CHAP. I 



The Cell 187 



though not the infiltration of non-living material on which 

 Naegeli's views were based. 



The hypothesis of Wiesner, that the cell wall is at first 

 living and continues for some time at any rate to contain 

 a certain proportion of living matter, was supported by 

 Molisch's investigation on the growth of thyloses, carried 

 out in 1888. As the protrusions into the cavity of the 

 vessel enlarge to fill it, they in time touch one another, 

 and their walls thicken and form pits ; the latter are 

 formed always opposite to each other in adjacent walls. 

 Molisch pointed out that this almost certainly involves 

 the presence of protoplasm in the cell wall. 



The composition of the substance of the cell wall, com- 

 monly spoken of as cellulose, was known by the earlier 

 writers from 1825 onwards to be a mixture of several 

 compounds distinguished as cellulose and pectose respec- 

 tively. Prior to 1860 little had been ascertained about 

 their distinguishing characteristics, their relation to each 

 other, or their distribution in the cell-wall substance. The 

 latter was known to show a certain differentiation, its 

 middle region being in many respects unlike the rest. The 

 distinguishing reagent, ammonio-cupric sulphate, which 

 dissolves out the cellulose from the wall, leaving the pectose 

 constituents unaffected, was first employed by Fremy in 



1859- 



Kabsch ascertained certain facts about the relative dis- 

 tribution of the two components in 1863. He found that 

 when a cell-membrane is swollen by the action of chlor- 

 zinc-iod, the inner layers only take on the blue coloration 

 of cellulose, and that the middle region, or middle lamella 

 of Naegeli and later writers, varies in different cases, some- 

 times remaining colourless, sometimes appearing yellow, 

 and occasionally taking a pale blue tinge. He consequently 

 located the pectic bodies in the middle lamella, as they are 

 not coloured by the iodine, and claimed that the layers 



