THE CARBOHYDRATES OF THE CACTI. 43 



10 c. c. of the solution with 1 to 2 drops of 15 per cent alphanaphthol and 

 3 c. c. concentrated sulphuric acid (Goldschmiedt reaction) gave the beau- 

 tiful emerald green coloration characteristic of glucuronic acid. Special 

 experiments with a large variety of substances apt to be found in plants 

 showed that none of these except nitrates produce the green color. The 

 absence of nitrates was established by the diphenylamine reaction. The hot 

 aqueous solution was distinctly acid. 



The substance under discussion melted sharply at 176 and burned with- 

 out leaving any ash. The specific rotation was [a]f> = +20.0 ; that is, 

 1.25 grams in 100 c. c, in a 1 dm. tube gave [a] = -(-0.25. These prop- 

 erties establish the identity of this substance with glucuronic acid. After 

 28 hours the same solution showed a specific rotation of [<*]D = 3.2. 



It is a well-known property of the type of acid to which glucuronic acid 

 belongs, that the lactones of the acids are usually strongly rotatory, but the 

 free acids are only feebly so. Nef * has worked out the conditions of equi- 

 librium for d-mannonic acid. The normal /?-lactone of d-mannonic acid in 

 water solution has a specific rotation of -{-111.1. On standing at 20 

 this solution changes, so that after 24 hours about 75 per cent of the lactone 

 is converted into free d-mannonic acid and the rotation has dropped to 

 [a] 20 =-(-28. The free d-mannonic acid has a rotation of [a]^ = 

 1.0. From the data given it appears that glucuronic acid behaves in 

 a similar manner and the reduction of the rotation is to be ascribed to the 

 conversion of a portion of the dextro-rotatory lactone into the feebly levo- 

 rotatory free acid. 



This is the first time that glucuronic acid has been reported as a plant 

 constituent. 2 It is, however, probably a rather common component of 

 plants, though its isolation is associated with considerable difficulty. Re- 

 peated attempts with the cacti have yielded only very small amounts. From 

 100 grams of the dry material, or about 500 grams of the fresh cactus, 

 0.1 gram of pure glucuronic lactone was obtained. The role of this sub- 

 stance in the carbohydrate metabolism will be discussed in the section on the 

 origin and role of pentose sugars. 



One of the most striking characteristics of the cacti is their high mucilage 

 content. This property adds enormously to the difficulty of working with 

 them and necessitates quite a different procedure of analytical and experi- 

 mental technique from that commonly employed in work of this nature. 

 Some of these peculiarities have already been discussed in the preceding 

 chapter. The presence of mucilages or slimes has been very generally 

 observed in succulent plants and many fleshy vegetable tissues, and is 

 undoubtedly of great physiological importance. In fact, in one form or 

 another, these colloidal substances are present in all plants to a greater or 

 less extent. The exact nature of these substances has not been definitely 



1 NEP, J. U. Liebig's Ann. d. Chem., 403, 308, 1913. 



3 PAT.T.ADIN, W. Glucuronic acid, glucuronides and glyoxylic acid in plants. Chem. 

 Abstracts, 11, 48, 351, 1917. 



