THE METABOLISM OF PLANTS. 225 



are capable of using them in the reconstruction of proteid ; 

 and partly to the fact that any nitrogenous waste-products 

 (alkaloids) which may be formed are not excreted. This 

 important point is illustrated by Boussingault's comparative 

 analyses of seeds and of seedlings given on page 176, and his 

 results agree with those of all other observers. 



We must not forget, too, that protoplasm contains sulphur 

 and phosphorus ; we will briefly consider what becomes of 

 these elements in destructive metabolism. It appears that 

 the sulphur is set free as sulphuric acid. Schulze and others 

 have found that the proportion of sulphates gradually in- 

 creases during the germination of seeds (Lupins, Pumpkins, 

 Vetches), and it seems that the amount of sulphates present 

 bears some relation to the activity of the proteid-metabolism. 

 Probably the same is true of phosphorus. 



In certain plants sulphurised ethereal oils are found; in the Onion, 

 Garlic, Horse-radish, etc., allyl sulphide (C 8 H 5 ) 2 S; in the seeds of the 

 Black Mustard, allyl sulpho-cyanide, C 3 H 5 , CN, S. Attention has been 

 already drawn to the fact that a phosphorised fat, lecithin, has been 

 commonly found in plants. 



We will now turn our attention to the probable mode of 

 origin of the vast number of other, principally non-nitro- 

 genous, waste-products which have been found in plants, 

 such as the organic acids, the aromatic substances, the colour- 

 ing-matters, etc. 



The organic acids are very generally present in plants, 

 either free, or in combination with inorganic bases, forming 

 frequently acid salts, or in combination with organic bases 

 (alkaloids). It is to the presence of these acids or of their acid 

 salts that the acid reaction of plant-tissues is due. 



The following enumeration includes the organic acids most commonly 

 found in plants : 



i. Fatty acids ; general formula C n H 2n +i, CO OH. 



Formic acid CH 2 O 2 , acetic acid C 2 H 4 O, propionic acid C 3 H 6 O 2 , butyric 

 acid C 4 H 8 O 2 , valerianic acid C 5 H 10 O 2 , caproic acid C 6 H 12 O 2 , have been 

 found in various plants by many observers. Bergmann concludes from 

 his researches that formic and acetic acids at least, are always present in 

 living plant-cells. 



The higher members of this series such as caprylic acid C 8 H 16 O 2 , 



V. 15 



