464 Profs. Percy Frankland and Marshall Ward. 



The external factors are: (1) temperature. Variations in the 

 curve are produced by sudden variations in the temperature, and, 

 apparently, the variations are the more pronounced the quicker the 

 temperature changes and the more extensive their range ; but the 

 amount of variation in the curve, due to any given rise or fall of 

 temperature in constant time, appears to depend on the distance of 

 the temperature (from which the variation is reckoned) from the 

 optimum. In other words, the sensitiveness of the organism to a rise 

 or fall of a degree centigrade varies according to the temperature 

 from which the rise or fall occurs, for, if it has been growing at 30 C., 

 constant temperature, for an hour, it shows a more marked deflection 

 on the curve for a sudden rise or fall of 1 C. than for the same 

 sudden rise or fall from 25 C. 



That the variation in rate of growth which has been going on at 

 any hitherto constant temperature is more pronounced when the rise 

 or fall is 2 C. than when it is only 1 C. will be obvious, and similarly 

 for any other range ; but, again, it must be noted that the amount of 

 deflection of the curve for any range of variation depends on the 

 mean temperature, or the hitherto constant temperature, at which 

 the growth has been going on. 



The practical importance of all this on experiments on light is very 

 great, and the difficulty, amounting almost to impossibility, of 

 arranging two cultures differently illuminated (if the sunlight is at 

 all intense), so that each shall be growing at exactly the same tem- 

 perature during the observations, limits the absolute value of the 

 determinations, so Far as the curve of growth is concerned. 



But there are other points besides (1) the actual temperature when 

 constant, (2) the suddenness of any rise or fall, and (3) the range of 

 temperature, referred to above. 



As we have seen, the time during which the organism is exposed 

 to any given (constant) temperature above the optimum affects the 

 growth considerably. The general result of the observations is that 

 the higher the temperature is above the optimum the more rapidly the 

 organism completes its _/" -shaped curve of growth i.e., the quicker 

 it passes through the phase of maximum rapidity of growth into that 

 of cessation and consequently the smaller the crop produced from a 

 given amount of food-materials. 



For instance, if the spores germinate out at 22 C., and are made 

 to complete their growth at 30 C., the amount of growth (or, in 

 other words, the crop produced) will be much greater if the exposure 

 to 30 is delayed till the eighth hour after sowing than if it is made 

 at the sixth hour. In both cases the growth at the higher tempera- 

 ture is at maximum rapidity at first, and then rapidly declines. 

 Suppose two cultures at 22 C., the spores having been sown at 

 12.0 noon : one is put in at 30 at the sixth hour (6 P.M.) after sowing, 



