540 THE STINGING PROPERTY OF THE GIANT NETTLE-TREE, 



As is well known, free oxalic acid is found in Opuntias, free 

 malic acid in Cacti, etc., and free tartaric and citric acids in fruits. 



Formation of Organic Acids. — Liebig and his followers held 

 that organic acids were formed by plants in their constructive 

 metabolism, e.g.:— C0 2 + H 2 0= HCOOH + O 

 2C0 2 + H 2 = (COOH) 2 + O 



Now, although the formation of acids is accompanied by the 

 evolution of oxygen, yet the above explanation of the process 

 does not correspond with the accumulated evidence of recent 

 observations, and Liebig's theory has been discarded. 



During the respiration of plants the absorbed oxygen has been 

 found to enter in some obscure manner into combination with 

 the protoplasm, and then follows a long series of katabolic 

 changes. The breaking down of these unstable protoplasmic 

 molecules results most generally in the production of carbonic 

 acid, but in some plants other and more complex organic acids 

 are formed instead of carbonic acid. And so it is now generally 

 held that the production of organic acids by plants is a result of 

 destructive metabolism. 



Carbohydrates form the chief synthetic products of plants, and 

 these are used both for storage of reserve food-material, and also 

 to supply the energy for the formation of the various products of 

 metabolism. From the carbohydrates organic acids are formed(7), 

 the acids are neutralised by bases taken up from the soil, and 

 these organic salts in certain rare instances are found to pre- 

 dominate, as the reserve food-supply of the plant. 



Among the various ways in which carbohydrates can be con- 

 verted into organic acids the most prominent seems to be by the 

 agency of bacteria(8). Certain of these produce enzymes, and the 

 latter can form carbonic, formic, or acetic acid by oxidation of 

 carbohydrates. Also formates have been produced by the decom- 

 position of bicarbonates at ordinary temperatures(9). 



Quite recently, Usher and Priestley of Bristol, have studied 

 the mechanics of carbon assimilation(l7), and have definitely 

 proved that the photolysis of carbon dioxide always produces 

 formic aldehyde (in some cases formic acid) and hydrogen peroxide. 



