White mustard roots when treated with a slightly alkaline solution of silver nitrate (Tollen's reagent) show, on sectioning the meristem, the accumulation of small black stained particles in the intercellular spaces and intervening cell walls. The production of these particles in the intercellular spaces of treated roots and their complete absence in those of untreated roots indicates the presence of a substance capable of reducing alkaline silver. The presence of this substance strengthens the viewpoint that in the meristem radiating rows of intercellular spaces in white mustard roots provide for the more rapid movement of soluble materials from differentiating vascular strands to the outside. Thus cell position as related to a more accessible food supply is an important adjunct to the general problem of cell development and cell behavior.