ENVIRONIC FACTORS 



429 



plant grows fifteen to twenty times as rapidly at its optimum tempera- 

 ture as it does within ten or fifteen degrees of its zero or minimum. 

 The nature of the experiments upon environic effects, for which some 

 method of temperature evaluation was necessary, demanded greater ex- 

 actness, and it was finally decided that the actual activity of some plant 

 should be used as a standard of measurement, as the effect of tempera- 

 ture upon growth is one in which chemical equilibrium is disturbed in 

 a score of ways and is therefore not expressible by any single or simple 

 formula. This will be obvious upon the inspection of the graph which 

 shows the relation of temperature to the growth of the hypocotyle of 

 wheat plants determined by measuring the rate at constant tempera- 

 tures for 48 hours (Fig. 6). 



From this it may be seen that growth of the stems takes place at a 

 rate of about 4 to 6 mm. in 48 hours at temperatures between 40° and 

 65° F. Above this the rate rises precipitously until the temperature 

 reaches 80° F., and if it becomes warmer than this a drop ensues during 

 the next few degrees of rise, then the increase is resumed and carried 

 until a temperature of 86.5° F. is reached. Any further rise in tem- 

 perature definitely checks growth, which ceases entirely at temperatures 

 of 108° F. 



This plant was fixed upon because it is widely grown from sub- 

 tropical to subarctic localities, reliable measurements have been made of 

 its rate, and it may accompany nearly all of the experimental cultures 

 made in our researches. It is proposed, therefore, to integrate the tem- 

 perature factor in climate in terms of growth of wheat. Any other 

 suitable species might be used as well. The scheme in brief consists in 

 fixing upon an averaged rate of growth between 40° and 65° F. and 

 then for the five-degree intervals up to the optimum and upper limit. 

 The sheet in which the thermograph has made its tracing of 

 the course of the temperature is now ruled into figures bounded bv a 



SATURDAV 



Fig. 7. Thermograph Chart Ruled for Integration in Terms of Rate of 

 Growth. The areas of the separate figures are to be determined by a planimeter and 

 multiplied by the factor expressing the rate of growth prevalent during the period 

 covered by the figures. Record from Coastal Laboratory, June 16-23, 1913. 



