CHAPTER VI 



THE VEGETABLE ACIDS 



Though the name "vegetable acids" might strictly be applied to all 

 acids found in plants, it is, as a rule, restricted to certain acids and 

 hydroxy-acids of the methane, ethylene and acetylene series. 



We may take first the acids of the methane series which biologically 

 fall into two groups, the simpler members associated with fundamental 

 metabolism and the more complex ones associated with fat formation. 

 The first six members, at least, may be included among the vegetable 

 acids in the narrow sense. They are liquids, readily volatile in steam, 

 and several of them, without doubt, are closely involved in some of the 

 most fundamental and important reactions of plant metabolism. In fact 

 their relationships to certain of the amino-acids which are constituents 

 of most proteins, cannot be too strongly emphasized. The higher 

 members (with ten and more carbon atoms) are solids insoluble in water. 

 The glycerol esters of certain of these higher members are important 

 constituents of the plant fats and will be considered in the following 

 chapter. The first six representatives of the series are : 



Acids of the methane series Corresponding amino-acids 



Formic acid H • COOH 



Acetic acid CH3 • COOH amino-acetic acid or glycine 



Propionic acid CH3 • CH2 * COOH amino-propionic acid or alanine 



Butyric acid CHg- CHg •CH2- COOH 



Valeric acid CH3 • CH2 ' CH2 * CH2 ' COOH amino-iso-valeric acid or valine 



Caproic acid CH3 • CH2 * CHg • CH2 • CH2 ' COOH amino-iso-caproic acid or leucine 



Formic acid can be obtained by submitting plants to steam distil- 

 lation. This indicates that it probably exists in the free state in plants, 

 though there is the possibility of its being formed from other substances 

 during distillation. There is good evidence (Dobbin, 1), however, that it is 

 present in the stinging hairs of the Nettle ( Urtica dioica). It is a liquid 

 which is volatile with steam and can be readily reduced to formaldehyde 

 with nascent hydrogen. 



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