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, ag 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 toit. 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 micelle, 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 
