404 LECTURE XXV. 



the carbon and hydrogen must first be torn asunder from their molecular combi- 

 nations. In any case, so much is certain, that heat is produced by respiration in 

 the plant, just as the heat of the animal body is produced by respiration. While 

 this remark depends on general natural laws, it may appear somewhat paradoxical 

 that respiring plants possess either only the temperature of their environment, 

 or, if living in the open air, may even be cooler than it is. On closer con- 

 sideration, however, this result is seen to be quite natural, since the temperature 

 of a body depends not only upon the heat developed in it, but quite as much upon 

 the causes which carry away this heat, and thus effect cooling. Apart from special 

 cases, it is the cooling influences which are particularly energetic in plants, because 

 the very large surfaces and relatively small mass facilitate the exchange of heat 

 with the environment. In submerged aquatic plants and subterranean organs 

 every increase of temperature produced by respiration is very easily equalised 

 by means of the bulky surrounding medium, so that such parts of the plant 

 exhibit simply the temperature of the environment. Leaves and their shoot-axes 

 in the open air, however, are more exposed to cooling than these. They lose such 

 considerable quantities of heat, not only by radiation, but also by the absorption 

 of heat in the formation of aqueous vapour, that they are usually colder than the 

 surrounding air. During clear nights the temperature of the leaves may fall by 

 radiation several degrees below that of the air ; and when the latter is some 2 or 3° 

 above zero, for instance, the former may cool down 3 or 4° below zero, and the 

 aqueous vapour of the surrounding air be precipitated in the form of ice crystals 

 (hoar frost) on the plants. On the other hand, the leaves may be even warmer than 

 the surrounding air in strong sunshine ; and this of course does not depend upon 

 respiration. In observing the temperature of the wood of trees earlier observers 

 allowed themselves to fall into much confusion. They found, for instance, that the 

 temperature of the wood at night is higher than that of the surrounding air, and 

 many held this to be an indication of spontaneous heating, although, as later and 

 more exact observations showed, this phenomenon, as well as the opposite difference 

 of temperature in the day, depends simply upon the slow conduction of heat by 

 wood. As a matter of fact,- also, the wood of a tree is the least suitable of all 

 materials for the observation of the spontaneous heating of plants ; since in the 

 wood, apart from the relatively small quantity of parenchyma, respiration (and con- 

 sequently also spontaneous heating) does not occur. 



However, if cooling— i.e. the removal of the heat of respiration— is prevented, 

 it is easy to demonstrate rises of temperature in the most various parts of plants by 

 means of suitable thermometric observations. The fact longest known in this 

 connection is the rise of temperature, often very high, of germinating barley in the 

 preparation of malt : here the respiring seedlings are accumulated in large quantities, 

 and protected from cooling to any great extent. Göppert's experiments made in 

 1832 with various seeds, bulbs and tubers, depend on the same principle: the 

 seeds etc. were accumulated in large quantities, and rose in temperature several 

 degrees, though great sources of error were overlooked. Evidently it is not sufficient 

 simply to accumulate large quantities of germinating seeds, bulbs and tubers in 

 the neighbourhood of a thermometer ; it is also necessary to afford access to the 

 atmospheric air in the process, in order that respiration may proceed. This may 



