[341] SCIENflFIC MANUAL. 55 



surrounding circumstances of warmth and moisture are 

 favorable. 



Leaves imbibe from the air, carbonic acid ga c , which is 

 decomposed under the influence of sunlight into its ele- 

 ments, carbon and oxygen, the former retained, and the 

 latter, to a large extent, exhaled into the air again. 

 During the night carbonic acid is exhaled, and oxygen re- 

 tained. 



The volume of oxygen exhaled during the day is about 

 equal to that of the carbonic acid imbibed during the 

 same time. 



Sunlight is necessary for the healthy development of 

 vegetation, as is shown by the white appearance of potato 

 sprouts which grow in dark cellars. 



In cases of drouth the mineral food held in solution by 

 the liquid imbibed by the roots is too highly concentrated, 

 and often produces injurious effects upon vegetation apart 

 from those resulting from drouth proper. This is fre- 

 quently seen where highly ammoniated commercial fertili- 

 zers have been applied in large quantities in the drill, to 

 summer crops. 



CHAPTER III. 



CHEMICAL COMPOSITION OF PLANTS. 



The whole physical world is divided into organic and in- 

 organic substances. 



The first embraces all bodies which have resulted from 

 life, and includes both the vegetable and animal kingdoms. 



The second embraces all bodies not the result of life, as 

 well as the remains of organic bodies reverted by complete 

 decomposition to mineral form. 



All organic substances are composed of volatile and fixed, 

 or ash, ingredients. 



