MERISTEM: Types M. Bot- 201 UNIT –IV MSc Botany Semester II Prepared by: Dr Jyoti Pandey, Assistant Professor, Botany, A.N. College, Patna 1 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. 2 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 3 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. 4 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 5 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. 6 7 .