Fig. 4.

3-D visualization underlines the substitution and transitions of metaxylem vessels along the wheat nodal root axis. (A) Cross-sections of the stele and 3-D reconstructions and visualization of the vessels. As the presence of a lateral root interrupts the progressive change of anatomical structure of the main root, the 3-D reconstruction and visualization of the root segment have to be performed separately on two parts: segment 1 towards the root base and segment 2 to the root apex. On the left are presented the cross-sections of two root segments: proximal end (towards the root base) of segment 1, proximal end of segment 2 and distal end (towards the root apex) of segment 2. The images in the middle of (A) are 3-D reconstructions in each segment, illustrating the transition of vessels along the root segment. Vessels in the segments can be distinguished one from the other by colours, note that vessel 07 (violet) that was visible in segment 1 and in the proximal end of segment 2, did not appear in the distal end of segment 2, meanwhile the yellow one (vessel 01) was newly formed, as it does not occur in segment 1. On the right of (A) is a side view of the two segments performed by section interpolation described within a VRML file. (B) Longitudinal section showing two vessels are connected by pits on their lateral walls. (C1) Evolution of the metaxylem vessels along a nodal root axis. The length of the root is 25 cm. The slice images are taken at each segment indicated by the arrows. The number of metaxylem vessels decreases from root base to root tip. (C2) Evolution of the number of metaxylem vessels along the root axis – data from cross-sections of 12 roots. In most roots, the number of metaxylem vessels decreases to one at 10 cm from the root base.