1. The total nitrogen content and the total phosphorus content of the three apical 5-mm. segments of the onion root were determined both before and after respiratory measurement. The most apical segment (meristem) showed the highest levels of both nitrogen and phosphorus. 2. The inorganic phosphorus content of the root segments was also determined in some cases. A series of these determinations was made when the roots were sprouted in growth solutions containing different quantities of phosphate. 3. The gas exchange (oxygen uptake and carbon dioxide output) was measured on these segments and is recorded on the basis of wet weight, individual root segment, dry weight, milligrams of total nitrogen, milligrams of total phosphorus, and in some cases milligrams of organic phosphorus. The root apex showed significantly greater activity except when recorded on the basis of total nitrogen content and of both total and organic phosphorus content. For example, oxygen consumption reported on the basis of wet weight showed the 0-5-mm. segment to be 3.3 times as active as the more basal segments; on the basis of individual root segments, the apex was 3.4 times as active as the other segments; and even on the dry-weight basis the apical segment was 1.5 times as active as the more basal segments. These ratios decrease to 1.1 for values reported on the basis of nitrogen content and a maximum of 1.3 for values recorded on the basis of phosphorus content. Thus the respiratory gradient along the onion root axis decreases significantly when gas exchange is reported on the basis of tissue nitrogen or tissue phosphorus.