Primary wall in plant cells is composed of an intricate network of micro-fibrils in a gel-like matrix arranged in the following manner: cellulose micro-fibrils, pectin polysaccharides and structural proteins. When a cell is young and small the cellulose fibrils are loosely packed and cross linking of the cellulose fibrils is not complete. However, in mature cells due to cross linking of fibrils present in the wall, the cell is able to grow further. However, a new class proteins called expansion has been found to be responsible for wall loosening and cell expansion by addition of cellulose molecules to cellulose micro-fibrils. While the secondary wall is made up of cellulose fibrils in parallel rows at an angle with the other fiber bundles. Sometimes, hemicellulose, pectin and lignin may also be deposited in the secondary wall.
Primary cell wall are thin wall of the growing cells, while secondary cell walls are thick walls of grown cells, deposited between primary wall and plasma membrane.
Sclereids or stone cells are special types of sclerenchyma. Which are developed in certain plants to impart mechanical strength. It sclereids may occur in the cortex, pith, phloem, hard seed, nuts, stony fruits and in the leaf and stem of many dicotyledous and gymnosperme. There are four types of sclereids based on their shape:
- Brachysclereids: Brachysclereids are more or less isodiametric usually found in phloem, cartex, bark of stem, flesh of fruits intermingled with soft parenchyma, for example-pears.
- Macrosclereids: Macrosclereids are rod shaped such sclereids often form continues layer in the testa of seeds e.g. in the seed coats of legumes.
- Osteosplereids: Osteosciereids are bone or spoal shaped sclereids, the needs of which are enlarged, lebed and sometimes even some what branched formed in the seed coats.
- Asterosclereids: Asterosclereids are variously branched sclereids that are often star shaped. Such sclereids are found in leaves of Nymphaea plant.