In view of the growing practice of testing crop plants for cold endurance under controlled conditions, the following studies with winter wheat at the Nebraska Agricultural Experiment Station may be of interest in connection with the technic of such investigation. These data were secured in connection with a wheat breeding project which has been reported elsewhere. The control equipment described by Peltier was available for the freezing of these plants. In part these tests correspond with those reported for alfalfa by Peltier and Tysdal.

Winter wheat seedlings in four 1-week interval stages of development from emergence to past tillering were compared for cold tolerance. Very young seedlings, presumably still dependent upon the endosperm to a considerable degree were found to excel all other more advanced stage of development groups in cold tolerance, whereas seedlings emerged from 7 to 12 days prior to controlled hardening and freezing, and probably on the verge of endosperm independence, were least cold tolerant. Hardening by exposure to a constant or an alternate (warm day and cold night) temperature near freezing revealed that actual cold resistance increased materially from exposure up to three weeks and none thereafter up to four weeks. The ranking of varieties was not affected by the duration of the hardening periods employed. Differences in varietal cold resistance relationships are indicated from (a) dissimilar growing temperatures or (b) dissimilar ages and stages of plant development at the time of initiating uniform hardening and freezing. These results, together with the seasonal effects reported in a previous paper (6), all point to the great importance of factors other than exposure to low temperatures and resultant high specific cold tolerance as determinators of the relative resistance of winter wheat varieties to cold.

Daily measurements were made of the increase in the height of the plants at the tip of the spike and the ligule of the upper leaf during the heading period of 18 varieties of winter and spring wheat grown at Moscow, Idaho, during one or more of the 3 years, 1931 to 1933, inclusive. Wheat culms elongated rapidly during the heading stage, the rate of elongation increasing with the temperature. The maximum daily elongation during the heading period of varieties of common and club wheat ranged from 4.3 to 9.1 cm. The average increase in the height of the culm after the tip of the heads emerged above the ligule in the common and club wheats was 30.6% of the final height of the plants in 1931, 35.6% in 1932, and 39.4% in 1933. No significant varietal differences were observed except those clearly associated with the final height of the plant. The culm continued to elongate for 5 to 12 days after the tip of the head emerged above the ligule. No consistent relations were found between increase in height after heading, final height, exsertion of the head, or type of growth curve on the one hand and adaptation on the other. There were some indications that the amount of growth during the heading stage was less variable from season to season in the better adapted varieties, but this observation needs further verification. The time required for the spike to exsert its full length from the boot was 2 to 4 days in the common wheats, 1 to 3 days in the short-spiked club varieties, and nearly 4 days in White Polish wheat, which has very long spikes. Elongation of the wheat culm continued as long as blossoming continued and little or no growth occurred after blossoming was completed. The average blooming period for individual culms of the wheat varieties ranged from 1 to 3 days in 1932 and from 3 to 6 days in 1933.