Shoot Apical Meristem

Apical Meristems:

                                       The apices of stem and root are called as growing points. They are occupied by meristematic tissues. These meristems are called apical meristems. All the primary permanent tissues of the plant body are derived by the sole activity of apical meristems. The apical meristem consists of upper promeristem and lower primary meristem. In primary meristematic region the cells are differentiated into three fundamental units. These are protoderm (gives epidermal tissue system) ground meristem (gives ground tissue system) and procambium (gives vascular tissue system). Different organs of plant body are initiated by promeristem and built up by primary meristem.

                                    Shoot Apical Meristem

Shoot Apex

                                     In higher plants, the meristematic cells at the tip of a main shoot axis, or a branch there, constitute a primary shoot meristem, or shoot-apical meristem. The terminal part of the shoot that lies above the level of young leaf primordium is called as shoot apex. The shoot apex is radially symmetrical. As the shoot apex has leaf primordia, nodes and internodes, it is more complicated than the root apex. The shape and size of the shoot apex varies among vascular plants. In majority of angiosperms the shoot apex is cone like with rounded apex. But in some members (Hibiscus sp.) it is concave. At the time of initiation of leaf primordium, the shoot apex shows rhythmic changes. It widens (maximal-phase) before initiation of leaf and gets narrow (minimal- phase) afterwards. The width of shoot apex varies from 90 (grass) to 3.5 mm (Cycas revolute).

A) Types of shoot Apices:

                                         Newman (1961) has recognized three types of shoot apices in vascular plants on the basis of number of cells or number of rows of apical cells.

  • Monoplex Type:

                                          This type of shoot apex is reported in ferns and allied members. The shoot apex consists of generally one or few initial cells. The cells face inwardly. The cells divide parallel to the surface of stem apex and give rise to tissues.

  • Simplex Type:

                                   This type of shoot apex isnfound in gymnosperms. Shoot apex consists of one layer of initial cells. The cells divide anticlinally and periclinally and give rise to different tissues of primary plant body.

  • Duplex Type:

                         In this type of shoot apex, the initials are arranged in two rows. It is found mostly in angiosperms. The outer layer cells divide anticlinally, whereas the inner layer cells divide anticlinally as well as in periclinal manner

B) Regions of Shoot Apex:

                                           Wardlaw (1968) distinguished five integrated and interrelated zones in shoot apex.

  • Distal Region:

                                          The extreme tip portion of the shoot apex is called distal region. It is composed of one or two tiers of apical initials.

  • Distal Region:

                                 The extreme tip portion of the shoot apex is called distal region. It is composed of one or two tiers of apical initials.

  • Organogenic Region:

                                                 The meristematic region that is responsible for the initiation of first organ i.e. leaf primordium is known as organogenic region. Early meristematic tissue differentiation begins in this zone.

  • Sub-Apical Region:

                                              In this region continuous cell division and cell elongation take place The cells differentiate into different tissues of primary plant body.

Region of Maturation:

                                   In this zone the cells lose meristematic nature. As the name indicates cell maturation begins in this zone.

Theories of Shoot Apical Organization

                                                               Number of scientists have studied extensively the shoot apical organization in vascular plants and postulated different theories from time to time. Three outstanding theories have been discussed in this text.

A. Apical Cell Theory

                                                                Apical cell theory is proposed by Hofmeister (1852) and supported by Nageli (1859). Scientists have observed single apical cell in Algae, Bryophytes and Pteridophyta members. The single cell is structural and functional unit of apical meristem. The inverted tetrahedral apical cell divides and gives rise to entire plant body in cryptogams. The scientists have believed that similar shoot apical organization was present in phanerogams too. But further investigations clarified that apical cell theory is applicable to the shoot apical organization of cryptogams but not phanerogams. Because, in phanerogams group of apical cells is present.

B. Histogen Theory:

                                                              Histogen theory is proposed by Hanstein (1868) and supported by Strassburgur. He recognized three distinct zones, dermatogen, periblem and pleurome in shoot and root apical meristems. He called them as histogens or tissue builders. They perform different functions. Each zone is composed of group of initial cells. Further he believed that these zones are derived from separate sets of initials.

Dermatogen:

                     The outermost tissue builder is called dermatogen. The cells of dermatogen divide anticlinally and give rise to epidermal layer of plant organs.

Periblem:

                It is the multilayered zone that lies beneath the dermatogen. The periblem cells divide in anticlinal and periclinal planes and give rise to cortex and endodermis.

Pleurome:

                Pleurome is the multilayered central core. It gives rise to pericycle, vascular tissues, pith and medullary rays.

Recent experiments revealed that there is no clear demarcation between different histogens and rigid relationship between histogens and different tissues formation in shoat regions. Hence this theory does not explain shoot apex organization very clearly. However, even today this theory is applicable to root apex. Haberlandt proposed the terms protoderm, ground meristem and procambium for dermatogen, periblem and pleurome respectively.

C. Tunica Corpus Theory:

                                         This theory was propounded by Schmidt (1924) to explain shoot apex organization phanerogams. According to this theory two distinct zones, tunica and corpus can be recognized in the apical meristems. The tunica and corpus originate from a separate set of initials. The two zones are clearly distinguished from each other on the basis of cell size and plane of cell division.

The Tunica:

                          In apical meristem, the outer enveloping zone is called as tunica It may be single layered or two to three layered structure. In this zone the cells are very small and are arranged in definite rows. In this zone the cells divide in anticlinal plane only. If the tunica is single layered it produces outer covering, the epidermis of the plant body. If it is two to three layered the outer produces epidermis and inner layers produce outer cortical layers. In the tunica, the number of layers of initials is equal to the number of layers of tunica.

The Corpus:                     

The multi-cellular central core which is covered with tunica mantle is called as corpus This zone is composed of larger cells. The cells are not arranged in definite rows as in tunica. The corpus cells divide in all planes. All the tissues of the organ other than the tissues formed from tunica are produced by corpus (viz., Cortex; endodermis; pericycle; vascular bundles & pith). Tunica and corpus theory is applicable to shoot apex organization in spermatophytes. The corpus arises from a single tier of initials.

Related posts:

  • Meristem and its types
  • Root Apical Meristem

 

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