In this article we will discuss about the classification of meristem based on position.
According to the position in plant body meristems are divided into apical meristem, intercalary meristem and lateral meristem (Fig. 7.3).
Apical meristem:
It is located at the apices of roots and at the apices of main and lateral shoots. It is also located in trichomes, glands and in other structures that they produce. Primordial meristem (= promeristem) composes the apical meristem and consists of undifferentiated homogeneous cells.
Promeristem exhibits zonations in the apical meristem. The zones can be distinguished from each other by their relative thickness of cell wall, cell size, nuclear size and relative frequency of mitoses.
Apical meristem represents the region of initiation of primary tissue organization in roots and shoots. Promeristem transforms into primary meristem that consists of protoderm, procambium and ground meristem. Apical meristem is the growing point of shoot and forms leaves and branches in genetically predetermined sequences.
As a result shoot elongates. Protoderm, procambium arid ground meristem respectfully initiate the dermal, vascular and fundamental tissue system to form primary body of shoot and root. Flowers also differentiate from apical meristems.
Intercalary meristem:
It is located at the base of internode. Intercalary meristem is short lived and divides frequently. It donates cells above and below. Vascular tissues are also differentiated from this meristem. The derivative cells differentiate into permanent tissues in basipetal succession; as a result stem elongates.
Intercalary meristem is an isolated region in the internode and has permanent tissues above and below it. It remains widely separated from apical meristem. Ex. grasses (Fig. 7.4) and some plants belonging to the families chenopodiaceae, caryophyllaceae and polygonaceae etc. In monocots elongation of shoot axis occurs by random cell divisions and differentiation throughout the youngest internodes.
This region of diffuse cell division is termed as uninterrupted meristem. Uninterrupted meristem is continuous with apical meristem and the derivative cells differentiate into permanent tissues in acropetal succession.
Lateral meristem:
It is located parallel to the long axis of root and shoot. Examples of lateral meristems include vascular cambium, cork cambium, primary thickening meristem (PTM) and secondary thickening meristem (STM). Pericycle of root also could be referred to as lateral meristem because adventitious root originate from pericycle.
In dicotyledonous stem vascular cambium consists of fascicular and interfascicular cambium. The cambium divides tangentially and donates cells on the peripheral and inner side. The derivative cells differentiate into vascular tissues. Thus new cells are added to primary vascular, tissues. As a result a stem increases in girth. Root also increases in diameter by the activity of cambium.
These new tissues supplement the vascular and mechanical tissue system of root and stem. Cork cambium (= phellogen) is located either in the epidermis or adjacent to epidermis or in the peripheral or deeper layer of cortex. Phellogen divides tangentially and donates tissues on the peripheral and inner side. The derivative cells form periderm that supplements the protective tissue system in stems and roots.
Primary thickening meristem (Fig. 7.5) and STM are present in monocotyledonous stems. Monocots lack vascular cambium. PTM and STM donate cells that cause the increase in diameter of monocot stems.
Primary thickening meristem (PTM) is located near the vegetative shoot apex. PTM is a narrow multiseriate zone having a concave form in young plants. The zone consists of meristematic cells that donate cells both centrifugally and centripetally. The centrifugal (peripheral) derivatives are differentiated into parenchyma cells only.
The centripetal (inside) derivatives are differentiated into both parenchyma and discrete vascular bundles. PTM, in addition to stem thickening, also forms vascular tissues that link between the vascular tissues of root, stem and leaf. PTM is also responsible for the production of adventitious roots in some species.
In Dracaena, Yucca and Aloe etc. further increase in stem thickness occurs by the activity of secondary thickening meristem (STM). STM is located in the pericyclic region and is distant from the primary apex. STM divides tangentially and the inner derivatives are differentiated into parenchyma and vascular tissues.
The vascular tissues are secondary vascular bundles that are leptocentric (= amphivsal) and radially elongated. In Yucca whipplei PTM and STM are axially continuous whereas in Beaucarnea recurvata they are axially discontinuous.
PTM, STM and vascular cambium all are meristematic tissue but PTM and STM are not homologous to vascular cambium. There exist differences between thickening meristems (PTM and STM) and vascular cambium.
Vascular cambium exhibits the following characteristics:
(i) It is present in dicotyledons
(ii) It is uniseriate
(iii) It originates within vascular tissue
(iv) It is distantly situated from apical meristem.
(v) Cambium donates cells both on peripheral and inner sides and the derivatives differentiate respectively into secondary phloem and secondary xylem.
In contrast PTM and STM have the following characteristics:
(i) They are present in monocots;
(ii) They are multiseriate;
(iii) They are present below the apical meristem;
(iv) PTM occurs near the apical meristem
(v) PTM and STM donate cells mostly centripetally. The inner derivative cells differentiate into ground parenchyma and discrete amphivasal vascular bundles.