OUTLINES OF THE PHYSIOLOGY OF NUTRITION 231 



are in active life. It is only when ths protoplasm is 

 completely at rest, as in a dry seed or bulb, that 

 respiration ceases. If respiration is stopped, all the 

 work of the plant is stopped also ; all the chemical 

 changes in the plant come to an end ; growth ceases, 

 and so do the protoplasmic movements, such as we 

 studied in Elodea. Just as the combustion of the 

 coals in the fire-box of a steam-engine supplies the 

 energy for the work of the engine, so does the com- 

 bustion of carbon in the plant's protoplasm supply 

 energy for the growth of the plant, and for such 

 movements as it performs. The plant, in fact, like 

 an animal, must breathe in order to live, but as in 

 most cases the plant does not have to execute such 

 active movements as an animal, the actual amount of 

 oxygen which it has to consume is so much less. 



Kespiration being a form of combustion, heat must 

 of necessity be set free. As a rule, however, no con- 

 sequent rise of temperature is observed in the case of 

 plants ; for the plant in most cases loses heat more 

 rapidly than it gains it. An ordinary leafy plant has 

 an enormous surface in comparison with its bulk, and 

 is thus exposed to very rapid cooling, both by radiation 

 and conduction. The evaporation of water during 

 transpiration also uses up more heat than respiration 

 can supply. It is only when actively breathing organs 

 are crowded closely together in a confined space that 

 a rise of temperature can be detected. If the bulb of 

 a thermometer be thrust into the midst of germinating 

 seeds, a temperature about 1*5 C. higher than that of 

 the surrounding air may be observed, and in some 



