Lateral meristems are of two types – the vascular cambium and the cork cambium. The activity of the vascular cambium gives rise to secondary vascular tissues increasing the thickness of vascular tissues to fulfil the increasing needs of the growing plant for minerals, water, and food.
The action of the cork cambium, also referred to as the phellogen, leads to the formation of secondary cortex and cork tissue exterior to the vascular tissues to recharge the outer layers of injured and dead cells that are sloughed off periodically with the increasing girth. Both, the cork cambium and the vascular cambium, have highly vacuolated, cuboidal cells.
The vascular cambium is responsible for generating the phloem and xylem of the vascular system, utilized in supporting and transporting. These are single layers of meristematic cells which experience an expansion at the time of transition from primary to the secondary growth.
The fusiform initials and ray initials – two types of cells, generate the axial and radial xylem and phloem derivatives. The various sequence and patterns of the fusiform initials are indicated in the final axial xylem and phloem which is generated.
The typical vascular cambium is a single ring surrounding the stem. Abnormal variations seen in lianas, vines and the storage tissues produce many forms deviating from the standard transport and support functions seen in normal stems. The vascular cambium of the root stock and of the scion in grafting are facilitated to grow together and produce a functional vascular connection. The parasitic plants conduct a similar thing by growing and fusing its vascular system with the host.
Vascular cambium – Origin
The primary vascular tissues and the vascular cambium in the stems differentiate from the procambium. The procambium develops from the derivative cells of the apical meristems. The transverse parts of the growing vegetative shoot apex discloses a cylinder of cells which are greatly cytoplasmic and stained densely.
The residuum of the meristematic tissue of apical meristems is constituted by these rings of cells and hence called residual meristem. In the residual meristem, even more dense staining sites are seen, such sites have a topographic relationship with the leaf primordia. Such sites make up for the procambium which develops as the leaf trace.
The remaining part of the residual meristem constitutes the interfascicular parenchyma. In the longitudinal part of the vegetative apical shoot of the angiosperms and the gymnosperms, it is seen that the procambial strand or ring is continuous, developing acropetally. The procambial strand shows two waves of differentiation – differentiation of the protophloem on peripheral sides and the differentiation of the protoxylem towards the inner edges in typical angiosperms.
Role of Vascular Cambium
The vascular cambium are meristematic tissues involved in the lateral growth and their continued production of the new phloem and xylem, the shoot vascular cambium in woody plants constitute the wood.
An increase in the diameter results from the activity of the vascular cambium and the cork cambium. Between the primary phloem and the xylem lies the vascular cambium, dividing for the formation of phloem on the exterior and xylem on the interior.
The arrangement of primary phloem and xylem in roots averts the vascular cambium initially from forming circular configuration. In a year or so, various rates of cell division in the vascular cambium forms a cylinder. The secondary xylem expands the capacity of the plant to conduct minerals and water up from the roots, adding to the structural support aspect.
The secondary phloem elevates the transportation of food from the leaves. As the root or stem grows in thickness, the mature primary phloem and xylem tissues are further pushed away. It is the secondary xylem which is called wood and majorly comprises dead cells. It is just the more recent layers which are formed of the secondary xylem that are involved in the conduction of minerals and water. Likewise, the more recently formed layers of the living secondary phloem are involved in the conduction of food.
The old phloem cells cannot conduct as they are stretched and broken when the new cells that are generated by the vascular cambium are pushed towards the outside. The xylem cells which are old do not conduct, as an increasing number of vessels possess broken water columns and an increasing number of tracheids have air contained.
Vascular cambium gives rise to secondary vascular tissues. It snips cells in the inside that form the secondary xylem and cells toward the exterior which differentiate into the secondary phloem. In the stem, the interfascicular and fascicular cambium together form the vascular cambium while in the root, vascular cambium forms from the pericycle and conjunctive tissue. The annual rings seen in the wood are as a result of the seasonal variations in action of the vascular cambium.
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