The exudation rates of fluid and potassium ions from isolated maize roots were determined before and after excision of certain lengths of root tip. The results of this study suggest that excised maize roots possess the ability to absorb potassium (and presumably chloride) ions and concomitant amounts of water over a considerable distance (10 cm) from the tip. Moreover, the observed power of absorption of ions and water into the translocatory pathway decreases in passing from the tip towards the base of the root. Both light and electron microscope techniques were used to examine the anatomy of primary roots similar to those used in the physiological experiments. The principal observation was that the xylem vessels near the root tip contain membrane-bounded cytoplasm with organelles. The number of mature xylem vessels, i.e. without cytoplasm, progressively increased in transverse sections cut from 1 to 10 cm from the root tip; above 10 cm from the root tip all of the xylem vessels were found to be completely mature. It is evident that prima facie a connexion exists between this single aspect of root anatomy and fluid exudation from excised roots. The uptake of tritiated water by roots and its transport into exudates was examined. These data were analysed on the assumption that the exchange of external labelled water with the exudate was achieved by the fluid exudation itself; this analysis indicated that an operational volume, similar to that of the total xylem volume within the root, must become labelled during the formation of the exudate.