In this article we will discuss about the definition of meristem.
Meristem can be defined as mitotically active centre that is restricted to specific portion of higher plants and the cells of meristem divide indefinitely by multiple mitoses thus resulting in continuous addition of new cells to the developing plant organs.
The zygote, in all sexually reproducing higher plants, divides and subdivides to produce an undifferentiated packet of cells. After differentiation these cells form root, stem, leaves and other organs. But among these a group of cells always remains undifferentiated and retains the capability of cell division. These cells undergo frequent divisions by multiple mitoses and add new cells to the developing organs.
Such cells are collectively termed as meristem or meristematic tissue. The term meristem is derived from the Greek word ‘merizein’ meaning to divide (en(dot)wikipedia(dot)org/wiki/Meristem). A cell of meristem is regarded as totipotent; that is the cell has the capability to develop the different types of cells that compose a plant body.
Meristems occupy the growing regions of organs concerned. The regions include stem tip, leaf tip, leaf bases and root tip. Meristem in the form of cambium occurs as fascicular cambium (present within vascular bundle), interfascicular cambium (present in between vascular bundles), cork cambium (= phellogen) and accessory cambium (originates within stele). Meristem also occurs in reproductive apex and flower buds.
The cells of meristem are isodiametric in shape. They are tightly packed living cells with little intercellular spaces. The cell wall is thin and composed of cellulose only.
Cells have large nucleus, rich cytoplasm with less ergastic inclusions, crystals and reserve food; vacuoles if present are very small and scattered throughout the protoplast; plastids occur in rudimentary form (proplastid); cells have smaller amount of endoplasmic reticulum and less elaborate structure of mitochondria.
Though meristematic cells exhibit the above mentioned criteria, exceptions are observed in cambial cells. The cells of fusiform initial of vascular cambium are long and narrow. Cork cambium or phellogen contains ergastic substances, reserve food materials and chloroplasts. The ray initials of vascular cambium may contain starch and tannin. Vacuoles occur in the cells of apical meristem of Ginkgo.
Fusiform —and ray initial cells of vascular cambium may be highly vacuolated (Fig. 7.1). In Ginkgo the cells of the apical meristem have thick primary cell walls with prominent primary pit fields. The cambial initials also exhibit thick cell walls with depressed primary pit fields.
There exists much variation in the characteristics of meristem. It appears from morphological and physiological point of view that there exists no typical meristematic cell.
So Kaplan in 1937 suggested the term eumeristem (= true meristem) to designate those meristems that have only the following characteristics:
(a) Small and isodiametric in shape;
(b) Cell wall is thin, and
(c) The cell has rich cytoplasm. The term was coined for descriptive purposes only.
The derivative cells of meristem transform into permanent tissues of plants. Permanent tissue includes epidermis and periderm that compose the dermal tissue system, xylem and phloem that compose the conducting tissue system and ground tissue system that consists of all the remaining tissues apart from epidermis, periderm and vascular tissues.
Meristem consists of a group of cells that continuously divide and the derivative cells differentiate into respective tissues. The undifferentiated group of cells is termed as promeristem. Promeristem is also referred to as primordial meristem or embryonic meristem. Promeristem occupies the apical portions of root, stem and leaf. When differentiation starts, the meristem is not regarded as promeristem.