MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 311 



and 43° C, in the plant which has been cited, with a final limit of 

 temperatures of the body of 51.5° C. in some other extreme cases. 



Growth of etiolated shoots of a nearly related species in a dark 

 room was i mm. or less per day in members at body temperatures 

 of 15° C. or 16° C. Rates of 2 to 2.6 mm. daily at 16° C. to 18° C. 

 were followed by 8 to 12 mm. daily at 2y° C. and 28° C. yielding 

 values of 3 to 4 mm. for a rise of 10° C. Rates of 5.6 to 7.8 mm. 

 daily at 24° C. and 25° C. being compared with 8.4 to 10.2 mm. daily 

 at 29° C. to 2,2° C. show a similar coefficient at 29° C. to 31.5° C. 

 The meager records at 35° C. and 36° C. yield rates of 10.2 to 13.2 

 mm. daily. Observed rates at temperatures above 32° C. or 33° C. 

 in the shoot showing such rates were not readily to be integrated 

 with these results, and growth ceased at 41.5° C. in the shoot yield- 

 ing them. 



The second shoot of the same plant showed rates of .85 to 1.2 

 mm. daily at 16° C. to 18° C. ; 2.9 to 3.4 mm. at 26° C, and 13.2 

 mm. daily at 35° C. ; 20.4 mm. daily at 46° C, and 18.5 mm. daily 

 at 48.5° C. 



The highest observed rates, both in green plants and in etiolated 

 shoots, were those immediately preceding cessation of growth; a 

 daily occurrence in plants exposed to normal sunlight. 



Accepted conclusions as to growth include an optimum at which 

 growth proceeds continuously at a high rate, and above which the 

 rate is higher for a brief period then falls off. Some of the records 

 are conformable to such ideas and others are not. The two shoots 

 of the same plant subjected to the same treatment did not agree in 

 this matter, as may be seen in the preceding pages. It is conceded 

 that our experiments were not arranged to bear critically on this 

 point. It is to be noted that growing shoots in the open may cease 

 to elongate at temperatures as low as 26° C. which would be below 

 any optimum hitherto suggested. Hundreds of observations of 

 such cessations under external conditions supposedly favorable to 

 continuous growth are available. The facts in question seem to 

 lessen the importance and the usefulness of the term optimum tem- 

 perature. 



The results of measurements of growth of the apical part of the 



