112 CELLULAR TISSUE. 



described by Sanio, which have an inner layer of the thickened wall of low re- 

 fractive power, may be assigned to this category. 



The presence within the totally suberised layer, which shows no cellulose reaction, of 

 one showing cellulose-blue after treatment with potash, may explain IMohl's statement ', 

 according to which the cork-cells consist of cork substance and cellulose, and the 

 presence of the latter may be proved by reagents after treatment with potash. The 

 statement is correct for the cases just mentioned (I have not investigated all those 

 enumerated by JNIohl), but the blue coloration does not affect the whole suberised mem- 

 brane, as Mohl asserts, but only the above described layer within that totally suberised. 

 Also after treatment with Schultze's mixture no cellulose reaction appears in the latter. 

 Moderate treatment with the mixture causes decolorisation and separation of the totally 

 suberised membranes from one another. Stronger heating with it gradually changes the 

 latter into a greasy disorganised mass^ Still stronger treatment with it dissolves it 

 entirely. 



Between the totally suberised outer layers of the wall of contiguous cork-cells there 

 always lies a very thin limiting layer, the material of which differs in some way from 

 them. Sanio describes this definitely for Uhnus effusa and Sorbus aucuparia, and 

 figures the limiting lamella, from, which in this case the totally suberised layers often 

 separate in the section-cutting. It is distinguished from the latter by lower refractive 

 index, but shows in other respects the same reactions. The fact that cork-membranes 

 are separated from one another without injury when warmed with Schultze's mixture 

 (Quercus suber), or even with potash (Boswellia papyrifera), is further evidence of the 

 presence of such a delicate and distinct limiting lamella. On the question raised by 

 Sanio of the possible growth of the membrane by apposition, these facts have at present 

 no decisive bearing. 



With the exception of a few points of difference, what has been above stated 

 coincides with the facts recently discovered by Haberlandt ^. He found in the cork 

 of the cork oak, potato, elder, and maple a separation of the cells from one another 

 after treatment with Schultze's mixture or chromic acid, and appearance of the 

 cellulose reaction after treatment with solution of potash ; the latter therefore, 

 according to him, dissolves the cork substance combined with the cellulose, and 

 the former reagents the ' Intercellular substance,' or limiting lamella, which he 

 identifies with ' ligneous substance.' Whether the relations are so simple, needs 

 further investigation, the more after what has been stated, since Haberlandt does 

 not sharply distinguish the distinct cellulose-containing layer, which is present in 

 the cell while still part of a continuous tissue. 



As INIohl has shown, the fibrous thickened cells in Boswellia papyrifera are 

 the only case as yet known of siUcified cork membranes. 



The colour of the cork membranes is independent of the stage and extent of 

 the suberisation. The totally suberised membranes of old birch bark, of Salix 

 viminahs, aurita, caprea, are colourless, those of Q. suber, &c. are bright brownish 

 yellow, the internally thickened walls of Platanus greenish yellow : those of Salix 

 alba, purpurea, and fragilis yellow ; in general the colour of the membranes them- 

 selves is always very slight, and the bright brown colour of very many masses of 

 cork is chiefly due to the cell-contents. The silicified walls of Boswellia, as far 

 as it is possible to determine, are quite colourless. 



1 Botan. Zeitg. 1847, p. 503. ^ Compare Schacht, Lehrb. I. p. 14. 



- Ueber Xachwtisiing v. Cellulose im Korkgewebe. Oesterr. hot. Zeitschr. 1874, No. 8. 



