Extension of the internode below the point of insertion of the flag leaf of Lolium temulentum L. takes place in the lower 10 to 15 mm. Extension of the long epidermal cells also takes place in this region. However, fibre cells show little extension in the lower 10 mm and maximum extension occurs at about 30 mm above the node. Thus, there is extension of fibre cells within that portion of the internode which has ceased extension and is bounded by an epidermis, the cells of which have also ceased elongation. This suggests the occurrence of intrusive growth of the fibres and evidence for this is shown from the measurement of macerated fibres and from electron micrographs.
The walls of fibre cells measured in internodes of different lengths and ages thicken only very slightly in the lower 30 mm and most cell wall thickening occurs between 45 and 90 mm above the node. Thus most cell wall thickening occurs after the fibre cells have ceased to elongate.