35-' SCIEXCE PROGRESS. 



behaviour beinir due to infiltrations of nitrogenous matters 

 into it. 



The intercellular substance was thus reQ^arded bv some 

 of the older investigators as a primitive, jelly-like material, 

 ultimately giving rise to cellulose, and by others as a pro- 

 duct of secretion formed by the protoplasm and made to 

 accumulate in the outer part of the membrane. By some 

 its composition is not stated, especially by Schacht, which 

 is rather surprising, as he carefully studied its reactions. 

 As already mentioned, Mulder, Harting and Payen were 

 agreed that pectic compounds entered into its structure. 

 Dippel says it is not cellulose, but gives no opinion as to its 

 nature. 



In the writings of Xaegeli. Sachs, and other later writers, 

 the term "middle lamella" replaces '' intercellular substance" 

 It was proposed to call it the "primary membrane," but as 

 this suggested a definite origin and was not exactly in line 

 with the theory of growth by intussusception, which was 

 then coming into favour, the term " middle lamella" came 

 to be adopted. This certainly had in its favour the fact 

 that it was essentially non-committal. The writers of this 

 period leave its nature undetermined. Sanio (20) in 1873 

 speaks of it in Pinhis sylvesti'is as '' Zwischensubstanz '' 

 and describes it as a gelatinous material intercalated between 

 the radial rows of the generating cells. Both Sanio and 

 Dippel lean to the opinion that whatever its chemical nature 

 it arises from decomposition of the original cell-wall, a view 

 exactly opposite to that advanced by Unger some years 

 previously. 



When Wiesner put forward his hypothesis of the com- 

 position of the cell-wall he recognised the existence of this 

 middle region and pointed out that very young cells could 

 be separated easily by its severance ; much more easily, 

 indeed, than older ones, and concluded that the threads 

 uniting the dermatosomes were much more easily ruptured in 

 that region than in the other layers. The rupture could be 

 caused in some cases by mere mechanical traction, in others 

 by chemical reagents such as strong h)"drochloric acid. 

 Wiesner did not consider there was necessarily any chemical 



