BESPIBATION. 109 



tendency to propel the fluids in the direction of least 

 resistance, which is also that in -which they are consumed." 



RESPIRATION. 



139. The respiration of plants consists in the absorp- 

 tion of atmospheric oxygen and the liberation of carbonic 

 dioxide. This cannot be readily detected while assimila- 

 tion is going on, for then much larger quantities of oxygen 

 escape. Nevertheless, respiration is constantly performed, 

 causing oxidation of the assimilated substances and other 

 chemical changes resulting from this. " The loss of 

 assimilated substance caused by respiration would appear 

 purposeless if we had only to do with the accumulation of 

 assimilated products ; but these are themselves produced 

 only for the purpose of growth, and of all the changes with 

 life; the whole life of the plant consists in complicated 

 movements of the molecules and atoms; and the forces 

 necessary for these movements are set free by respiration." 

 In the absence of oxygen, the chemical changes connected 

 with growth, the movements of the protoplasm, and the 

 power of motion of motile and irritable organs cease. 

 The heat generated by oxidation seldom causes a sensible 

 increase in the temperature of the tissue. But in a mass of 

 germinating seeds, or unfolding flowers, or a spadix during 

 anthesis, the elevation of temperature may be observed. 

 The phenomenon of phosphorescence also depends — in 

 a manner not clearly demonstrated — on the respiration of 

 oxygen. 



140. The degree of temperature at which assimila- 

 tion and metastasis may take place varies in difierent 

 plants. Some plants live wholly in very low temperatures, 

 as the Red-snow plant. lu Polar waters myriads of 



