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 micelle, 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 micelle attracting 
each other and tending to squeeze out the water. The 
latter, on the other hand, tends to separate the micelle. 
According to Strasburger, the molecules of the solid 
matter are held together by chemical affinity, and there is 
