1. Plant Anatomy by Dr. Intedhar Abbas

2. Meristematic tissues
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8. Several theories shoot apical meristem
1- Apical cell theory
This theory proposed by Nagli According this theory, a single apical cell is the structural and functional unit apical meristem which governs the entire process of apical growth. Such organization has been found only in lower plants like algae, ferns.
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9. 2- Histogen theory
Hanstein postulated this theory revealing that three distinct meristematic zone (layer) are found in apical region. Each zone consists of a variable number of layers called histogen or tissue builder.
A - Dermatogen
The outermost uniseriate layer. Dermatogen cells divide anticlinally and develop into one layer epidermis.
B - Periblem
The middle region composed of isodiametric cell, forms the cortex.
C - Plerome
The central or inner mass, forms the stele (vascular tissues, pericycle, pith, rays).
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10. 2- Histogen theory
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11. 3- Tunica-corpus theory
Schmidt 1924 proposed tunica – corpus theory to explain shoot apex organization.
According to this theory, there are two zone in apical meristem.
a- Tunica: the outer zone consisting of one or more peripheral layers of cells, forming the outer region by anticlinal divisions.
b- Corpus: the central undifferentiated multilayered mass of cells surrounded by tunica which forms the central part of shoot by irregular divisions.
The tunica cells are smaller and corpus cells are larger. The number of tunica layers may vary even in same species due to influence of seasonal growth changes.
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12. 3- Tunica-corpus theory
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13. 4- Cytohistological zonation(Growth of zones)
4- Cytohistological zonation(Growth of zones)
It was found that in gymnosperms, tunica like layer is not found which led Foster 1939 to divide shoot apex organization of ginkgo biloba on basis of cytohistological zonation, which react differently to staining. He recognized four inter related zones.
a- Apical initial zone
The outermost layer of the apical meristem undergoes periclinal and anticlinal divisions and contributes cells to the peripheral and interior tissues of the shoot.
b- Central mother cell
The zone located under Apical initial zone, its divided to peripheral cells are actively, and central is slower, peripheral cells form Peripheral layer, and the center is the rib meristem and the cells of this area are small and strongly pigmented.
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14. 4- Cytohistological zonation(Growth of zones)
c- Peripheral layer
Encircles the other zones. The peripheral zone typically is the most meristematic of all three zones and has the densest protoplasts and the smallest cell dimensions. It may be described as a eumeristem. Leaf primordia and the procambium arise here, as well as the cortical groundTissue.
d- Rib meristem
Appears directly below the central mother zone and is centrally located in the apex. It usually becomes the pith after additional meristematic activity has occurred.
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15. شكرا لحسن الاصغاء
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