56 CARNEGIE INSTITUTION OF WASHINGTON. 



brought into the chemical reactions and by the concentration of the 

 products of the reactions. The rate and amount of expansion of a 

 growing organ depends upon osmosis and imbibition, which are modi- 

 fied in a manner not closely conformable to the van't Hof law. A 

 statement of definite temperatures for any of the critical points will 

 laclv final value unless given as the resultant of a formula expressive 

 of the intensity of the contributory factors. Attention has been paid 

 to the upper and lower limiting temperatures of growth. It might be 

 possible in either case to find some theoretical point where one of the 

 processes would be stopped absolutely with all of the other contribu- 

 tory factors at an optunal intensity. It does not follow, however, 

 that growth mil begin when a rising temperature passes the lower hmit, 

 or that it will carry on to the upper limit. 



Extensive tests and measurements of about 40 growing shoots of 

 Opuniia in their native habitats showed that the temperature of the 

 body at which enlargement might begin ranged from 10° to 25° C. 

 Growth ceased under varying conditions at temperatures ranging 

 from 26° to 43° C. 



The rates of growth of an etiolated shoot of Opuntia during a month 

 in a dark room is illustrated by the following excerpts from the notes : 



Varying rates of 1.2, 1.2, 1.15, 0.85, and 1.15 daily at 16 to 18° C. 



Increasing rate of 2.9, 3, 3, 3, 3.2, and 3.44 mm. daily at 26° C. 



Increased rate of 3.6, 6.7, and 9.6 mm. daily at 30° C. 



Increased rates of 11.4, 11.4, 8.4, 9.2, and 11.4 mm. daily at 31.5° to 32° C. 



Decreased rate of 5.3 to 5.7 mm. daily at 18° to 19° C. 



Increasing rate of 8.4 to 16.8 mm. daily at 39° C. 



Decreasing rate of 5.4 to 9 mm. at 39° to 40° C. 



Increased rate of 13.2 mm. at 35° C. 



Increased rates of 16.8 to 19.2 mm. daily at steady temperature of 38° C. 



Decreased rate of 13.2 mm. daily while temperature rose from 38° to 45° C. 



Cessation of growth at 45° C. 



Resumption of growth at 46° C. 



Increased rate of 25 mm. daily during the first hour at 46° C. 



Decreased rate of 20 mm. daily for 4 hours at 46° C. 



Increased rate of 20.4 mm. daily during an hour at 45° C. 



Decreased rate of 18.5 mm. daily during hour temperature was raised to 48.5° C. 



Cessation of growth at 48.5° C. 



Resumption of growth at rate of 19.2 mm. daily at temperature" of 48° and 46° C. 



Growth of 15 mm. daily as temperature rose to 47.5° C. 



Cessation of growth at 49° C with air-temperature 40° C. 



Resumption of growth after 20 minutes at 49° C, but soon stopped. 



Temperature of plant 52° C. for half an hour; air 43° C. 



Resumption of gro\vth at 49° C. with air at 41° C. at rate of 9.6 mm. daily. 



Similar results with many plants at high temperatures were obtained. 



The above records show a gradual progression of the rate as the 

 development of the shoot proceeded. The comparison of the rate at 

 any point, as for example, at 16° to 18° C, is made with the next 

 observed rate at 25° or 26° C. to obtain the temperature coefficient. 

 The advanced stage of development when the temperature again 



