Given the diversity of grasses and the habitats they occupy, it was predicted that a broad survey of root anatomy would reveal a range of variation. A comparative survey of trans-sectional root anatomy of the mature stele in the Poaceae, sampling 76 species representing all 12 subfamilies from herbarium and fresh-fixed specimens has been undertaken. The species are a subset of the 157 species of grasses for which plastomes (plastid genome), leaf shape data, and leaf anatomical data are available. Quantitative and qualitative characters including endodermal cell radial thickness and inner-tangential wall thickness, stele area, and metaxylem vessel diameters were measured. Additionally, some characters, such as the number of endodermal and pericycle cell layers, were scored, and some ratios, such as total metaxylem vessel area to stele area, were calculated. This study represents the first comprehensive survey of trans-sectional root anatomy in grasses in a phylogenetic context, to document existing variation and to better understand the functional significance of anatomical variation in the evolution and diversification of this ecologically successful and economically important family.

More variation than expected from relatively sparse reports in the literature was uncovered and two unique features, internal phloem and an apparently multiseriate endodermis, were documented. Internal phloem is confirmed in a few taxa of the Bambusoideae, as well as newly reported in a few species in the Micrairoideae. An apparently multiseriate endodermis (of up to seven cell layers) was found in many species in several subfamilies; the presence of a Casparian strip in any of the layers was not confirmed, but the characteristically asymmetrical cell wall thickening was similar or identical among layers. However, there were a few taxa that exhibited an apparent multiseriate endodermis with varied asymmetrical cell wall thickening among the layers. Characters were coded and tested for phylogenetic signal, photosynthetic pathway signal, and potential functional correlations with climatic factors. Characters showed no phylogenetic or photosynthetic pathway signal but some aspects of root anatomy showed an ecological (moisture, precipitation, and temperature) signal. Anatomical variation, especially in the complete root, should be documented in pursuit of the potential for cereal or other crop development, in the context of global climate change.