Cambial derivatives differentiating as xylem fibres characteristically increase in length by intrusive growth. Radial expansion of contiguous cells is required for this intrusive growth which, in consequence, is restricted to the expansion zone of the newly formed wood. The amount of intrusion depends primarily on the position of the tip of a fibre in relation to the lines of junction of tangential and radial walls in the surrounding cells; these lines of junction are potential sites for intrusive growth. In general, the greatest intrusion occurs where there are two potential sites for intrusion (usually one on either radial face of the fibre's tip); tips with one site show less intrusive growth and those with none show least of all. These arrangements may be modified by the configuration and activity of neighbouring cells.
The hypothesis formulated is that the tip of a fibre secretes an enzyme which weakens the middle lamella between the cells adjacent to it; where this occurs, these cells, during their turgor expansion, round off from each other at the corners. The thin–walled tip of the fibre fills this space as it is created. The notion that the intruding tip pushes its way into the cellular matrix is contested.