Advanced vascular function discovered in a widespread moss The evolution of terrestrial plants capable of SUCCESS OF VASCULAR PLANT EVOLUTION growing upwards into the dry atmosphere The evolution of features capable of maintaining photosynthetic profoundly transformed the Earth. A tissues hydrated and functional in a dry atmosphere is considered transition from small, ‘non-vascular’ critical for explaining the success of vascular plant evolution. However, what are these fundamental engineering requirements bryophytes to arborescent vascular plants for maintaining photosynthesis aloft in the atmosphere? Classical during the Devonian period is partially theory emphasizes the evolution of a robust water transport attributed to the evolutionary innovation system (hence the characterization of “vascular plants”) yet the key underpinnings of these theories remain untested. Here of an internal vascular system capable of we examine one of the most productive living representatives functioning under the substantial water of the primitive “non-vascular” plants (Poytrichum commune, tension associated with vascular water a bryophyte species) to test the dogma linking plant success to vascular evolution. transport. Here we find that classical assumptions about the inability of non- HYPOTHESES vascular plants to transport water using a During the last century, it has been proposed that the following powerful suction are incorrect, highlighting three characteristics were unique and indispensable for vascular plants [2]: (i) a lignified water transport system conducting in particular that Polytrichum's vascular water to photosynthetic structures, (ii) photosynthetic tissues ducts resist buckling and cavitation while encased in impermeable cuticle, and (iii) adjustable stomatal transporting water under tension. valves regulating CO and water vapor exchange with the2 atmosphere. Like their common ancestor with vascular plants, we hypothesize that bryophytes are devoid of these vascular EVOLUTIONARY HISTORY OF PLANTS plant components, confining their photosynthetic activity to 400 million years ago a profound change in the engineering ofmoist microclimates close to the substrate. The moss family plants enabled the “vascular plant” group to grow photosytetcPolytrichaceae represents an exceptional group of bryophytes, structures upwards into the atmosphere [1]. This plant evolutionaryequipped with well-developed water-conducting cells (hydroids) transition from green mats to 3-dimensional forest transformedand includes species with the capacity to grow >200mm in the planetary surface of Earth into the vegetated state we knowheight. These qualities provide an excellent system in which to today. Mysteriously, only this globally dominant “vascular plant”investigate the prerequisite elements necessary for the evolution clade ever evolved an arborescent stature. of areal photosynthetic tissues. FIGURE 1 50