342 Development of Opinion on the Nature [BOOK n. 



throw von Mohl's account of the intercellular substance and the 

 cuticle, but they had not proved that the intermediate laminae 

 are in fact the primary partition- walls on which von Mohl's secon- 

 dary thickening-layers had been deposited, on both sides in the 

 case of the intercellular substance, on one side in that of the 

 cuticle. The structure of the partition-walls and the existence of 

 the cuticle could be explained in a totally different way from the 

 point of view now opened by Nageli's theory of intussusception; 

 there was no need now to see either a secretion or a primary 

 cell-wall in the intermediate lamina of thickened cells or in the 

 cuticle, for it was possible that this lamination might be due to 

 subsequent chemical and physical differentiation of membranes 

 thickened by intussusception. As phytotomists are not yet 

 quite agreed as to the correctness of this view, we must be con- 

 tent with observing here that in the matter of the cuticle and the 

 intercellular substance lies one of the points, the determination 

 of which will involve the question of von Mohl's earlier theory 

 of apposition. It is not the purpose of this history to give the 

 more modern views that have asserted themselves since 1860, 

 especially where the question is still in debate. 



It was a part of von Mohl's idea of the cell-tissue and one 

 to which he had firmly adhered since 1828, that except in the 

 cross walls of genuine wood-vessels and some very isolated cases 

 the partition-walls in cellular tissue are never perforated ; that 

 both simple and bordered pits always remain closed by the 

 very thin primary lamina of cellulose. But between 1850 and 

 1860 several cases were discovered which were at once excep- 

 tions to von Mohl's rule, and of great importance to physiology. 

 Theodor Hartig, in his ' Naturgeschichte der forstlichen Kul- 

 turpflanzen Deutschlands ' (1851), described peculiar rows of 

 cells in the bast-system, in which the transverse and sometimes 

 the longitudinal walls appear to be pierced like a sieve by 

 numerous minute holes, and to these cells he gave the name 

 of sieve-tubes. Von Mohl (1855), while in other points con- 

 firming and extending Hartig's discovery, declared against 



