1. Data are presented which support the conclusion of Emerson (1929) that the rate of photosynthesis is proportional to the chlorophyll content when the latter is varied by varying the iron supply. These data give a straight line passing through the origin, which is not true of Emerson's results.
2. Similar data are presented which show that a similar relation exists when nitrogen controls the chlorophyll content.
3. Evidence is given which indicates that magnesium plays a part in the process of photosynthesis in addition to its effect upon the chlorophyll content.