The plant vascular system II: From essential functions in resource allocation, inter-organ communication and defense, to evolution of the monocot cambium.

In this Special Issue, a focus is also placed on the role of the xylem as an essential conduit for the long-distance delivery of water and mineral nutrients from the soil to the vegetative (above-ground) regions of the plant. Xylem cells destined to form tracheids or vessel members, which will make up the conduit for this water and mineral transport from the roots to the shoots, undergo apoptosis, a process of programmed cell death. In their review, Venturas et al. (2017) provide an in-depth analysis of the processes underlying the passage of water through these mature, and thus, dead, xylem conducting cells. They elegantly describe the physics associated with this transport of water from the soil to the above-ground tissues and organs of the plant. A tensional gradient, within the water column, pulls the transpiration stream through the plant and Venturas et al. (2017) address the challenges that this presents to the plant, including the unthinkable, in that the water column can rupture, a process termed cavitation. The authors provide an up-to-date analysis of the debate as to how plants might refill such cavitated xylem cells, an important topic with respect to tissue hydraulics. This review closes with an insightful section on the impact of climate change on xylem function.