Types Meristem

MERISTEM: Types M. Bot- 201 UNIT –IV MSc Botany Semester II

Prepared by: Dr Jyoti Pandey, Assistant Professor, Botany, A.N. College, Patna

MERISTEM: TYPES • Meristem is the plant tissue consisting of young cells that are in a continuous state of division. • Meristem is found in growing parts of plant body like apex of root and shoot, at nodes etc. • Meristem contributes cells for plant growth indefinitely and also perpetuates itself. • Term meristem was first used by Carl Wilhelm von Nägeli in his book “contribution to scientific Botany”, Beiträge zur wissenschaftlichen Botanik . • Term meristem is derived from Greek word, meristos which means “divisible”. • Meristematic tissue is commonly called as meristem.

Characteristics of meristematic tissue/ meristem

• Cells are immature and undifferentiated. • Meristematic tissue is a group of cells that has power of continuous division. • Cells are small in size with oval, rounded, polygonal or rectangular shape. • There is no intercellular space between cells and cells are compactly arranged. • Cells have large and prominent nucleus. • N/C (nuclear to cytoplasm) ratio is high as cells are capable of division. • Cells are metabolically very active. • Cells have dense protoplasm but lack ergastic substances, crystals and storage food material. • Cell wall is thin and elastic. It has primary wall only and there is no secondary thickening. • Organelle like ER and mitochondria are feebly developed and chloroplast is present as proplastid. Vacuoles are small and few if present or absent altogether.

TYPES OF MERISTEM

1. Based on origin and development 2. Based on position 3. Based on function 4. Based on plane of division

1. Based on origin and development: a. Promeristem/primordial /embryonic meristem • A group of young meristematic cells of a growing organ. • It is the early embryonic meristem from which other advanced meristems are derived. • In a plant, it occupies a small area at the tip of stem and root. . • It represents Embryonic Axis and is the initial stages of meristematic cells. • It further divides to form primary meristem b. Primary meristem: • These are derived from promeristem. • They are present below the promeristem at shoot and root apices. • They appear early in life. • Primary meristem gives rise to primary permanent tissues and secondary meristem. c. Secondary meristem: • Develop later on in life. • It is developed from primary meristem. • They are lateral in position like cork cambium, vascular cambium in dicot root and interfascicular cambium in dicot stem. • It gives rise to secondary permanent tissue after primary growth.

2. Based on position

Meristems are divided on their location or position in plant body

a. Apical Meristem: ▪ It is present at apical parts of plant such as root tip and shoot tip

▪ Cells remain active indefinitely.

▪ It helps to increase the height of plants. 3 zones are distinguished a) outer protoderm –it forms epidermis b) innermost zone in the core is procambium – forms vascular tissue and c) between above 2 zones lie ground meristem – forms cells of cortex, pith.

b. Intercalary Meristem: ▪ It is present in intercalary position in the leaves and internode and is commonly found in internodes of vascular plants. ▪ It is found intercalated between permanent tissues above and below. ▪ It is actively growing region behind apical meristem. ▪ It also adds to height of plants. ▪ It is commonly present in monocots, grass and pines.

c. Lateral Meristem: ▪ It is present on lateral side of stem and root. ▪ It helps in increasing the diameter or thickness of plants. It is thus responsible for secondary growth of plant. ▪ Example: vascular cambium (primary meristem) and cork cambium (secondary meristem)

3. Based on plane of division Meristems are classified on the plane of divisions:

a. Mass meristem (or block meristem): • The cells of this meristem divide in all planes thus the tissue increases in volume. • It is found in young embryos, endosperm, and reproductive organs during the formation of spores and sperms etc

b. Rib meristem (or file meristem): • The cells of this meristem divide at right angles to the longitudinal axis of the plant organ. As a result parallel files of cells (= ribs) are produced. • Cells divide in one plane only. • Rib meristem is the characteristic of organs that have cylindrical form. Thus, Rib meristem forms the elongated cylindrical structures like root, stem and petiole etc. • It is found in cortex and pith of root and cortex of stem.

c. Plate meristem: • The cells of this meristem divide and increase the number of cells in one layer only. • Cells divide in two planes. • The cells of meristem grow as a sheet but not in thickness • Plate meristem is the characteristic of plant organs that have flat forms. • Their activity leads to formation of flat organ like leaf. . 4. Based on function

a. Protoderm meristem: • It is the outermost layer of the meristematic tissue • Cells divide and develop to form epidermal tissue system.

b. Procambium meristem: • It is the innermost layer of meristematic tissue. • It is composed of narrow, elongated, meristematic cells that give rise to the vascular tissues system.

c. Ground Meristem: • It is located between protoderm and procambium. • It gives rise to cortex, endodermis, pericycle and pith in dicot and hypodermis, ground tissue in monocot.