648 PHYSIOLOGY. 



evolved, is constantly accompanied by a rise of temperature, which 

 is satisfactorily accounted for on chemical principles ; and the 

 " heating" of heaps of germinating seeds is the more marked in con- 

 sequence of the " fermentation" (or decomposition through contact- 

 action) by which it is always accompanied. In the case of Fungi, 

 as shown by MclXab (Gard. Chron. 1871, p. 1256) in Bovista 

 gigantea, the temperature is higher than that of the air, owing pro- 

 bably to the formation of carbonic dioxide. Similar phenomena had 

 been previously observed by Dutrochet in France and Marcet in 

 England. 



Heat evolved during Flowering. The most remarkable instances 

 of evolution of heat are those which occur during the flowering of 

 plants. This rise of temperature appears to take place in all cases ; 

 but it is most strikingly displayed, of course, in plants which have 

 a crowded inflorescence, and above all when this is surrounded by a 

 structure confining the liberated heat, as in the spathes of the 

 Araceae, on which many observations have been made. 



Arum maculatum, A. italicum, A.Dracunculus^Itichardia cethiopica, Colo- 

 casia odora, c., the male inflorescence of Cycas circinalis, the large solitary 

 flowers of Cactus grandijlorus, JBiynonia radicans, Victoria regia, c., are 

 among the recorded plants in which the rise of temperature has been 

 observed. 



The greatest evolution of heat, after the opening of the spathe 

 of AracesB, is at the part where the male flowers are situated : after 

 the pollen has been discharged, the upper parts of the spadix grow 

 warmer, and the lower parts cool gradually upwards. 



Experiments made to ascertain the cause of this heat lead to the 

 conclusion that it is attributable to a process of combustion. Saussure 

 found that the flowering spadix of Arum maculatum abundantly ab- 

 sorbed oxygen ; and Vrolik and De Vriese, comparing the tempera- 

 tures attained by a spadix placed in oxygen gas and by one placed in 

 common air, found that the former always exceeded the latter, while 

 a spadix kept for a longer time in nitrogen gas did not manifest 

 any increased temperature. 



Connected with the subject of the temperature of plants are the in- 

 teresting observations of Boussingault, Alph. DeCandolle, and others, on 

 the different external temperatures required by different plants to stimu- 

 late them into either vegetative or reproductive activity. It is well 

 known that any given plant will require a certain sum of heat during a 

 season to enable it to go through its whole course of development, and 

 that under certain limits the course of development will be passed through 

 proportionately more quickly in warmer climates. Further, it is known 

 that most plants of warm climates are killed by certain degrees of cold. 

 But it is further observed that certain temperatures, insufficient to injure 

 plants, are at the same time insufficient to " start " them into growth, and 

 that different species of plants have different constitutions in this respect ; 

 while, on the other hand, excessive temperatures may render plants barren 



