THE SKELETON OF THE PLANT 43 



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. 



Cellulose, as we have seen, is a member 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 cellulose 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 micelles, of crystalline form, the long axis 

 of the crystals being arranged at right angles to the sur- 

 face of the wall. He supposed each micella to be surrounded 

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

 some considerable internal strain, the micellae attracting 

 each other and tending to squeeze out the water. The 

 latter, on the other hand, tends to separate the micellae. 



According to Strasburger, the molecules of the solid 

 matter are held together by chemical affinity, and there is 



