Tissues Meristematic

6.1 Introduction to Plant Anatomy

You can very easily see the difference in the external structure of root, stem, leaf, flower, fruit, and seed. Within the angiosperms, the monocots and dicots are also seen to be anatomically different. Internal structures also show interesting variations. Anatomy is the study of internal structure of organisms. Plants have cells as the basic unit, cells are organised into tissues and in turn the tissues are organised into various parts of the plant.

A group of cells with similar origin and similar function is called a tissue. A plant is made up of different kinds of tissues. Plant tissues are broadly classified into meristematic tissues and permanent tissues, based on whether the cells are capable of dividing or not.

6.2 The Tissues

6.2.1 Meristematic Tissues

Growth in plants is largely restricted to specialised regions of active cell division called meristems. Plants have different kinds of meristems. The meristems which occur at the tips of roots and shoots produce primary tissues and are called apical meristems.

The root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis. During the formation of leaves and elongation of stem, some cells "left behind" from shoot apical meristem, constitute the axillary buds. Such buds are present in the axils of leaves and are capable of forming a branch or a flower.

The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores. Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and contribute to the formation of the primary plant body.

The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appear later than primary meristem is called the secondary or lateral meristem. They are cylindrical meristems. Examples are fascicular vascular cambium, interfascicular cambium and cork cambium. These are the meristems which cause the increase in the diameter of organs in which they are present.

6.2.1.1 Classification of Meristems by Position

6.2.1.2 Classification by Origin

6.2.1.3 Three Zones of Apical Meristem (Hanstein's Theory)

Following Hanstein's classical concept (Histogen Theory), the shoot/root apical meristem can be divided into three zones:

• Dermatogen — outermost layer; gives rise to epidermis (protoderm)
• Periblem — middle layer; gives rise to cortex
• Plerome — innermost; gives rise to vascular tissue (procambium)

6.2.2 Meristematic vs Permanent Tissues

The cells of the meristem may divide for some time and then lose the ability to divide. Such cells assume specific roles, that is, they undergo differentiation. Differentiation leads to development of a permanent shape, size, and a function. Cells derived from both apical and lateral meristems form the permanent tissues.

6.2.2.1 Differentiation, Dedifferentiation, Redifferentiation

• Differentiation — cells derived from root apical, shoot apical, and cambium differentiate to form mature tissues with specific functions.
• Dedifferentiation — living differentiated cells regain the capacity for division (e.g., interfascicular cambium and cork cambium form from already differentiated parenchyma cells).
• Redifferentiation — dedifferentiated cells lose the ability to divide once again, becoming permanent (e.g., secondary xylem and secondary phloem from cambium).

Worked Examples