Quiescent centre concept was proposed by Clowes ( ) to explain root apical meristem activity. This centre is located between root cap and differentiating cells of the roots. The apparently inactive region of cells in root promeristem is called quiescent centre. It is the site of hormone synthesis and also the ultimate source of all meristematic cells of the meristem.
. Permanent Tissues The Permanent tissues develop from apical meristem. They lose the power of cell division either permanently or temporarily. They are classified into two types: .
Simple permanent tissues. . Complex permanent tissues. Intercellular spaces Starch grains a.
b. Figure . : Types of Parenchyma a) Aerenchyma, b) Storage parenchyma c) Stellate parenchyma, d) Chlorenchyma, e) Prosenchyma Intercellular Spaces Chloroplasts Palisade Parenchyma Spongy Parenchyma Intercellular Space c. d.
e. Collenchyma (Gk. Colla-glue; enchyma – an infusion) Collenchyma is a simple, living mechanical tissue. Collenchyma generally occurs in hypodermis of dicot stem.
It is absent in the roots and also occurs in petioles and pedicels. The cells are elongated and appear polygonal in cross section. The cell wall is unevenly thickened. It contains more of hemicellulose and pectin besides cellulose.
It provides mechanical support and elasticity to the growing parts of the plant. Collenchyma consists of narrow cells. It has only a few small chloroplast or none. Tannin maybe present in collenchyma.Based on pattern of pectinisation of the cell wall, there are three types of collenchyma.
. Aerenchyma: Parenchyma which contains air in its intercellular spaces. It helps in aeration and buoyancy. Example: Nymphae and Hydrilla.
. Prosenchyma: Parenchyma cells became elongated, pointed and slightly thick walled. It provides mechanical support. Parenchyma .
Storage Parenchyma: Parenchyma stores food materials. Example: Root and stem tubers. . Chlorenchyma Parenchyma cells with chloroplast.
Function is photosynthesis. Example: Mesophyll of leaves. . Stellate Parenchyma Star shaped parenchyma.
Example: Petioles of Banana and Canna . Types of Collenchyma . Angular collenchyma It is the most common type of collenchyma with irregular arrangement and thickening at the angles where cells meet. Example: Hypodermis of Datura and Nicotiana .
Lacunar collenchyma The