plant grows fifteen to twenty times as rapidly at its optimum temperature 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 exactness, and it was finally decided that the actual activity of some plant should be used as a standard of measurement, as the effect of temperature 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 temperatures 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 temperature definitely checks growth, which ceases entirely at temperatures of 108° F.
This plant was fixed upon because it is widely grown from subtropical 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 temperature 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 by a
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.
