<P> The apparent rigidity of the cell wall thus results from inflation of the cell contained within . This inflation is a result of the passive uptake of water . </P> <P> In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity . Additional layers may be formed by lignin in xylem cell walls, or suberin in cork cell walls . These compounds are rigid and waterproof, making the secondary wall stiff . Both wood and bark cells of trees have secondary walls . Other parts of plants such as the leaf stalk may acquire similar reinforcement to resist the strain of physical forces . </P> <P> The primary cell wall of most plant cells is freely permeable to small molecules including small proteins, with size exclusion estimated to be 30 - 60 kDa . The pH is an important factor governing the transport of molecules through cell walls . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (October 2013) </Td> </Tr> </Table>

Why cell wall is present in plant cell