BY JAMBS M. PETRIE. 543 



Functions of the Organic Acids. — These may serve for — 



(a) Osmotic substances. 



(b) Plastic material. 



(c) Protection from natural enemies. 



(d) Liberation of other acids by mass action. 



(e) Solvents, or neutralising agents. 



(a) Formic and acetic acids and their salts have a strong 

 affinity for water, and therefore a high osmotic pressure. In 

 consequence, they are especially favourable to the production of 

 great turgidity. Even the thick stem of the Laportea is green 

 and succulent, and when cut or wounded it bleeds very profusely. 

 This occurs in most members of the Urticacese, and shows the 

 high exudation pressure. It has been generally found that the 

 sap in all rapidly growing parts of plants has an acid reaction, 

 for the purpose of maintaining and regulating the turgidity of 

 the cells, which are therefore kept succulent during the period of 

 their growth. Astruc (12) finds that a definite relation exists 

 between the formation of acids and the intensity of growth in 

 plants. 



(6) Under certain conditions formic and acetic acids can be 

 used by plants as food, though they have but a small nutritive 

 value. When a decrease of turgidity closes the stomata on the 

 leaves and causes the assimilation of carbon dioxide to cease, the 

 plants may then draw on their store of organic acids for the 

 manufacture of carbohydrates. This is especially the case in 

 green succulent plants, when organic acids acted upon by light 

 produce carbohydrates, oxygen, and carbon dioxide (13), and the 

 latter is at once assimilated. 



Again, many common forms of bacteria are able to 

 decompose formates and acetates directly into hydrogen and 

 carbon dioxide (14); formates yield equal volumes of the two 

 gases. In this way carbon dioxide is made available for assimi- 

 lation. 



Kraus (15) has shown how certain plants store up organic acids 

 as reserve food-supplies from which they can draw as required. 



