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Apex of shoot refers to the apical portion of the stem. It comprises multipotent stem cells that generate leaf primordia and give rise to all of a plant’s aerial parts, including leaves, branches, and flowers. It is apical meristem-containing and continues to grow. The stem of a plant is the aerial portion that bears leaves, branches, flowers, and fruits. The stem is composed of nodes, which are the locations of leaves, and internodes, which are the segments between two nodes. The apical meristem is found on the root and shoots apex. It is the zone of active cell division in plants that is responsible for growth and the formation of primary tissues. Apical meristem is located at the apex of the shoot. Terminal or axillary buds are present on the stem. Axillary buds develop into branches or blooms in the axils of leaves. These are apical meristem cells that remain after stem extension or leaf development.
Theories of Shoot Apical Meristem
There are three theories about how the shoot apex is organised. They are as follows:
1. Apical Cell Theory
In 1944, Nageli pushed for this hypothesis. According to this view, the apical meristem is composed of a single apical cell (sometimes referred to as the apical beginning), which serves as the structural and functional unit of the apical meristem’. The cell is enormous and has the shape of an inverted pyramid. The apical cell is tetrahedral in shape and contains three or four cutting faces, one of which faces upward and the rest of which faces downward. The upward-facing side of the apical cell is triangular or square in shape and forms a portion of the shoot apex’s outer surface.
The cutting sides of the apical cell divide in a helical pattern. The cell splits asymmetrically, resulting in the formation of a narrow and flat cell. The apical cell’s subsequent division is also asymmetric. This sort of asymmetric division is repeated on the apical cell’s downwardly pointed faces.
As a result, each of the cutting faces has its own daughter cells. Daughter cells divide as well, forming enormous packets of cells. The packet of cells develops and generates the shoot’s several segments. As a result, the apical cell is considered a reserve of one genetically sound cell.’
In vascular cryptogams, a single apical cell forming an apical meristem is present. Following the finding of a solitary apical cell in vascular cryptogams, it was assumed that a similar apical meristem existed in higher plants as well.
Subsequent considerable research disproved the uniform existence of a single apical cell in a meristem. In higher plants, the apical cell idea has been superseded by the concept that the various components of a plant’s body originate independently. As a result, the apical cell idea was eventually supplanted by histogen theory.
2. Histogen Theory
Hanstein proposed the histogen hypothesis in 1868. (Histogen means tissue builder). According to this hypothesis, a plant’s tissues start from a multitude of meristems from which the following three (histogens) can be differentiated.
Dermatogen: (Greek for skin). It is the meristem’s outermost layer. It gives birth to root and stem epidermis.
Periblem: (In Greek, this term refers to apparel). This location is located within the dermatogen but on the periphery of plerome. This histogen is responsible for the formation of the cortex of the root and shoot, as well as the inner tissues of the leaves. It encircles plerome.
Plerome: (In Greek, this means that it fills). This region produces the vascular cylinders of the stem and root, as well as the pith. It is the stem and root’s centre core, and the cells that comprise this zone are highly irregular. This region is surrounded by a variable number of mantle-like layers composed of dermatogen and periblem.
According to Hanstein, dermatogen, periblem, and plerome all originate from independent apical meristem initials.
Subsequent research reveals that the sub-divisions — dermatogen, periblem, and plerome — are not universally applicable for two reasons:
There is no apparent differentiation between periblem and plerome in gymnosperm and angiosperm
The three histogens’ respective responsibilities cannot be established
The primary shortcoming of Hanstein’s idea was the inability to ascribe precise fates to histogens. The histogens – dermatogen, periblem, and plerome — are irreversible and give rise to the epidermis, cortex, and stele, respectively. Later, the tunica-corpus hypothesis supplanted this theory due to the noncommittal nature of the zones.
Though histogen theory is rejected, it is nevertheless considered ‘classic’ since Hanstein viewed the shoot and root apex as a composite entity composed of distinct groups of histogens, each of which produced distinct tissues. It’s worth noting that Hanstein anticipated this far before the concepts of gene and DNA were created.
The current paradigm is that the several zones at the apex of the shoot are fundamentally identical and capable of creating all tissues. The position of derivatives in the meristem dictates their fate. This is because in zones, certain genes are activated while others are inhibited, resulting in the formation of distinct tissues.
3. Tunica-Corpus Theory.
Schmidt proposed the tunica-corpus idea in 1924, based on investigations of angiosperm shoot apices. This idea is concerned with the apical planes of cell division. Unlike apical cell and histogen theory, tunica-corpus theory is only applicable to the shoot apex and not to the root. Schmidt designates two distinct tissue zones in the shoot apex as tunica and corpus.
The majority of angiosperm shoot apexes have tunica, which is composed of two layers of cells and a corpus. The layers are designated as L1, L2, and L3 to describe the tunica’s outer, inner, and corpus layers, respectively.
Plasmodesmata exist between tunica and corpus cells. It is believed that plasmodesma regulates gene expression in the protoderm, ground meristem, and provascular tissue.
Structure and Organisation of Shoot Apex
The shoot apex is the tip of the stem where meristematic cells and the growth area are located. It grows into lateral branches, leaves, and flowers, among other things.
Different plant groups, such as pteridophytes, gymnosperms, and angiosperms, have distinct sizes, shapes, organisational patterns, and patterns of tissue differentiation. The following are some of the properties of the shoot apex:
It contains the apical meristem, which differentiates into various tissues.
It is located directly above the earliest leaf primordium. It is self-sustaining and produces leaf primordia
When an inflorescence or flower forms at the shoot apex, the growth becomes determinate. When the stem is changed with tendrils or thorns, the growth also becomes determinate or constrained
Flower embryogenesis occurs in the shoot apical meristem. Primordia of leaves, sepals, petals, stamens, and ovaries grow sequentially over a time interval called the plastochron.
The shoot apical meristem is composed of four distinct cell types. They are stem cells, daughter stem cells, initiators of organ development, and an organising centre
Three distinct types of primary meristems arise from apical meristems. They are as follows:
Protoderm: develops into epidermis.
Procambium: produces primary vascular tissues, including the xylem and phloem.
Ground meristem: Cortex, pith, medulla, and pericycle develop from the ground meristem.
Conclusion
The shoot apex is the developing point of the plant shoot and plays an important function in morphogenesis: it’s here that new leaves or flowers emerge and increase quickly. The stem has nodes, where leaves grow, and internodes, which are the segments between nodes. Located at the root and shoot apex. Plants use active cell division to develop and generate basic tissues. The apical meristem is at the shoot’s tip. The stem has terminal or axillary buds. Axillary buds sprout branches or flowers in the leaf axils. Apical meristem cells that do not extend stem or leaf.
