THE VOLATILE PAET OF PLANTS. 85 



that in the living plant cellulose passes into pectoso and 

 pectin. Without doubt, also, the reverse transformations 

 may be readily accomplished. 



4. THE VEGETABLE ACIDS. The Vegetable Acids are 

 very numerous. Some of them are found in all classes of 

 plants, and nearly every family of the vegetable kingdom 

 contains one or several acids peculiar to itself. Those 

 which concern us here are few in number, and though 

 doubtless of the highest importance in the economy of 

 vegetation, are of subordinate interest to the objects of 

 this work, and will be noticed but briefly. They are 

 oxalic, tartaric, malic, and citric acids. They occur in 

 plants either in the free state, or as salts of lime, potash, 

 etc. They are mostly found in fruits. 



Oxalic acid, C 2 H a O 4 2 H 2 O, exists largely in the com- 

 mon sorrel, and, according to the best 

 observers, is found in greater or less 

 quantity in nearly all plants. The pure 

 acid presents itself in the form of color- 

 less, brilliant, transparent crystals, not Fio> 15 

 unlike Epsom salts in appearance, (Fig. 

 15,) but having an intensely sour taste. 



Oxalic acid forms with lime a salt the oxalate of lime 

 which is insoluble in pure water. It .nevertheless exists 

 dissolved in the cells of plants, so long as they are in active 

 growth, (Schmidt, Ann. Chem. u. Pharm., 01, 297.) To- 

 wards the end of the period of growth, it often accumu- 

 lates in such quantity as to separate in microscopic crystals. 

 These are found in large quantity in the mature leaves and 

 roots of the beet, in the root of garden rhubarb, and espe- 

 cially in many lichens. 



Oxalate of potash is soluble in water, and exists in the 

 juices of sorrel and garden rhubarb. It was formerly 

 used for removing ink-stains from cloth and leather, under 

 the name of salt of sorrel. Oxalic acid is now employed 

 for this purpose. Oxalate of soda is soluble in water, and 



