Fig. 7.5-1. Longitudinal section of root of cattail (Typha). The large white cells in the two prominent rows are precursors to vessel elements. Whereas surrounding cells are still dividing as they grow, these precursors are enlarging without dividing. As they are growing, they have only a primary wall, not a secondary one, so they are parenchyma cells at this stage. Once they achieve the proper size, they will stop expanding and begin depositing the secondary wall. Because these will become vessels not tracheids, the cross walls can be called perforation partitions: their margins might become covered with lignified secondary wall (then it will be the perforation plate rim) but the center will be digested away and become the perforation, the actual hole in the wall.