Permeability 51 
separated by a pectic middle lamella fall apart. Such a maceration 
of tissue certainly takes place in many cases, but in many other cases 
it does not. Thus Professor Priestley informs me that in a bean stem 
after such treatment the epidermis, endodermis and cambium still 
remain as continuous layers, from which it is to be concluded that 
the radial walls of these cells do not possess a pectic middle layer. 
Professor Priestley also finds that the cells at meristematic apices of 
roots do not possess a pectic middle lamella, a very important finding. 
The bulk of the cell walls in soft tissues is composed of cellulose, 
a group of substances which on hydrolysis yield glucose, and which 
are therefore to be regarded as glucosans. In addition to cellulose, 
or rather, celluloses, there may also be substances representing inter¬ 
mediate stages between cellulose and glucose, to which the substances 
termed “ amyloid ” appear to belong. Condensation products of other 
sugars such as mannose and galactose (mannosans and galactosans), 
included in the inappropriate term “hemicelluloses,” may also be 
present, in addition to pectin. These substances are all complex 
carbohydrates, for information with regard to which reference may 
be made to standard chemical textbooks of the subject (e.g. Tollens, 
1914; Haas and Hill, 1920). In these so-called cellulose walls other 
organic substances are also present. Thus Cranner (1914) has extracted 
fatty substances from the cell walls of the young parts of Ricinus 
communis, Vicia Faba, Cucurbita Pepo and a number of other species. 
Apart from these organic constituents, the cell wall often contains 
a certain amount of mineral matter, silica, calcium oxide and calcium 
carbonate being the inorganic substances most frequently met with. 
It is important for our purpose to know how these various sub¬ 
stances found in the cell wall are there related to one another. It is 
clearly a feasible view to suppose the cellulose forming a solid frame¬ 
work, in the meshes of which the pectin and lipoid and other sub¬ 
stances constitute the disperse phase of a hydrosol or hydrogel. This 
view, to which the writer inclines, would account for the rigidity and 
definitely solid character of the cell wall, and at the same time account 
for its powers of imbibition. But there are others who regard the 
cellulose itself as also forming part of the colloidal system. Thus 
Cranner, who thinks the fatty substances found by him in the cell 
wall are there probably in the form of salts of fatty acids, that is, as 
soaps, regards the cell w'all as a hydrogel complex of which the more 
solid phase is composed of the colloid cellulose + pectin + colloidal 
soap. He was indeed able to prepare a compound of pectin and lauric 
acid and to manufacture membranes of this substance. 
4—2 
