SPECIAL METHODS. 139 



system of the constituents of the cell-wall, although the 

 macrochemical study of the cell-wall has been taken up 

 in various aspects in recent years. 



243. I prefer, then, to discuss first in the following pages 

 the kinds of membranes which may primarily be distin- 

 guished by their microchemical relations. For some time 

 there have been pretty generally distinguished the pure 

 cellulose wall, the lignified wall, the cuticularized or su- 

 berized wall, the gelatinized wall, and fungus-cellulose. In 

 connection with the gelatinized wall may be discussed the 

 remaining plant mucilages and gums and the jelly-formation 

 of the Conjugates. As related to these various modifications 

 of the membrane may be mentioned the paragalactan-like 

 substances which serve as reserve-materials, callose, and the 

 pectins. Finally, this chapter may describe the preparation 

 of ash and siliceous skeletons, and some methods which 

 have been used in the investigation of the development and 

 finer structure of the cell-walls. 



i. The Cellulose Wall. 



244. Cellulose is a carbohydrate whose empirical compo- 

 sition corresponds to the formula C 6 H 10 O B . It is especially 

 characterized by its solubility in cuprammonia and in con- 

 centrated sulphuric acid, by its blue or violet color with 

 iodine and sulphuric acid or with chloro'iodide of zinc, and by 

 the fact that from its hydrolytic splitting with sulphuric 

 acid there finally results a fermentable sugar (glucose). 



245. But it should be remarked that there are very prob- 

 ably different substances which give the same reactions,' and 

 which perhaps represent nearly related isomeric compounds. 

 Thus, according to W. Hoffmeister (I and II), cellulose 

 shows very -varying relations, especially toward a 1-5$ 

 caustic soda solution, in which it is partly soluble, partly 

 insoluble. But an exact microchemical distinction of the 

 kinds of cellulose has not yet been possible, and it is there- 

 fore best for the present to call all membranes or parts of 

 membranes which show the above reactions pure cellulose 



