Shoot apex organization and its theories

Shoot apex organization and its theories

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  SHOOTAPEX ORGANIZATION Apical meristems- shootapex in Pteridophytes, Gymnosperms and Angiosperms, theories By, Ms. MEGHASHREE A M MSc. KSET (Ph. D.) Assistant Professor PG Department of Botany JSSCACS, Mysuru
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  Apical meristem: • Apicalmeristems are completely undifferentiated meristems in plant. • It includes the meristematic initials and their derivatives at the apex of the shoot or root. • There are two types of apical meristematic tissues: 1. Shoot Apical Meristem (SAM): Shoot apical meristem gives rise to organs like the leaves and flowers. 2. Root Meristem (RM): Root meristem produces the meristematic cells for further root growth.
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  • The meristemlocated at the apices of the shoot are termed as shoot apical meristem. • The shoot produces cells towards the axis. • The shoot meristem is terminal in position • The shoot apical meristem forms the lateral appendages to form leaves and branches. • The shoot apex show periodic changes in shape and size. • The growth of the shoot is not uniform because the internodes elongate much more than the nodes.
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  Tissue zones inthe shoot apex: • Protoderm: Protoderm gives rise to the epidermis of the plant. • Procambium: Procambium gives rise to the primary vascular tissue (xylem and phloem). • Ground meristem (fundamental meristem): It give rise to the cortex, endodermis, pericycle, medulla and pith region.
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  Organisation of ShootApicalmeristem Several theories have been putforth from time to time to explain mode of growth in shoot apical meristem. Apical cell theory Histogen theory Tunica corpus theory
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  Apical cell theory: •Apical cell theory was the first theory to explain the apical organization of the plants. • This theory was proposed by Nageli in 1858. • The apical cell theory says that “a single apical cell constitutes the growth point in most of the cryptogams, this single cell is called apical cell. • But this has been found only in cryptogams (Bryophytes and pteridophytes) but not in phanerogams.
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  Histogen Theory: • TheHistogen theory was proposed by J. Hanstein in 1870. • The Histogen theory is based on the two considerations: 1. The plant body does not originate from a single cell but from a mass of meristematic cells. 2. The meristematic cells consists of three distinct zones called histogens. 1. Dermatogen: the outermost layer which give rise to epidermis. 2. Periblem: located inner to the dermatogen, consists of few layers of cells that give rise to the cortex. 3. Plerome: The central core of cells which give rise to vascular tissues and endodermis. • According to this theory, each plant part originates from a group of initials.
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  Tunica-corpus theory: • Tunica-corpustheory was proposed by Schmidt in 1924. • Tunica corpus theory was given for vegetative shoot apex. • According to this theory, there are two zones of tissues in the apical meristems. • The tunica (Tunic = cover) consisting of one or more layers of peripheral layers of cells, and the corpus (corpus = body) a mass of cells enclosed by the tunica. • The layers of tunica show anticlinal (perpendicular to periphery) divisions and bring about surface growth. • In the corpus, cell division is irregular and at various planes resulting in growth and volume of the mass. • Tunica gives rise to epidermis. • Corpus gives rise to Cortex/ground tissue.
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  REGIONS OFTHE SHOOTMERISTEM: • CZ - Central zone  Center of shoot meristem; low rates of cell division; cells small, cells remain undifferentiated.  Can initiate any organ • PZ - Peripheral zone  Surrounds the CZ; rapid rates of cell division; cells can differentiate.  Positional information in the PZ triggers differentiation into primordia.  Once differentiation has begun, cell fate is determined.  In other words, cells in the PZ might only be able to differentiate into leaf cells.
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  • OZ-Organ zone Site at which organ primordia (eg. leaf, flower) become distinct.  As cell division continues, the older PZ cells begin to develop into discrete primordia (eg. leaf primordia), and are now referred to as OZ cells • RM - Rib meristem  Gives rise to vascular and interior stem structures.
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  Shoot apex organizationin Pteridophytes 1. Fern type (Single apical cell) 2. Transition type (Two to Five apical initials) 3. Lycopodium type (Shoot apices with three zones) Shoot apex organization in Gymnosperms 1. Ginkgo type 2. Abies- Cryptomeria type Shoot apex organization in Angiosperms 1. Opuntia type 2. Usual Angiosperm type
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  1.Fern type • Pteridophyteshaving a single apical cell located at the summit of the shoot apex. • The apical cell may be two-sided, both sides being convex, or it may be an inverted three or four-sided pyramidal cell. • For example, two convex sides apical cell are seen in Selaginella hortensis, S. galeotii, Azolla sp., Salvinia natans. • Three-sided apical cell observed in Equisetum arvense, Marsilea vestita, Psilotum sp.
