Fig. 7.5-5a. Transverse section of vascular bundle in stem of ragweed (Ambrosia). The narrowest vessel elements here (marked by arrows) are protoxylem. They stopped enlarging and began depositing their secondary wall (in either an annular or helical pattern) while they were still so young that they were very small. Once mature, they digested away their protoplasts and died. But the surrounding cells remained alive and growing, and because all the cells are glued together by their middle lamellas, the growth of the living cells caused the dead protoxylem elements to be stretched into larger and larger sizes. Finally, the primary walls were stretched to their maximum and were torn apart, creating the holes visible here. Once the primary walls ruptured, the vessel elements were no longer able to conduct water. But by that time, other cells had matured into newer, larger vessel elements and conduction in this part of the stem was still possible.