In the design of experiments where plants are to be grown under controlled radiation conditions, it is necessary to have information on the relative power efficiency and the type of growth obtainable with various lamp sources, for the proper selection of equipment. It is well known that the growth form of plants is dependent upon the incident irradiance, photoperiod, and the spectral characteristics of the radiant energy (1, 2, 11, 14, 15,16). Before the introduction of the gaseous discharge lamps, most of the studies were made using the incandescent tungsten filament lamp as a source (7, 9,14). While the growth rate obtained with the bare incandescent lamp frequently was comparable with that often secured under greenhouse conditions when sufficiently high irradiances were used, the plants usually had thin stems and longer internodes than those grown with solar irradiation. With the advent of the modern type gaseous discharge lamps, Roodenburg (17), Arthur.and his co-workers (1, 2, 3), and others studied the growth obtainable with these sources. Arthur and Stewart (1) observed production of buckwheat plants under incandescent, neon, mercury, and sodium vapor lamps. They found that, with equal irradiances as measured with a Weston illuminometer, the efficiency in production of dry matter decreased in the following order : neon, incandescent, sodium, and mercury. When the radiant energy was calculated on an equal energy basis, they concluded that the actual order of efficiency was sodium, neon, incandescent, and mercury. When sodium alone was tried (2), it was found that the plants did not grow well over long periods of time. Supplementing the sodium with two hours per day from an 85-watt capillary mercury vapor lamp, caused an improvement in growth with a number of varieties of plants. Mitchell (12) grew tomato plants, using a carbon arc and an incandescent filament lamp as sources. He found that total synthesis of dry matter was greater per unit of time under the incandescent lamp when equivalent energies, as balanced with a Weston cell, were used. The plants synthesized more dry matter, although the height was less, under the carbon arc when equal total radiant energies, as measured by a nonselective detector, were used. ? Journal Paper no. 272, Purdue University Agricultural Experiment Station.