Conquering the terrestrial environment: the evolution of xylem anatomy in early tracheophytes

Abstract

Since the first land plants appeared 480-360 million years ago, natural selection has resulted in continued colonization into increasingly drier and harsher environments. The evolution of traits associated with water transport and avoidance of embolisms allowed plants to conquer increasingly more seasonal and drier terrestrial environments. However, it is unclear just how the xylem anatomy of the first Embryophytes differed from extant (living) taxa and if these differences translated to differences in plant functioning. I measured and compared hydraulic traits from stem cross-sections from extant plants and extinct fossil specimens. For 231 stems comprising 115 extant and 116 extinct taxa, measures of xylem conduit diameter/frequency and segment diameter were measured. Comparing these measures of xylem conductive traits indicates that extant plants, for their size, have more and wider conduits leading to a larger total conductive area and higher rates of water conductance. Further, the combination of xylem traits found in extinct paleo plants suggests that they were less efficient at water transport and likely more restricted to less seasonal and more wet environments. Together, these results reveal a unique insight into the functioning of extinct paleo plants and the evolution of xylem form and function.