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Lesson 7 bio101 (c)Dr. Evangelista

The document discusses apical meristems, which are found at the tips of roots and shoots. It defines key terms like initials, promeristem, and protomeristem. It describes the histogen theory of plant development and different zonation patterns observed in meristems. It also covers the origin of leaves and branches from the shoot apical meristem as well as differences between shoot, root, and reproductive meristems. The quiescent center is identified as an important source of new initial cells in the meristem.

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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
Cyto-histologic zonations •  Zonations based on: – Planes of division – Degree of meristematic activity – Cytologic and histologic differentiation •  Shown in the apex of gymnosperms
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 •  bserved also in many angiosperms O
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
Reproductive apex •  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
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
Lesson 7 bio101 (c)Dr. Evangelista
Lesson 7 bio101 (c)Dr. Evangelista
Lesson 7 bio101 (c)Dr. Evangelista