THE RESERVE MATERIALS OF PLANTS. 483 



vegetable tissues. When dissolved in alkaline carbonates 

 its solutions are mucilaginous and hard to filter ; when 

 oxalate of ammonium is the solvent they are perfectly fluid 

 and filter with ease. 



Metapectic acid has been identified with arabic acid. 

 It has an acid reaction, is soluble in water and forms soluble 

 salts with the alkaline earthy bases, differing thus from 

 pectic acid. This body and its compounds are met with 

 very frequently in living tissues. It can easily be prepared 

 by boiling pectine, or pectic acid, with excess of alkali. 



When heated with sulphuric acid Schreibler (44) says 

 it splits into an organic acid and a dextro-rotatory sugar, 

 which crystallises in beautiful prisms and is identical with 

 arabinose. It may therefore be a glucoside. 



These pectic bodies differ thus materially from the 

 cellulose with which in cell walls they are so closely 

 associated. Like the latter they can be claimed as re- 

 serve materials, from the localities of their occurrence and 

 their ultimate transformations. The relation in which they 

 stand to cellulose on the one hand and to such carbohy- 

 drates as starch on the other is not very clear. Probably 

 they had best be regarded as forming a third group by 

 themselves, approaching rather the gums and mucilages. 

 Indeed, many of the reactions which they show have for a 

 long time been described as those of a certain number of 

 the latter. 



Mangin has published a considerable number of re- 

 actions with different staining fluids which are distinc- 

 tive of the pectic bodies, and enable their distribution in 

 the cell wall to be ascertained with fair accuracy. They 

 are of course only suitable for micro-chemical determina- 

 tion. The best of them are neutral or slightly acid 

 solutions of phenosafranin, methylene blue, Bismark brown, 

 fuchsin, and methyl violet B. They stain the pectic 

 bodies, as well as lignified and suberised membranes, 

 but the colour can be washed out from the latter in alcohol, 

 glycerine, or acids. Further distinction between these two 

 groups can be drawn by using other stains which act in the 

 reverse manner, leaving the pectic bodies uncoloured while 



34 



