It has been the practice in measurements of low temperature resistance to expose plants to continuous low temperatures 1 for a certain number of days and then to determine their hardiness by freezing them at constant temperatures considerably below the freezing point of the tissues. By making several tests at a series of freezing temperatures, comparisons can be made between the relative hardening capacity of varieties and also of the rate of acquiring hardiness. If a single uniform variety of plant is used, a comparison can be made also between the effectiveness of different treatments for hardening plants. In testing the relative effectiveness of various temperatures and time exposures in evoking hardiness, cabbage plants were used because they can be hardened easily, the evidences of freezing injury are readily seen, and varieties with known and relatively uniform hardening capacity are available. The plants were exposed at 20? C., IO' C., 50 C., and o0 C. for five days continuously, and then the hardiness was determined by freezing for twelve hours at various constant temperatures, 40 C., 50 C., and 70 C. When the critical freezing temperature had been determined, it was used for the test and the relative amount of injury after freezing at this temperature was recorded and used in photographs, because this injury criterion gives a more accurate comparison between treatments than the determination of the freezing temperature which is just sufficiently low to give one hundred percent killing of the plants. Furthermore, the critical killing point requires much more experimentation for its accurate determination. Early Jersey Wakefield cabbages were grown to a height of about four inches in four-inch pots. They showed good uniformity in size and rate of growth; they also gave uniform performance in hardening trials. Five plants were taken for each exposure treatment at 200, IO, 50, and o0 C. continuously for five days. Also similar sets of cabbages were alternated between various high temperatures and o0 C., being kept twelve hours at o0 C. and twelve hours at the higher temperatures, IO0 and 200 C. This alternation of the temperature exposure of the plants was accomplished by moving the plants between rooms kept constantly at 00, 50, IO0, and 200 C. 'Harvey, R. B. Hardening process in plants and developments from frost injury. Jour. Agr. Res. 15: 83. I9I8. . Varietal differences in the frost resistance of cabbages and lettuce. Ecology 3: