CELL-CONTENTS AND CELL-WALLS. 81 



state or, more commonly, combined with bases to form 

 acid or neutral salts. 



The chief non-nitrogenous organic acids in plants are 

 oxalic, malic, citric, and tartaric. Soluble potassium oxa- 

 late occurs in Rumex (Docks, Sorrel Docks) and Oxalis 

 (Wood Sorrel) ; sodium oxalate in Salsola and Salicornia ; 

 while crystals of insoluble calcium oxalate are the most 

 frequent mineral deposits found in plant tissues. Malic 

 acid and malates occur in the juice of many fruits (e.g. 

 Apple, Gooseberry, Eowan, where they are abundant), in 

 the tissues of various succulent plants (especially in Cras- 

 sulaceae), in Fern prothalli, etc. ; citric acid occurs in the 

 juice of Lemons, Oranges, etc., and in Lycopodium pro- 

 thalli ; tartaric acid (generally as acid potassium tartrate) 

 in Grapes, Pine-apples, etc. 



91. Oxalic Acid. Any soluble calcium salt, added to a solu- 

 tion of oxalic acid or a soluble oxalate, gives a white precipitate of 

 calcium oxalate, soluble in hydrochloric or nitric acid, but almost 

 insoluble in potash or ammonia. This is a delicate test, and is 

 hastened by warming, if the oxalic solution is very dilute. 



Repeat this test with oxalic acid or potassium oxalate, using 

 calcium chloride for the reagent. Apply this test to juice pressed 

 from leaves and petioles of Rumex, Oxalis, Salicornia, Salsola. 



92. Tartaric Acid. Potassium chloride produces in a solution 

 of free tartaric acid a white precipitate of hydrogen potassium 

 tartarate, readily soluble in mineral acids and alkalis ; calcium 

 chloride, added to tartaric acid or an alkaline tartarate, gives a 

 white precipitate of calcium tartarate, distinguished from calcium 

 oxalate by being soluble in potash ; silver nitrate gives a white 

 precipitate of silver tartarate filter, dissolve the precipitate off 

 the filter with a little dilute ammonia, heat the solution in a test- 

 tube for a few minutes, when the glass becomes coated with a silver 

 mirror (characteristic reaction for tartaric acid). 



Apply these tests (especially that with silver nitrate) to a solu- 

 tion of tartaric acid or a tartarate. Repeat the tests with some 

 Grape juice pressed into a test-tube, after addition of a little 

 caustic potash to the juice. 



93. Citric Acid is readily distinguished from tartaric, since 

 no precipitates are given with potassium salts, nor with cold 

 lime-water (on heating with lime-water, white calcium citrate is 

 thrown down) ; the silyer citrate precipitate (given on adding 

 silver nitrate solution) does not form a mirror when dissolved 



P. B. 6 



