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Uploaded bySunita Sangwan
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2. Shoot Apical Meristem and theories of shoot apical meristem

The lecture by Dr. Sunita Sangwan discusses the significance of shoot apical meristem (SAM) in plant growth and development, highlighting its role in forming all above-ground tissues. It reviews various theories explaining the organization of SAM, such as the apical cell theory, histogen theory, tunica-corpus theory, and others, illustrating the complexities of meristematic zonation. Additionally, it classifies shoot apices into three types based on their meristematic structure: monoplex, simplex, and duplex types.

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Plant Tissue Lecture-2 Shoot Apical Meristem By: Dr. Sunita Sangwan Assistant Prof. Deptt. of Botany MNSGC Bhiwani
Introduction Apical meristem are present at terminal positon in plants. These are at the tips of root and shoot. They first appear in embryonic root and shoot and all the primary tissues are derived from this root or shoot apical meristem. These both apical meristem may differ from each other in their organization.
Shoot Apical Meristem (SAM) It is located at the terminal position of the shoot in contrast to sub terminal position of root apical meristem. It is dome shaped or somewhat flattened. The cells in this region have unlimited capacity of growth and divide continuously to form new tissues or organs. All the above ground tissues are originated from shoot apical meristem.
1. Apical Cell Theory 2. Histogen Theory 3. Tunica-Corpus Theory 4. Mantle Core Concept 5. Cytohistological Zonation Theories of Shoot apical meristem
Apical Cell Theory Nageli proposed the theory in 1878. It suggests the presence of single tetrahedral apical cell in the shoot apex and that cell is considered as the Structural and functional unit of apical meristem. The single apical cell divides to give rise to all the tissues of the shoot. This theory is applicable only to the cryptogams not to the phanerogams because it is proved experimentally that shoot apical meristem of gymnosperms and angiosperms consist of a group of cells. So this theory was discarded.
Histogen Theory This theory was postulated by Hanstein in 1868 by stating that three different meristematic zones which arise from the independent initial of the apical meristem. These layers are termed as histogens (layers). The three histogen are: This theory also discarded as these layers are not specific in their functions and these layers are not distinct in gymnosperms and angiosperms. • It is the outermost histogen and develop the epidermal tissue. Dermatogen: • It is middle histogen layer and give rise to cortical tissue. Periblem: • It is the innermost histogen and vascular cylinders are originated from it. Plerome:
A. Histogen Theory B. Tunica & Corpus Theory
Tunica-Corpus Theory Schmidt (1924) postulated the Tunica-corpus theory for shoot apex organisation. This theory was proposed based on the plane of cell division in the apex. According to this theory, there are two distinct layers in the shoot apex of angiosperms named tunica and corpus.
Tunica It is the outer zone and consist of one or more layers of cells that form outer region by anticlinal division. Therefore, tunica develops as a layer and not increase in thickness. However, in some species, variations in the number of tunica layers have been observed in the different stages of development of shoot apex. These variations may be due to the plastochron periodicity. The cells of tunica are smaller. Usually tunica develop the epidermis.
Corpus It is the inner zone of shoot apex and form the central part of stem by irregular divisions in all planes. Therefore, shoot apex increases in thickness. It is covered by tunica from outside. Cells of corpus are larger as compare to tunica. From Corpus cortex and vascular tissue has been originated.
The shoot apex of most of the gymnosperms does not possess tunica-corpus organization, because their shoot apex does not have a surface layer that divides anticlinally. The number of tunica layers varies highly in species and genera. Distinction between two layers is also not clear. Longitudinal Section of Shoot Apex: A. Two Tunica Layer and corpus, B. One Tunica Layer and corpus.
Mantle Core Concept To assist fluctuations in tunica and corpus, Popham and Chan (1950) proposed mantle core hypothesis. They separated the shoot apex into two zones but not based on cell division. Mantle consisted all the outer layers of the apex and tunica included only the anticlinally dividing layers. The inner mass of cells surrounded by mantle known as core. Mantle constitutes the dome-shaped outer layers of the apex, and the core represents the inner cell mass covered by the mantle. In this hypothesis, the term ‘mantle’ is used for tunica and the ‘core’ is used for corpus.
Fig. 1.6: Shoot Apex A. Mantle core concept, B. Cytohistological Zonation
Cytohistological Zonation The shoot apex organisation of gymnosperms cannot be explained by tunica corpus theory because gymnosperms do not possess layer of cells that divide anticlinally. Foster (1939) divided the shoot apex of Ginkgo biloba in different zones based upon cytological characteristics and staining. These zones are related with the growth processes in the shoot apex. The four growth zones recognised by him are as below: Apical initial group Central mother cell zone Rib Meristem Peripheral Zone (flank Zone)
Cytohistological Zonation 1. Apical initial group: This zone includes the group of initials present at the apical surface and their lateral derivatives. 2. Central mother cell zone: This cell lies just below the apical initial group derived only from it. The cells of the distal zone show low mitotic activity and have large number of vacuoles and due to which these stained lightly. Central mother cells have thick primary cell wall and pitted. 3. Rib Meristem: It consists derivatives of central mother cells situated in centre. These derived cells divide transversely to form vertical files. This region is also known as pith or file meristem as pith is formed in this zone.
Cytohistological Zonation 4. Peripheral Zone (flank Zone): This zone arise partly from the lateral derivatives of the apical initials and partly from the central mother cells. The cells of this zone are densely stained and mitotically active and results in. Localized activity of this zone result in the formation of leaf primordial. The periclinal divisions results in shoot elongation. Anticlinal divisions increase the width of the shoot. • These four zones are morphologically and biochemically differentiated. This type of zonation has been observed in many angiosperms. The shoot apices of angiosperm consist of cytohistological zonation along with tunica corpus zonation.
In addition to above four zones, Popham and Chan proposed the fifth zone as explained below: Cambium like zone: It is cup shaped zone and lies between the central mother cell and rib and flank meristem. It present below the youngest leaf primordium. It is not found universally.
Types of Shoot Apices Newman (1965) proposed this concept. He gave the term continuing meristematic residue for a group of permanent initial cells in the shoot apex. He classified shoot apex into three types based on the form this meristem.
Types of Shoot Apices Monoplex Type: It is present in cryptogams, ferns and related genera. In this type of shoot apex meristem is the only outer layer that is pointed inwards and a single division is required for growth. Simplex Type: It is commonly present in gymnosperms; it comprise of one or more initial cells arranged in a single superficial layer. These cells divide both anticlinally and periclinally. Duplex Type: It is present in the shoot apex of angiosperms; it consists of at least two continuing meristematic layers of cells. The cells of superficial layer divide anticlinally and that of inner layer divide both anticlinally and periclinally.
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