1884.] of the cell-wall and middle-lamella. 103 



layers, as the case may be, which during the subsequent thickening, 

 have become physically and chemically altered 1 . 



In the light of Schmitz 2 and Strasburger's 3 researches, it 

 will be readily understood that the formation of such a differ- 

 entiated portion in the thickening cell-wall, is naturally ne- 

 cessitated, on account of the amount of tension and pressure 

 produced during the process of surface growth, which although 

 it may be small in the last or most internal layer deposited, 

 in the region of the middle lamella reaches its maximum, and 

 becomes very appreciable. As regards the chemical phenomena, 

 we know that it is admitted on all hands that the various changes 

 accompanying lignification, cuticularisation, &c. are most marked 

 in the more external layers of the cell-wall, and in consequence 

 we find that the substance of the middle lamella has usually 

 undergone the maximum amount of alteration from its primitive 

 cellulose character, so that both chemically and physically it may 

 be truly regarded as the most modified portion. 



From the standard text-books generally in use, one is ap- 

 parently led to infer that the middle lamella consists of a certain 

 definite substance — the substance of the middle lamella — which is 

 distinguished by the fact that it is insoluble in sulphuric acid, 

 but soluble in Schultze's mixture. From these properties such a 

 substance might consist either of lignin or cutin or finally of cork 

 which we regarded as a mixture of both, for all these substances 

 would be soluble in Schultze's mixture. There is therefore great 

 want of definiteness in the statements with regard to this sub- 

 stance, which has been thus separated in botanical terminology, 

 and I shall therefore endeavour by means of the evidence afforded 

 by direct observation, and by recent research, to state the matter 

 somewhat more clearly. 



In certain cells, e.g. bast prosenchyma cells of the pulvini 

 of Mimosa and many palm endosperms, the cell- walls consist of 

 pure cellulose, and the middle lamella is but little developed. 

 In such cases one finds that the middle lamella although more 

 resistant than the rest of the cell-wall in consequence apparently 

 of its greater density is distinctly soluble in sulphuric acid, and 

 as far as one can observe consists simply of cellulose. In other 

 instances, e.g. lignified prosenchyma cells of the cortex of Ly- 

 copodium, the well-defined middle lamella equally with the layers 

 of the cell- wall assumes a gold-yellow colour when treated with 

 aniline chloride and hydrochloric acid, and with any other test 



1 This latter fact was insisted upon by Solla, "Beitrage zur naheren Kenntniss 

 tier chemischeu und physikalischen Beschaffenheit der intercellular Substanz. 

 {(Ester reiclrische bot. Zeitschrift, November, 1879.) 



2 Schmitz, loc. cit. 



8 Strasburger, Zellhaute, pp. 1—146 and 175—200. 



