THE SKELETON OF THE PLANT 47 



from cellulose convert it into pectine, or into pectio acid, 

 the former being soluble in water, the latter in alkalies. 

 The cell -wall can be shown to contain the two constituents 

 by the action of Schweizer's reagent, which, when used 

 with proper precautions, dissolves out the 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 liocculi. 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 can be 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 alkalies, 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 metapectic 

 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. Meta- 

 pectates are coloured yellow when they are warmed with an 

 excess of alkali. This body and its compounds are probably 



