inner zone )- appears directly below the distal zone; usually becomes the pith

• Peripheral zone or outer zone (flank meristem) –shows the smallest dimensions and densest cytoplasm; most meristematic

• Transition zone –like a cambium; actively dividing derivatives of the distal zone

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• Zonations based on: – Planes of division – Degree of meristematic activity – Cytologic and histologic differentiation

• Shown in the apex of gymnosperms

Cyto-histologic zonations

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Cycas type - without central mother cells zone

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Ginkgo type- with 5 meristematic zones

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Cryptomeria-Abies type- without transition zone

e.g. Cryptomeria japonica, Abies concolor

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Cyto-histologic zonations

• Observed also in many angiosperms

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Classification of meristems based on the number of initials

1. one initial – many vascular cryptogams e.g. Equisetum

2. several initials in one tier – most gymnosperms; in Gnetales tunica-corpus organization

3. several initials in more than one layer – most angiosperms

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Inactive center in the meristem • Waiting meristem • Quiescent center -Ultimate source of all other cells

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Origin of leaves

• Plastochron – time interval between two successive similar events

• plastochronic changes in size of shoot apex

-Minimal- area phase -Maximal –area phase

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Origin of branches

•  formed in close association with leaves; in the axils

•  initiated later than the leaves subtending them usually in the second plastochron

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Reproductive apex

• Modified shoot apex • Changes in shoot apex:

– Nature of foliar organ subtending the bud changes from foliar leaves to bracts

– the growth of the bud is earlier and more vigorous than subtending bract

– Rapid increase in internodes in plants that are rosette

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• Changes in shoot apex: – Shows uniform densely staining, small celled

mantle-like zone enclosing a lightly stained core

-Disappearance of the rib meristem

-Indistinguishable plastochronic rhythms

Reproductive apex

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Root apex •  subterminal in position •  no branches initiated at the apex -arise endogenously •  no lateral appendages comparable to

leaves •  do not undergo periodic changes in

structure

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Types of root apical meristem organization

single apical cell or from several initials in one tier (vascular cryptogams)

open type- lacks clear differentiation of initials

closed – characterized by the presence of 3 tiers of initials

1st tier – apex of the central cylinder 2nd tier – terminates the cortex 3rd tier – gives rise to the rootcap

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based on origin of epidermis •  rootcap and epidermis have common origin

– dermatocalyptrogen

•  root cap has independent initials – calyptrogen

• epidermis has its own initials –dermatogen

•  cortex and epidermis have common initials

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Functions of the cells in the quiescent center

• 1. reservoir of cells resistant to damage e g. radiation

• 2. site of auxin synthesis • 3. source of diploid cells • 4. permanent source of active initials

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