406 LECTURE. XXV. 



The spontaneous evolution of heat is very easily observed in a most instructive 

 manner in the inflorescence of the Aroideae. Nature has here brought together 

 in a confined space a large number of very actively respiring flowers. The 

 rise of temperature of such inflorescences, especially when large, amounts to 

 4 or 5 to 10 or 12 or even 15 and more degrees Centigrade, and therefore may 

 be perceived by the senses even vi^ithout the thermometer. Older observers had 

 already made close observations on these exceptionally favourable objects as to 

 the relation between the respiration of oxygen and the evolution of heat: we owe 

 the most exact of these to Garreau, who has done much for the theory of respiration. 

 He found in the inflorescence of Arum Italicum, for example, a spontaneous heating 

 of 3'2°C.; I IT c.cm. of oxygen being respired by i gram of substance in one hour. 

 The same spike showed a spontaneous heating of 8-3° C, i gram of the spike con- 

 suming 28-5 c.cm. of oxygen (i.e. converting it into carbon dioxide) in one hour. 

 These examples at any rate show that the respiration of plants may under certain 

 circumstances reach an intensity which may be compared with that of warm-blooded 

 animals. 



In large single flowers like that of Victoria regia, as well as in the flowering- 

 spikes of Aroideae, we find during the period of spontaneous evolution of heat, first 

 a rise of temperature up to a maximum, and then a decrease in the spontaneous 

 evolution of heat, evidently in consequence of the advancing development. Observers 

 give in addition also periodic oscillations of the spontaneous evolution of heat, the 

 true nature of which, however, has not yet been explained. 



Since respiration, like every other vital process, obtains in intensity as the 

 temperature increases, until an optimum of the latter is attained, it follows that the 

 spontaneous evolution of heat at higher but favourable temperatures of the surrounding 

 air must be more intense than at lower temperatures of the latter; and when the 

 temperature of the environment is so low that growth and respiration do not occur at 

 all, it is obvious that no spontaneous evolution of heat is to be expected. These 

 theoretical results find their confirmation in the observations before us. 



The observation of the rise of temperature in the respiration of green shoots is 

 more diflScult than in the cases hitherto referred to. Dutrochet in 1840 employed for 

 this purpose a thermo-electric pile of great delicacy, by means of which he succeeded 

 in demonstrating in the interior of individual growing shoots rises of temperature of 

 •Jg-th and- j^th of a degree. He found the greatest rise always in the case of 

 unfolding buds, which of course was to be expected. 



From the point of view of purely scientific discovery, however, all these observa- 

 tions have relatively little value, and I have only introduced them in confirmation of 

 the theoretical results. Much more important would be the determination of the 

 quantities of heat, expressed in thermal units, produced by the consumption of a 

 known quantity of oxygen in respiration. 



As akeady said, the production of heat is a universal and necessary consequence 

 of respiration. In rare cases, however, the production of light, or phosphorescence, 

 also occurs. Avoiding here many extremely doubtful statements, I confine myself 

 to mentioning several cases of luminous Fungi established by good observers, where' 

 it is essentially a matter of showing that phosphorescence may be a consequence of 

 the respiration of living plants. In this connection, the illumination of the ' Rhizo- 



