The two types of nodal roots of field-grown maize, sheathed and bare, were found to have such different water conductivities that an investigation of the anatomy of their large metaxylem vessels was made. While the vessels of the bare roots were open for scores of centimetres, those of the sheathed roots were found to be not vessels but developing vessel elements, with cross walls at I mm intervals, and protoplasts. The cross walls between the elements had several unique histochemical properties. Previous investigators have often failed to find the cross walls because they are very easily dislodged during the usual methods of tissue preparation. They are best identified by microdissection of fresh xylem. The living elements persist in the late metaxylem up to 20 – 30 em from the tip. As the roots become longer than this both the cross walls and the soil sheaths disappear and there is a transition to a bare root with open vessels in the proximal region. The soil sheath persists a little longer than the cross walls. The two types are thus stages in a developmental sequence through which all nodal roots pass. A fundamental difference between the two types is in their water status, since the estimated conductive capacity of a bare root is about 100 times greater than that of a sheathed root. These observations point to the need for a reassessment of the published work on transport of ions into the xylem of grass roots through a reinvestigation of the ‘maturity’ of their xylem vessels.