The annular and spirally thickened conducting elements generally develop in the protoxylem when the root or stem is

Step-by-step solution

Vessels or tracheae are made up of a row of cells, placed one above the other, with their intervening walls absent or variously pored. The walls of vessels are lignified and hard, but not very thick. The cell cavity or the lumen is wide. The thickening may be annular, spiral, scalariform, reticulate and pitted.