Meristems are the primary growth centers within plants, containing actively dividing cells responsible for forming all new tissues. These specialized tissues allow a plant to increase its size and develop complex structures. The intercalary meristem is a type of growth tissue found predominantly in monocotyledonous plants, such as grasses. This meristem enables growth to occur from within the plant structure rather than exclusively from the tips.
Defining Intercalary Meristem Location
Unlike the apical meristem, which is located at the tips of shoots and roots, the intercalary meristem is uniquely positioned or “intercalated” between regions of mature, non-dividing tissue. This strategic placement gives the tissue its name. In grasses, this meristematic tissue is concentrated at the base of the leaf blade and, more significantly, at the base of the internodes.
Internodes are the segments of the stem found between the nodes, which are the points where leaves attach. The meristem’s location at the base of these segments means the growing tissue is situated low down on the plant. This contrasts sharply with most other plants, where primary vertical growth happens at the top of the shoot. The placement of the intercalary meristem defines the growth pattern and resilience of grasses.
How Intercalary Meristems Drive Elongation
The primary function of the intercalary meristem is to facilitate rapid vertical growth and elongation. This growth is achieved through a two-part cellular mechanism involving mitosis and cell expansion. Cells within the meristematic zone are mitotically active, undergoing continuous division to produce new cells.
Once new cells are produced, they are pushed away from the meristematic zone and begin the process of differentiation and elongation. The newly formed cells absorb water and expand significantly in length, contributing to the lengthening of the leaf blade or the stem internode. As the cells mature and differentiate into permanent tissue, the meristematic activity in that area eventually ceases.
The Evolutionary Advantage in Grasses
The basal position of the intercalary meristem provides an evolutionary advantage to grasses, allowing them to thrive in environments subject to frequent disturbance. When a herbivore grazes on grass or a lawnmower cuts the turf, the top part of the leaf blade or stem is removed. In plants that rely on apical meristems at the tip, this removal would eliminate the primary growth point, halting vertical growth.
However, since the growth region in grasses is located near the base, the meristematic tissue remains below the damage and intact. This protected location allows the plant to immediately initiate regrowth and rapidly regenerate the lost leaf or stem tissue. The ability to quickly recover from defoliation has enabled grasses to colonize and dominate vast ecosystems, from prairies to lawns, despite constant pressure from grazing animals.