406 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



of growth, and the penetration of the new particles, whether produced by 

 chemical or physical attraction, may take place against considerable ex- 

 ternal resistance, and hence may render the plant capable of performing 

 pronounced external work. 



The production of heat by an ordinary plant has no direct importance 

 in metabolism, for this is not appreciably affected by a rise of a fraction 

 of a degree, and plants develop normally when kept slightly cooler by 

 transpiration than the surrounding medium. Although aerobic respiration 

 is always connected with a production of heat, this need not always be 

 essential, and it is not impossible that anaerobic organisms may exist 

 whose metabolism involves a lowering of temperature. It is in fact, as far 

 as our present knowledge goes, impossible to affirm that no existence is 

 possible without the production of heat by metabolism. In the case of all 

 physiological actions due to a chemical product, it is immaterial whether the 

 product is the 'result of an endothermic or exothermic reaction. Chemical 

 actions involving a liberation of heat are more readily induced than 

 exothermic^ones, and hence may be preferably employed by the organism l . 



Vital activity is inseparable from metabolism, and even adult organs 

 which have ceased to grow must respire as long as they live. The plant 

 may be compared to a factory in which all work ceases when the fire is 

 drawn, although the capacity for work may be retained during short 

 periods when the energy of the steam-engine is put to other purposes than 

 driving the different bench-machines. If the energy of the steam is mainly 

 employed in overcoming frictional resistance in the different mechanisms, 

 practically the whole of the chemical energy of the coal may be manifested 

 as heat. 



It has already been mentioned that the accumulation of waste products 

 has to be avoided as far as possible, and it is of interest to notice that for the 

 most part carbo-hydrates capable of oxidation into carbon dioxide and water 

 are used in metabolism. The nitrate and nitrite bacteria, which oxidize 

 ammonia and nitrous acid, as well as the sulphur bacteria which oxidize 

 sulphuretted hydrogen, derive in this way energy for the synthesis of 

 organic food and probably for the whole of their vital activity as well. 

 If this is so, and these organisms use carbon compounds solely as building- 

 materials and directly utilize the energy obtained by the oxidation of 

 inorganic compounds in place of ordinary respiration, then a close study of 

 these organisms should throw much light upon the nature of life 2 . 



It might also be possible to determine whether proteid molecules are 

 continually decomposed and regenerated during respiration, or whether 



1 Cf. Pfeffer, Studien zur Energetik, 1892, p. 174. 



a Nathansohn, Mittheil. a. d. zool. Station zu Neapel, 1902, Bd. XV, p. 655, finds that no 

 carbon dioxide is produced during the respiration of certain aerobic Bacteria, which cany out chemo- 

 synthetic assimilation by the aid of the energy derived from the oxidation of thiosulphates. 



