Fig. 11.5-9. Transverse section of parsnip stem (Pastinaca sativa). The xylem parenchyma is abundant and almost completely surrounds every vessel – there are few contact faces between vessels. Instead, the great majority of the surface area of each vessel element faces a xylem parenchyma cell. The vessel elements here have annular and helical secondary walls (arrows), which means that most of the surface area of each vessel is just primary wall, directly abutting the primary walls of parenchyma cells.