THE CELL-MEMBRANE. 357 



Hsematoxylin, methylene blue, vesuvian brown and 

 quinolin blue stain the pectic constituents of the wall and 

 not the cellulose as has hitherto been supposed. The com- 

 position of the wall in any case can be better ascertained, 

 however, by the action of solvents. If sections be placed 

 in freshly prepared Schweizer's solution and left for several 

 days, the fluid being renewed daily, the cellulose becomes 

 gradually dissolved, complete removal of it being effected in 

 the soft tissues in about three to four days. The sections 

 must then be washed several times with water and later with 

 a solution of acetic acid, containing from three to five per 

 cent, of the latter, till all trace of the copper has disappeared. 

 The shapes of the cells can then be seen to be unaltered, 

 although naturally the sections have become very fragile. 

 That the cellulose has been completely extracted is shown 

 by treating the tissue with iodine and phosphoric acid, when 

 the walls become pale yellow instead of blue. If the sec- 

 tions have been rather thick the cellulose will have disap- 

 peared from the membrane, but as the solution of this sub- 

 stance in Schweizer's reagent is slightly viscid, it will not all 

 have been removed from the sections by the washing with 

 water, and the subsequent treatment with the dilute acetic 

 acid will have precipitated it in little clots in the intercellular 

 spaces or sometimes in the interior of the cells. These 

 little masses will, after the treatment with the iodine and 

 phosphoric acid, stand out as blue patches among the yellow 

 membranes. If now the stains for which the pectic bodies 

 have an affinity be applied to the sections, the membranes 

 all take them up freely. 



The pectic compound thus remaining after removal of 

 the cellulose is not unchanged pectose, but has been con- 

 verted in great part into pectic acid ; for if the section be 

 irrigated with amnionic oxalate, in which this acid is soluble, 

 all the membranes rapidly disappear. If instead of the 

 oxalate the walls be dissolved in ammonia and then the 

 latter be neutralised by dilute acetic acid, a gelatinous pre- 

 cipitate falls, which can be stained with a basic stain such 

 as methylene blue. 



Another method of demonstrating the mixed nature of 



