Meristematic Tissues

NCERT grounding

NCERT Class 11 Biology, Chapter 6, states in its summary that plant tissues are "broadly classified into meristematic (apical, lateral and intercalary) and permanent (simple and complex)." The same chapter notes that secondary growth—the increase in girth—occurs in most dicotyledonous roots and stems through the activity of the cambium, a lateral meristem. The NIOS Biology lesson on Tissues adds the working definition: meristematic tissues are composed of immature, dividing cells, and lists their three positional types with location and function.

What a meristem is and how it is classified

A meristem (Greek meristos, "divisible") is a tissue of cells that retain the capacity for continuous division throughout the life of the plant. While most cells of a mature organ have stopped dividing and have differentiated into specialised permanent tissues, meristematic cells stay perpetually young. With every division, one daughter cell tends to remain meristematic—an initial—while the other is gradually displaced, enlarges and matures into a permanent cell. This unending supply of new cells is the engine of all plant growth, both the lengthening of axes and the thickening of stems and roots.

Because a plant has no fixed adult body and goes on growing at its tips and flanks, the position a meristem occupies is the single most important fact about it. Two independent classifications are therefore used: one by position in the plant body, and one by origin (developmental history). Both are examinable, and NEET frequently couples a location with the type of growth it produces.

Cytological features of meristematic cells

Before the types, fix the cell itself. Meristematic cells are built for division, not for storage or transport, and their cytology reflects this single-minded role.

Classification by position

Locate the meristem on the plant and you can immediately predict what it does. The three positional types occupy distinct sites and govern distinct directions of growth.

Apical meristems occupy the very tips of the main and lateral shoots and of the roots. They are responsible for the elongation of the plant axis—growth in length—and the body they build is called the primary plant body. The shoot apical meristem also gives rise to leaf and bud primordia, while the root apical meristem is protected by an overlying root cap. Cells just behind the apex are constantly cut off and begin to differentiate into the primary tissues.

Intercalary meristems are portions of the apical meristem that have become separated from the tip by mature tissue. They lie at the bases of internodes or at the bases of leaves and are characteristic of grasses and other monocots. Their activity allows internodes to elongate and grass leaves to regrow from the base after grazing or mowing. Both apical and intercalary meristems are early-formed and together constitute the primary meristems of the plant.

Lateral meristems appear along the sides of mature roots and stems, oriented parallel to the long axis. The two important examples are the vascular cambium, lying between xylem and phloem, and the cork cambium (phellogen) in the outer region. By dividing in the radial plane they add cells to the girth of the organ—this is secondary growth—and account for the thickening of woody dicot stems and roots.

Classification by origin

The second scheme asks when and from what a meristem arose. Primary meristems originate directly from the embryo and are present in the seedling from the start; the apical and intercalary meristems belong here, and their derivatives form the primary plant body. Secondary meristems develop later in life from cells that had already matured into permanent tissue and then resumed division. The cork cambium is the classic secondary meristem, arising in the cortical or epidermal region during secondary growth. The vascular cambium is partly primary (the fascicular strips between xylem and phloem) and partly secondary (the interfascicular cambium that forms from medullary-ray cells), which is why it is best learned as a lateral meristem rather than forced into one origin column.

The apical meristem and the promeristem

The earliest meristem of all is the promeristem—the small group of dividing cells in the embryo from which the mature apical meristems are derived. As the apex matures, the promeristem becomes organised into recognisable zones. The NIOS lesson sets out the classical histogen theory, in which the promeristem differentiates into three histogens: the dermatogen, which forms the epidermis of the stem and the epiblema of the root; the periblem, which forms the cortex; and the plerome, which forms the central tissues—the pericycle, pith and vascular tissue.

For the shoot apex specifically, NIOS also describes the tunica–corpus theory. Here the apex has a peripheral tunica of one or more cell layers that divide anticlinally to give surface growth, and an inner corpus, a mass of cells dividing in various planes to add volume. The tunica gives rise to the epidermis and the outer cortex, while the corpus produces the inner ground and vascular tissues. Either theory leads to the same practical point: the apical meristem continuously cuts off cells behind itself, and those derivatives mature in an orderly sequence into the primary tissues of the organ.

With these two classifications in hand, almost any NEET item on this subtopic resolves quickly: identify the position to predict the growth direction, and identify the origin to place a cambium correctly. The most reliable single anchor is the link between a lateral meristem and secondary growth—it appears in question after question.

Worked examples

Common confusion & NEET traps

The errors here are almost always about pairing the wrong meristem with the wrong outcome. Lock the position-to-growth mapping first, then guard the boundary cases below.