216 Variations in Growth-Rates [414 



graphically they give only comparatively slight variations 

 from a smooth curve and the actual graph may readily be 

 smoothed to give such a curve. After this has been done the 

 ordinates of the smoothed curve, corresponding to the various 

 culture periods, may be measured, and the series of graphi- 

 cally derived values thus obtained may be taken as a tentative 

 scale to indicate approximately the relative growth-rates to be 

 expected for this plant in these general surroundings. Of 

 course, the seasonal march of the climatic conditions in this 

 particular greenhouse must be expected to vary from year to 

 year, and it surely varies from greenhouse to greenhouse; 

 nevertheless, the tentative scale derived as just described may 

 be of value in several ways. 



For the first sixteen four-week periods of the present 

 study, beginning with Feb. 14, as given in the table pre- 

 sented above, these relative seasonal indices of growth-rate 

 (by either dry weight or leaf area, which appear to be propor : 

 tional, or by their average) are respectively as follows : 61, 71, 

 79, 86, 91, 96, 99, 100, 99, 96, 92, 87, 81, 75, 68, 61. In this 

 scale of growth-rate values the maximum (100) occurs for 

 the period ending June 19, and it represents actual average 

 growth-rates, as obtained in this study, of 1.24 g. of dry 

 weight and 209 sq. cm. of leaf area (one surface only), per 

 plant, per period of 28 days. While these derived results 

 are extremely tentative and probably only very roughly ap- 

 proximate, it is clear that we have here a new kind of descrip- 

 tion of the climatic conditions of this greenhouse for the 

 spring, summer and autumn of 1916, these conditions and 

 their seasonal march being described in terms of their ability 

 to produce dry material and leaf surface in the standard plant 

 here employed. 



By such a method as this the climatic plant-producing 

 power for any four-week period may be directly compared 

 with that of any other similar period, no matter when or 

 where these periods occur, the standard plant being used as an 

 automatically integrating instrument for the measurement of 



