It is believed that differentiating vessel elements increase their diameter by growing intrusively in the circumferential direction and symplastically in the radial direction in relation to the stem axis. On the basis of a detailed analysis of the cell arrangement observed in a series of semithin anatomical sections of cambial zone and the developing and mature secondary xylem of Terminalia ivorensis , Wisteria floribunda , and Millettia laurentii , we revealed a novel correlation of growing vessel elements with surrounding tissues. Rays seem to prevent the growing vessel elements from protruding laterally between the cells of adjacent rays. The growing vessel elements break the continuity of several neighbouring radial files of fusiform cell derivatives but not of ray cell derivatives. If a contiguous ray becomes an obstacle for the growth of vessel elements on any one side, the growing elements often start to grow in the opposite direction, consequently causing a deviation in the alignment of the vessel elements concerned. This mechanism explains why vessel elements may deviate from the array of their precursors, the fusiform cambial initials. Our models on the intrusive symplastic growth of vessel element mother cells have revealed that intrusive growth does not occur between radial walls of neighbouring cells.
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