Lesson 7 bio101 (c)Dr. Evangelista
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Transcript of Lesson 7 bio101 (c)Dr. Evangelista
Apical meristems
Definitions
Apical meristem • Synonyms root apex, shoot apex • Promeristem – includes the initials • Protomeristem/ metrameristem –
initials and most recent derivatives
• a cell is an initial because of its position in the meristem
Experimental evidence regarding existence of initials
Apical meristem organization
Apical cell – origin of all tissues of the plant body
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.
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.
Tunica- corpus organization • Tunica layer shows anticlinal divisions • Corpus –divides in various planes • Exhibited by angiosperms
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
• Zonations based on: – Planes of division – Degree of meristematic activity – Cytologic and histologic differentiation
• Shown in the apex of gymnosperms
Cyto-histologic zonations
Cycas type - without central mother cells zone
Ginkgo type- with 5 meristematic zones
Cryptomeria-Abies type- without transition zone
e.g. Cryptomeria japonica, Abies concolor
Cyto-histologic zonations
• Observed also in many angiosperms
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
Inactive center in the meristem • Waiting meristem • Quiescent center -Ultimate source of all other cells
Origin of leaves
• Plastochron – time interval between two successive similar events
• plastochronic changes in size of shoot apex
-Minimal- area phase -Maximal –area phase
Origin of branches
• formed in close association with leaves; in the axils
• initiated later than the leaves subtending them usually in the second plastochron
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
• 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
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
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
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
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