4.2: Introduction

Plant cells can be classified into three major categories based on their cell wall structure. Most plant cells are parenchyma cells (par- meaning equal), with an evenly-thickened primary cell wall. Some areas of a plant, particularly in young shoots, will require more flexibility, allowing them to bend without breaking. In these regions, you will often find collenchyma cells (coll- meaning glue), cells with strange, unevenly-thickened primary walls. Both of these cell types are alive at maturity. A third cell type, sclerenchyma cells (scler- meaning hard), develops a rigid secondary wall that is composed of lignin. This secondary wall provides structural support, but it also cuts off the exchange of water and other molecules permitted by the plasma membrane. Ultimately, this results in the death of the cell at maturity and loss of internal cell components.

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