48 VEGETABLE PHYSIOLOGY 



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 solu- 

 tions always being limpid. 



The two series of pectic bodies are closely related to 

 each other, for by the action of heat, acids, and alkalies 

 the various members of both can be prepared from pectose. 

 The final product of the action of the reagents is the freely 

 soluble metapectic acid. 



The cellulosic and pectic constituents of the cell-wall 

 show considerable differences of behaviour. The former 

 are soluble, the latter insoluble, in Schweizer's reagent ; 

 when oxidised with nitric acid the former yield oxalic, the 

 latter mucic acid. The celluloses when partially hydrated 

 stain blue with iodine ; the pectic bodies give no coloration 

 with this reagent. They behave differently also to staining 

 reagents and to dilute acids and alkalies. 



The celluloses, as we have seen, are members of the 

 group of carbohydrates. Various writers are not agreed 

 as to the relation of the pectic bodies to this group, some 

 holding that their reactions separate them from it entirely, 

 while others contend that they are closely connected with 

 it, if they do not actually belong to it. It has been suggested 

 that they are carbohydrates chemically combined with acids. 

 Like the celluloses, they yield some form of sugar when 

 hydrolysed with dilute mineral acids. 



All unchanged cell- walls contain a varying quantity of 

 water, and various views have been advanced as to the way 

 in which the latter is held by the other constituents. It is 

 probably not in a state of chemical union, as the quantity 

 present can be easily increased or diminished. 



Naegeli suggested that the wall contained particles of 

 solid matter or micellce, of crystalline form, the long axis 

 of the crystals being arranged at right angles to the surface 

 of the wall. He supposed each micella to be surrounded 

 by a thin film of water. Every cell-wall is thus under some 



