The major conclusions reached in my study are : 1. Through increased daily mean temperature, delayed planting affects the rate of accumulation of growing degree days. This results in changes in both rate of development and duration of growth of oat plants, and influences plant morphological and yield component characteristics. 2. Although changes in rate of development and duration of growth influence plant morphological characteristics and yeild component traits, no direct association occurs between yield and changes in either plant morphological characteristics or yield component traits caused by rate and duration changes. Neither does a direct association occur between yield and the accumulation of growing degree days. 3. The growing degree day method of accumulated thermal units more adequately predicts the rate of development and duration of growth in oats then does number of days. 4. Environmental factors other than temperature place serious limitations on the accuracy of the growing degree day method for predicting growth and development, and incorporation of these factors into growing degree day calculation might improve its predictive value. 5. The use of growing degree days for classifying oat cultivars into maturity groups has potential value; however, additional testing is recommended.