- The higher plants (vascular cryptogams and spermatophytes have highly complex bodies made up of various kinds of cells.
- These cells cluster together in order to perform specific function. These groups or cluster of cells are called tissues.
- A group or collection of similar or dissimilar cells that perform or help to perform a common function and have common origin is called a tissue.
- Plant tissues may be divided into three types:
- Meristematic tissues
- Permanent tissues
- Secretory tissues
- Meristematic tissue includes group or cluster of cells which remain in continuous state of division or retain their power of division.
- The meristematic tissues divide to form new cells which differentiate to give rise to permanent tissues.
Characteristics of meristematic tissues:
- The cells of meristematic tissues may be round, oval, rectangular or polygonal in shape.
- They are compactly arranged without intercellular spaces.
- They have thin and elastic primary wall made up of cellulose.
- Cytoplasm is dense with many small vacuoles and large nuclei.
- These cells don’t store reserve food materials.
- The plastids are present in the form of
- The crystals are absent and the endoplasmic reticulum is poorly developed.
- The cells usually divide in a particular plane.
Types of meristems:
- The meristems are classified into various types on the basis of origin and position in the plant body, their function, plane of division and stage or method of development.
Depending upon the position in the plant body, meristems are of the following three types:
1. Apical meristem:
- This meristem is located at the growing apices of main and lateral shoots and roots.
- The cells are responsible for the linear growth of an organ.
- This meristem is located in between the regions of permanent tissues.
- These are believed to be the tissues left behind due to tissue differentiation of an organ, which remain meristematic and continue to add new cells to the organ.
- These meristems are usually responsible for growth in length and present mostly at the base of node, base of internode and the base of leaf.
- The intercalary meristems usually disappear and give rise to permanent tissues.
- This meristem consists of initials(cells) which divide mainly in one plane (periclinal).
- They result in the increase in the diameter of an organ. E. g cambium, cork cambium and marginal meristem of some leaves.
Based on the origin and method of development, meristems are of the following types:
- The promeristem originates from the embryo and, therefore, called primordial or embryonic meristem.
- It is present in the regions where an organ or a part of a plant body is initiated and lay down the foundation of an organ.
- The cells divide repeatedly to give rise to primary meristem.
- The promeristem no more exists in the products of promeristem and it is restricted to a limited amount in a particular organ of a particular plant.
- For example, the promeristem is present at the extreme tip of a young stem consisting of only few cells which divide and give rise to a next zone of primary meristem.
- Primary meristem:
- A primary meristem originates from promeristem or embryonic meristem of the plant which is concerned with the formation of primary permanent tissues of the primary plant body.
- Due to activity of primary meristem, primary body is formed and the differentiation of various tissues and organs takes place.
- The protoderm, procambium and the ground meristems are the kinds of primary meristems.
- Secondary meristem:
- They originate as the new meristems from the permanent tissues which have already gone differentiation.
- They don’t have their own promeristem.
- The secondary meristems arise in plant organs whenever and wherever required.
- For example, they originate as vascular cambium from inter-fascicular regions in dicots when secondary growth is needed.
- When healing of wounds and formation of periderm is needed, they arise as cork cambium.
- The cambium of root and accessory cambium rings are the best examples of secondary meristems.