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  • New cellsare formed from the apical cell by sequence of divisions. • Each division of the apical cell results in two dissimilar cells, one of which enlarges and assumes the characteristic features and size of the original apical cell. • Anticlinal and then periclinal cell divisions occur in the other cell. • The cells thus formed later differentiate into the tissues of the shoot.
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  • Exception inSelaginella martensis Early sporeling stage – 4 sided pyramidal cell and older shoots show 3-sided pyramidal cell.
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  2.Transition/ tentative type •The second category includes a group of Pteridophytes such as Selaginella wallichii (2), S. grandis (2), Marattia fraxinea (4) whose shoot apices exhibit two to five apical cells. • These initials usually prismatic in form, not pyramidal, divide anticlinally and occasionally periclinally. • There is no definite apical cell.
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  • New cellsdivide anticlinally and periclinally and from these cells the tissues of the shoot are differentiate. • Selaginella kraussiana (Wand, 1914) exhibits both Type I and Type II organization in the same plant and therefore, in a sense, represents a transitional form. • The main shoot reportedly has four-sided pyramidal cells at the shoot apex while the lateral shoots have three-sided pyramidal cell at the apex.
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  3.Lycopodium type • Pteridophytesspecies belonging to this category have shoot apices which exhibit three zones. Zone 1, the surface meristem, • Unstratified surface cap of cells dividing by anticlinal, periclinal, and slanting walls. • The cells differ in size, shape, and dimensions. • The surface cap of cells appears very irregular. • It gives rise to epidermis, mother cell for zone 1 & 2.
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  • Zone 2,the central meristem. • At the junction of the two zones, periclinal derivatives lie in vertical tiers under the surface cells. • The basal cells of the tiers divide by periclinal, anticlinal, and slanting walls resulting in a mass of irregularly arranged cells. • Most of the enlarging cells of the zone become highly vacuolated and account for rapid growth in diameter very close to the shoot tip. • Cells of Zone 2 differentiate into the central tissues of the shoot, i.e., in Lycopodium, the stele.
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  Zone 3, theperipheral meristem, • Derives new cells from two sources (1) oblique dividing cells from zone 2 (2) Anticlinal divisions of cells in the zone 1. • In the peripheral meristem cells are smaller than those of Zone 2 and are often elongated. • Example: Lycopodium selago, L. alpinum, Isoetes japonica, Selaginella arborescens • Exception: Cycas revoluta
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  TYPE I
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  4.Ginkgo type 1.S -Surface meristem. 2.MO - Central mother cell. 3. C - Cambium like zone. 4.CM - Central meristem. 5.P - Peripheral meristem. Example- Ginkgo biloba Zamia integrifolia Microcycas calocoma
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  Z1: Surface meristem •It consists of a well defined surface layer and divide periclinally, forms tiers of cells • In Zamia, additional tiers may be formed by the anticlinal division of an cell of previous tier Z2: A group of mother cells • It lies in the central position directly below Z1. • The cells are irregularly arranged, polygonal cells of the zone is larger. • The polygonal cells have a very huge nuclei, much vacuolated cytoplasm and often much thickened walls particularly at the corners of the cell Z3: A Cambium like zone • In this zone, cells form a cup shaped transition between Z2 and the zones lying below. • The narrow zone is characterized by relatively frequent cell divisions in plane at right angle.
