54 NUTRITION. 



ascribe the function which they perform to the 

 immediate operation of the vital force." (Hen- 

 slow.) 



43. The influence of the atmosphere on the 

 nourishment of plants, or in other words, their 

 respiration, is the most complicated and perhaps 

 the most important of all the processes of vege- 

 table economy. Animal respiration, which is 

 in effect, that process by which the blood is ex- 

 posed to the action of the air, may shew us by 

 analogy how necessary it must be to consider 

 the relations of the nutritious juices of this class 

 also of organized beings with atmospheric action 

 in order to comprehend their physiology. Thirty 

 years after Bonnet (then occupied in researches 

 on the uses of the foliage of plants) had first 

 observed that air was given out by living green 

 leaves, Priestley's attention was turned to the 

 subject ; and he submitted the air thus obtained 

 to analysis : it proved to be either pure oxygen, 

 or to contain that gas in a much larger propor- 

 tion than atmospheric air does : other chemists 

 confirmed the details of Priestley's experiments. 

 The phenomenon is evidently connected with 

 the life of the plant, since leaves though still 

 green but no longer living, give out no gas at 

 all until the commencement of decomposition. 



