Collenchyma

Collenchyma (Gk. colla = glue, enchyma = infusion) is a living mechanical tissue - it supports growing organs the way parenchyma cannot, by combining a thickened wall with the flexibility to keep stretching as the organ elongates. It sits mainly in the hypodermis (just under the epidermis) of dicot stems, and also in petioles and pedicels, but is characteristically absent from roots, where mechanical demands and growth patterns differ. Its cells are narrow, elongated and appear polygonal in cross section, with an unevenly thickened wall rich in hemicellulose and pectin as well as cellulose - that unevenness in where the wall thickens is exactly what distinguishes the three named types. Angular collenchyma, the commonest type, concentrates its thickening at the corners where several cells meet (Datura, Nicotiana hypodermis). Lacunar collenchyma develops in tissue that already has intercellular spaces, and thickens specifically on the walls bordering those spaces (Ipomoea hypodermis). Lamellar collenchyma stacks its cells compactly in tangential rows and thickens the tangential (flat, parallel-to-surface) walls in successive layers (Helianthus hypodermis). A rarer fourth type, annular collenchyma (Duchaigne, 1955), thickens all the way around a roughly circular lumen, as seen in the petiole of Nerium. Functionally, collenchyma's liv …