308 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



12, 



The behavior of green opuntias in dayhght was tested in March, 

 1917, at Tucson. Preparations consisting of a rooted joint from 

 which a flower bud was arising were placed in the south end of a 

 glass house in an equatorial position. The temperature of the body 

 rose to 40° C. and 43° C. by the heat of the sun after i P.M. Addi- 

 tional heat was supplied by tungsten incandescent lights so that 

 the temperature was raised to 49° C. in an hour at which point 

 elongation ceased. The temperature following same rising curve 

 reached 51.5° C. a half hour later at which elongation was resumed, 

 and was maintained at temperatures of 51° C. to 51.5° C for an 

 hour and a half when it ceased. This behavior is in accordance 

 with that of etiolated shoots illustrated in Fig. 2. On the follow- 

 ing day the temperature near midday, which was above 40° C. by 

 the sun's heat, was raised to 48° C. and 49° C. for a half hour 

 by additional heat from a tungsten incandescent light bulb. Growth 

 continued at a rate near the maximum. In an additional prepara- 

 tion a bulb for heating not regulated properly raised the tempera- 

 ture of a portion of the joint 75° C. for a few minutes, resulting 

 in the death of a sector within the next two days. The young shoot 

 arising from the margin of the injured area probably reached a 

 temperature of 65° C. or 70° C. as some of the outer leaves were 

 blackened. Growth was checked at once but was resumed eighteen 

 hours later and continued for two days with the customary mid- 

 afternoon shortening. 



The gas interchange and variation in the concentration of the 

 residual acids has been worked out in detail in Opmitia versicolor. 

 Some available data show that the platyopuntia used so extensively 



