PART TWO. 



A BASIS FOR THE MEASUREMENT OF DEVELOPMENT. 



Former Methods of Estimating Progress in Life-History Stages. 



Formerly investigators have relied either upon natural phenomena 

 showing the seasonal progress of plants, c. g.. time of leafing, budding, or 

 blossoming, or upon ajjproximate accumulation of temperature as an 

 indication of opportune times for the performance of certain agricultural 

 operations, such as planting, spraying, and harvesting. For the greater 

 part of a century they have assumed that temperatures above the freezing 

 point or above the point at which a plant such as wheat starts growth, 

 can be used directly to ascertain the amount of progress made by plants 

 and animals at a certain date in the spring. Numerous investigators have 

 tried temperatures above various "starting points," some using sun tem- 

 peratures, others maximum temperatures or mean temperatures ; and 

 practically all have considered that the accumulated temperature, or "sum 

 of temperatures" above a starting point, is a measure of plant or animal 

 growth. This sum for a given period is obtained by adding together the 

 degrees by which each day's mean temperature exceeds the assumed 

 starting point. For many years the meteorological office of Great Britain 

 has used 42° F. as the starting point and published the monthly accumu- 

 lations above this temperature for various parts of the British Isles. A 

 mean temperature one degree above 43° F. continuing for a day has been 

 called a "degree-day" or a "day-degree." 



Various Europeans have carried on careful critical studies employ- 

 ing various detailed methods of determining the total accumulated tem- 

 perature necessary to bring a given plant into bloom or to ripen a crop 

 of grain. This total, however, was found to vary so greatly for the same 

 stage of development of the same variety of plant from season to season 

 and from year to year that there was little or no progress in the field 

 until the Danish physiologist Krogh ('14), while working on the effect of 

 temperature on the development of fish eggs and of frog eggs, made the 

 most important discovery on this subject in the present century, viz. : that 

 development goes on slowly even at temperatures below that commonly 

 considered as the starting point ; and that, as the temperature rises, the 

 time required to hatch an egg decreases to a minimum at a certain high 

 point, above which the time again increases. 



Glenn ('22), in his work on the codling moth, confirmed the finding 

 of Krogh relative to high temperatures (above 90°F.). He was first to 

 make corrections for the retarding effects of high temperatures. With 

 his correction applied, the accumulated temperature, or "sum of tempera- 

 tures," for the stages of the codling moth, varied much less from season 

 to season than the totals for the stages of European plants referred to 

 above. Wherever only temperature records are available, his work 

 affords a basis for estimation. 



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