42 VEGETABLE PHYSIOLOGY 



cellulose and leaves the framework of the cell apparently 

 unaltered ; it consists then, however, not of pure pectose, 

 but of a compound of pectic acid with some of the copper of 

 the reagent. 



Pectine swells up and dissolves in water, forming a 

 viscous liquid which soon becomes a jelly. It exists in 

 considerable quantity in many ripe fruits and in some 

 mucilages. It gives no precipitate with the neutral acetate 

 of lead, but is thrown down by the basic acetate in the form 

 of white flocculi. If it is boiled for some hours in water, it 

 is converted into parapectine, which is precipitated by 

 neutral lead acetate. Further boiling with dilute acids 

 converts it into metapectine, which is precipitated by 

 barium chloride. 



The acid series shows peculiarities similar to those of the 

 neutral one. Its most insoluble member is pectic acid, which 

 will not dissolve in water, alcohol, or acids ; it forms soluble 

 pectates with alkalis, and insoluble ones with the metals 

 of the alkaline earths, of which calcic pectate is the most 

 widely distributed. It dissolves in solutions of alkaline 

 salts, such as the carbonates of sodium and potassium, 

 alkaline phosphates and most organic ammoniacal salts, 

 forming with them double salts which gelatinise more or 

 less freely with water. Its solution in alkaline carbonates 

 is mucilaginous, but when ammonic oxalate is the solvent 

 it is perfectly limpid. 



The member at the other end of the series is meta- 

 pectic acid, a body with an acid reaction, freely soluble in 

 water and forming soluble salts with all bases, especially 

 those of calcium and barium, which precipitate pectic acid. 

 Metapectates are coloured yellow when they are warmed 

 with an excess of alkali. This body and its compounds 

 are probably very prominent in the gums ; when acted 

 on by dilute sulphuric acid they split up, one of their 

 products being a crystallisable dextro-rotatory sugar which 

 is apparently arabinose. Metapectic acid does not form a 

 jelly, its solutions always being limpid. 