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  Z4: The centralmeristem • The central meristem constituted pith initials or rib meristem or both, lies below the central portion of Z3. • The cells are more frequently arranged in continuous rows, cell elongation and enlargement is rapid. • Pith differentiates from the rib meristems or directly from pith initials. Z5: The Peripheral meristem • It forms a cylinder lying subjacent to the edge of Z3. • The epidermis, cortex, foliar primordia, pro- cambial ring and in some cases outer pith cells differentiate from the cells of the peripheral meristem
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  5.Abies- Cryptomeria type Thereare 4 distinct zones 1.S- Surface meristem. 2.SA - Subapical initials. 3.CM - Central meristem. 4.P - Peripheral meristem. Ex. - Sequoia gigantea
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  Z1: The surfacemeristem • It is a well defined layer of cells apparently uniform in thickness. • Periclinal division may be limited to or near the apical initial at the summit, except at loci of leaf and bud initiation, the periclinally dividing cells contribute to the subapical initial. • Frequent anticlinal divisions occur at the surface layer both at the summit and along the flanks Z2: A group of Subapical initials • Vertical and oblique division occurs but horizontal division are probably more common. • Cell division in periphery of the zone account for growth in length of peripheral zone, and basal derivatives contribute to the growth in the length of central meristem
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  Z3: The centralmeristem • It differentiate directly from the subapical initials thereby no intervening zone of cambium like cells • Cells of this zone account for the growth in length of the rib meristem • The highly vacuolated enlarging cells of the rib meristem differentiates into pith. Z4: The peripheral meristem • It forms a cylinder of elongating and dividing cells surrounding the central meristem zone. • Division occur in various planes but the cells often appears to be arranged in well defined layers due to higher frequency of anticlinal divisions. • The cells later differentiate into pro-cambial and cortical tissue and are responsible for initiation of lateral meristem
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  • In Chrysanthamum,presence of a cambium like zone occurs in the mid phase and is absent in the late phase of the plastochrone • Type IV organization is present in apices leading to shoots while type V organization occurs in apices of lateral branches
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  6.Opuntia type The Opuntiatype of shoot apex organization is seen only in angiosperms. It consist of 5 zones, 1. Mantle 2. Central mother cell 3. Cambium like zone 4. Central meristem 5. Peripheral meristem. Z1 Mantle : • It forms a cap of one or more discrete layer of cells over the summit. • In the surface layer division of the cells are of one type only i.e, anticlinal divisions. • If more than one layer of cells present, the anticlinal division takes progression towards the periclinal division.
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  Z2 Central mothercells It is composed of relatively small number of large closed, appressed , irregular shaped cells. • In species like Trichocerus and Opuntia, only one mantle layer occurs and this zone perpetuates itself independently and forms a zone of initiation for entire shoot, except epidermis. • Transverse and obliquely dividing cells along the inner boundary of the zone contribute new cells i.e, cambium like cells.
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  Z3 Cambium likezone: The cells of this zone are meristematic and are reminiscent of stelar cambium cells. New cells are contributed by central mother cell. Sometimes it may be through both the zone 1 &2 . Cells formed in cambium like zone accounts for the growth in length of central as well as peripheral meristem. Z4 Central meristem: The cambium like cells divide transversely and give rise to this zone. Transverse division of cells resulting in the formation of pith (Opuntia) Or into pith and medullary procambial strands (Phoenix and Trichocerus).
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  Z5 Peripheral meristem: Itforms a cylinder lying within the protoderm and around the central core of cells. Cells of this zone are usually smaller than that of zone 4. They divide both anticlinally and periclinally. Transverse division predominates resulting in the stratified layer of tissues. This zone gives rise to cortex, foliar organs etc. Ex : Opuntia cylindrical. Trichocerus spachianum Livistona chinesis Phoenix canariensis Phoenix dactylifera.
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  7. Usual angiospermtype There are 4 distinct zones in this type. 1. Mantle 2. Sub apical initials 3. The central meristem 4. The peripheral meristem. Z1 Mantle : It consists of at least one discrete, independent & self perpetuating surface layer whose cells divide exclusively in anticlinal division. The mantle may consist, a)Solely one tunica layer, b) 2 or more tunica layers plus sub tunica layer may be present. Ex : Zea mayas , Triticum vulgare.
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  Z2 Sub apicalinitials: It is composed of self perpetuating group of irregular shaped cells which may divide in various planes. The rate of cell division is relatively slow in the lenticular zone of subapical initials when compared with rate in the surrounding zones. Periclinally dividing central cells of inner mantle layers were also included as sub apical initials. This zone is the precursor for central meristem & peripheral meristem . Z3 Central meristem: Central meristem lies in the central part of the apex immediately below the sub apical initial zone. The cells of this zone is contributed by a)cells of zone 2 b)by transverse division of central meristem itself. c)by cell elongation. Due to successive division of these central meristem, an irregular tiers of cell results in the formation of rib meristem.
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  Z4 Peripheral meristem: It forms a cylinder lying beneath the protoderm and surrounding the zone 3. The cells of this zone is contributed by a) By mantle b) By sub apical initials c) By central meristem also. These cells may involve in the development of foliar initiation while other differentiate into cortex and procambial strands. Zone 3 and 4 may not exist in monocots (Graminae) Ex ; Agropyron repens Elodea canadensis Special cases: An exception is seen i,e. Araucaria brasiliana which shows angiospermic type of shoot apex organization