Physiological analyses of aspects of the functioning of vascular tissue in early plants

Summary Comparison of the vascular anatomy of the earliest known vascular plants (Upper Silurian/ Lower Devonian) with the functional anatomy of extant vascular plants permits the following conclusions (excluding previous analyses of xylem conductivity and biomechanics). The absence of an endodermis from early vascular plants may have hindered the regulation of the supply of soil-derived nutrients to the shoot. The absence of intraxylary parenchyma in early vascular plants may have interfered with the differential distribution of soil-derived nutrients among aerial branches. The earliest vascular plants, like extant plants, lacked gas spaces in vascular tissues, so that the early plants could have shared with extant plants some protection from the impact of embolism of xylem and, less importantly, reduce O2 damage to ‘phloem’. Early vascular plants also lacked the lignified UV-B screen found around the phloem in some extant plants, although this was probably not a serious lack in their relatively short-lived, nucleate phloem cells. An early vascular plant with a main axis and laterals shares with extant plants a xylem constriction at the base of lateral branches, which favours survival of the main axis during water shortage.