376 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



temperature became constant made an incision near one of the needles. In a par- 

 ticular case the temperature near to the fresh injury was higher than that at the point 

 of insertion of the other needle by 0-09 C. after 2 hours, 0-19 C. after 4^ hours, 

 0-31 C. after 8^ hours, 0-2 iC. after 1 2^ hours, and 0-02 C. after 40 hours, while 

 towards the end of the fourth day the difference of temperature was imperceptible. 

 The curves of respiration and heat-production are in this case very similar, the flatter 

 character of the latter curve being due to the rapid loss of heat by conduction and 

 radiation. The uninjured potato was o>i6C. warmer than the surrounding air, so 

 that the temperature in the injured region underwent a twofold increase. The pro- 

 duction of heat depends upon the respiration, and hence on cutting an average potato 

 into quarters the production of heat increases approximately tenfold. The whole 

 increase takes place in the tissues immediately bordering the injury, so that these 

 must respire with remarkable activity. 



Little is known as to the detailed course of the grand period of heat- 

 production and whether it exhibits secondary maxima or oscillations is 

 unknown. Daily variations of the excess of temperature do occur in 

 plants showing a marked production of heat, and Dutrochet observed slight 

 oscillations in shoots and fruits l . 



The existence of a daily periodicity in the warming of the spadix of Aroids has 

 been shown by Kraus and by the authors already quoted. Kraus has shown that the 

 same applies to the inflorescences of Cycads and Palms, and Knock to the flosvers of 

 Victoria regt'a 2 . 



Observations in the open seem to show a periodicity in the production of heat 

 independently of the air-temperature, and the same was shown under fairly constant 

 conditions by a plant of Colocasia odor a (Arum cordifolium) kept in a room at 1 7 C. by 

 van Beek and Bergsma 3 . Thus in the selected zone of sterile male flowers the rise of 

 temperature above the surrounding air was io-6C. at 2p.m., 14-7 C. at 5 p.m. of the 

 next day, 20- 2 C. at 2.30 p.m. of the third day, and ni C. at 2 p.m. on the fourth 

 day. Each morning the excess-temperature lay between i-3oC. and 5C., it rose to 

 a maximum during the day and fell to the morning temperature at evening. 



The maximum may be earlier or later on some days than on others, and 

 although it usually occurs during the daytime may also appear early in the morning or 

 during the evening hours. The spadices of Arum maculatum and A. italicum show 

 only a single pronounced period of heat-production, but, according to Dutrochet 4 , both 

 before and after this a feebler daily periodicity is shown. Here, as in the case of most 

 of the shoots and fruits used by Dutrochet, the excess temperature observed was 

 usually less than 0-3 C. 5 The maximal excess temperature observed in air saturated 



1 Dutrochet, Ann. de sci. nat., 1840, 2 e ser., T. xin, p. 41. 



3 G. Kraus, Ann. du Jard. hot. de Buitenzorg, 1896, T. xin, p. 217 ; Knoch, ibid. 



! The irregularities observed by Hoppe (1. c., p. 239) in the rise of temperature in the spadix of 

 Arum were due to changes of temperature in the surrounding air. These may also exercise a stimu- 

 lating action, for Kraus (1. c., 1884, p. 52) found that the spadix is very sensitive to external agencies. 



* Dutrochet, 1. c., 1840, p. 66. G. Kraus (1. c., 1884, p. 81 ; 1882, p. i) used an ordinary 

 thermometer, and hence was unable to detect any of these small oscillations. 



5 According to J. Schmitz (Ueber die Eigenwarme der Pflanze, 1870, p. 20) the buds of Aesculu 

 hippocastanuw show a daily periodicity of heat-production. 



