LECTURE XXV. 



THE RESPIRATION OF PLANTS. SPONTANEOUS EVOLUTION 

 OF HEAT. PHOSPHORESCENCE. 



Plants, like animals, must be continually making exchanges with the atmosphere, 

 and be able to absorb its oxygen in order to maintain life. The chemical changes 

 and molecular movements of which the life of plants as well as that of animals 

 consists, are accomplished only so long as the free oxygen of the atmosphere is able 

 to enter into them. If the supply of this gas is cut off from them, the internal 

 movements which effect growth cease, and the streaming of the protoplasm in which 

 w-e find the most direct expression of life is brought to an end ; the periodic move- 

 ments of foliage leaves and the parts of the flowers cease, and the organs which 

 respond to stimuli lose their irritability. If, when the conditions of vegetation are 

 otherwise favourable, the supply of oxygen is interrupted for a short time only, 

 the plants still retain their vitality, and the internal and external movements 

 brought temporarily to a standstill may return as soon as the access of the oxygen is 

 again permitted. If, on the other hand, the interruption of the vital movements 

 through lack of oxygen continues for a long time, destructive processes take place in 

 the cells in consequence of the so-called intra-molecular respiration, to which I shall 

 return subsequently ; the capacity for living is destroyed sooner or later, and a too 

 late access of oxygen no longer recalls those peculiar movements which are termed 

 vital '. 



' The most essential facts respecting the respiration of plants and its resemblance to that of 

 animals had already been clearly recognised by Ingenhouss and Theodore de Saussure before the 

 beginning of this century. The theory was further developed later by Dutrochet, Grischow, Meyen, 

 and others. But in consequence of a quite unwarranted dictum of Liebig's, which struck out the 

 respiration of plants from vegetable physiology, the matter simply passed into oblivion, at least 

 in Germany, from about 1840, and accordingly the universality of an evolution of heat in living plants 

 also was apparently put aside. An extreme confusion of ideas was at the same time, in spite of De 

 Saussure's clever work, brought about by the fact that, by a scarcely conceivable thoughtlessness and 

 obtuseness, people had accustomed themselves to speaking of a double respiration of plants — of a 

 so-called diurnal respiration, meaning assimilation, and a so-called nocturnal respiration, by which 

 was understood the evolution of carbon dioxide which occurs in true respiration. In spite of 

 Boussingault's splendid work, and Garreau's repeated and accurate putting of the difference between 

 assimilation and respiration, this confusion nevertheless persisted. By means of the very detailed 

 collection of the whole of the literature which had appeared up to 1865, and the putting forward of 

 the radical difference between assimilation and respiration which I accomplished in my 'Handbuch 



