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SUMMARYProgressive suberization and thickening of the endodermis along secondary seminal axes of Zea mays had little effect on the radial movement of phosphate into the vascular tissue. In all parts of the root, inhibitors of respiration, and low temperature, reduced uptake and xylem translocation of phosphate by more than 90%. With calcium, however, there appeared to be two components of the translocated fraction; one was sensitive to metabolic inhibition and was most prominent in young tissue near to the root tip, while the other was insensitive to metabolic inhibition. In older zones near the base of the root, where there was little calcium movement into the stele, only the second insensitive component of calcium transport was found.A marked maximum in calcium translocation, but not in the translocation of phosphate, was found 12 cm from the root tip. This was not associated with a marked increase in total absorption but was due to a high proportion of the absorbed calcium which moved into the stele. This region is one where lateral roots are initiated in the pericycle and where the structure of the endodermis may change transiently.At the extreme base of the roots the development of a suberized hypodermis appeared to restrict very severely the translocation of both phosphate and calcium.Iron and manganese were also absorbed and translocated by all regions of the root examined; the proportion translocated was much greater for manganese than for iron.

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The patterns of uptake and translocation of magnesium in different regions of the root are very similar to those of calcium. Once the endodermis has become suberized translocation of either ion to the shoot is greatly reduced and it is concluded that magnesium, like calcium, appears to move across the root cortex largely in the free space.