spiral thickening

(helical thickening) A type of secondary wall patterning in *tracheary elements in which the secondary cell wall is laid down in a helical pattern. Spiral thickening, like *annular thickening , permits further extension of the cell. It is therefore found in tissues such as protoxylem, which develop in young organs that are still elongating. Compare pitted thickening , reticulate thickening , scalariform thickening.