THE CELL-MEMBRAXE. 349 



cases easily visible, the view arose that it consists of a sort 

 of intercellular substance, or cement, joining contiguous cells 

 together. This was advanced by von Mohl and supported 

 by Mulder and Harting. These authors speak of its being- 

 very variable in composition and consisting very largely of 

 pectose, filling the spaces between the cell-walls and pene- 

 trating into the inner layers. They also held that it forms 

 the mass of the little projections or concretions which often 

 extend into the intercellular passages. The layers internal 

 to the middle lamella they regarded as chiefly cellulose. 



Harting's conclusions on the formation and growth of 

 the cell-wall are that the primary cell-wall forms the internal 

 layer and that it becomes thickened by a process of intus- 

 susception from within outwards, the primary membrane 

 being cellulose, while pectose is developed in the external 

 region as the wall thickens. Von Mohl on the other hand 

 holds that the intercellular substance as well as the pectose is 

 not formed in the wall, but infiltrates into the cellulose, mask- 

 ing the reactions of the latter. 



None of these writers aa^reed with Paven,that the so-called 

 intercellular substance was composed of alkaline pectates. 

 In a later paper, Pay en (12) points out a difference that can 

 be established in this substance in different places. He 

 found he could disintegrate the tissues and separate the cells 

 by treating pieces of the former by the successive action of 

 dilute acids and alkalies. When so treated the lateral or 

 radial walls could be easily dissolved, but not the tangential 

 ones, so that the cells separated in radial chains. Hence 

 he suggests that the lateral walls were separated by pectates 

 and the others by laminae of cellulose. 



A method of distinguishing between cellulose and the 

 various pectic bodies was pointed out by Fremy (13). This 

 consisted in treating the tissue with ammonio-cupric sul- 

 phate (Schweizer's reagent). This reagent dissolves pure 

 cellulose, w^hile the group of pectic bodies remains apparent- 

 ly unaffected, though really much modified, insoluble cupric 

 pectate being formed. Alkalies on the other hand do not 

 affect cellulose, but dissolve the pectose by converting it 

 into soluble pectates. He agrees with the other writers 



