352 Nagelis Theory of Molecular Structure [Bookii. 



any satisfactory conclusion. Agardh, who discovered some new- 

 instances of cell-striation, was still less successful in his specula- 

 tions. Von Mohl resumed the subject in 1853 in the 'Bota- 

 nische Zeitung/ and insisted on the fact that it was not possible 

 to separate the striae or apparent fibres by mechanical or chemi- 

 cal means, but he left it still undecided whether the lines which 

 cross each other in the surface-view belong to the same or to 

 different layers of membrane. The communications of Criiger 

 and Schacht, made shortly after, did not help to advance the 

 question; Wigand also took part in the discussion in 1856, but 

 wandered at once from the right path by supposing the cross- 

 striations to belong to different layers of membrane. As long 

 as botanists adhered to von Mohl's theory, that the concentric 

 stratification of cell-walls was due to deposition of new layers, 

 it was scarcely possible for them to arrive at a correct decision 

 with respect to striation ; it became possible, when Nageli proved 

 in his great work 'Die Starkekorner ' (1858) that the con- 

 centric stratification of starch-grains and of cell-membranes 

 generally does not mean, that similar layers lie simply one on 

 another, but that denser and less watery layers alternate with 

 layers that are less dense and contain more water ; and that it 

 is not possible to explain this mode of stratification by deposi- 

 tion as understood by von Mohl, but that it may be explained 

 by intercalation of new molecules between the old ones and by 

 corresponding differentiation of the amount of water. That 

 surface-growth in cell-walls does take place by this kind of 

 intussusception had been incidentally suggested by Unger, and 

 the appearance, known as the striation of the cell-wall, might 

 now be referred to the same principle as the concentric 

 stratification, namely to the intercalation of more and less 

 watery matter in regular alternation. But Nageli pointed out 

 a fact which had escaped other observers, namely, that the 

 difference of structure which usually appears on the surface- 

 view as double cross-striation, passes through the whole 

 thickness of a stratified cell-wall. Thus Nageli arrived at a 



