266 H. von Mohl on the Investigation of Vegetable Tissue 



difference of the cell-membranes of the periderm and the cuti- 

 cular layers of the epidermis, their incapacity to take a blue 

 colour with iodine and sulphuric acid, and their resistance to 

 the solvent action of sulphuric acid, do not depend upon their 

 consisting of a constituent substance different from cellulose, 

 but that their basis is likewise cellulose, and that this appears 

 with its characteristic reaction with iodine, when the compound 

 deposited in these membranes has been removed by caustic 

 potash. Hence it required to be examined whether these mem- 

 branes could be made to recover, by the same treatment, the 

 property of acting upon polarized light in the same way as 

 cellulose. Some experiments made with the epidermis of Aloe 

 ohliqua showed that this is the case completely ; for even after 

 only a few hours^ maceration in solution of caustic potash, the 

 reaction had changed perfectly into that of cellulose. Treatment 

 of the cuticle with oxidizing agents, for instance with a solution 

 of chromate of potash in dilute sulphuric acid, had the same 

 effect, but less perfectly, even when the maceration in this fluid 

 was prolonged for several days. There is no doubt, therefore, 

 that the optical reaction of the cuticle, like its chemical reaction, 

 is to be ascribed to the deposition of a foreign substance in 

 its membranes composed of cellulose. 



In the conversion of cotton into gun-cotton there is a change 

 of the optical conditions analogous to that of the cellulose layers 

 of the epidermis in their conversion into cuticular layers. 



The same opposite action on polarized light to that of cellu- 

 lose which is found in cuticle, is seen also in the cell-membrane 

 of Caulerpa (I examined in this respect C. prolifera, Freycinetii, 

 clavifera), and this not only in the tough, external, lamellated 

 cell-membrane of the stem, the leaves, and the radical fibres (if 

 these expressions may be used to denote the parts of a uni- 

 cellular plant), but also in the substance of the branched struts 

 which run across the cavity of the plant. 



The cell-membrane of Btyopsis, which I examined in B. Bal- 

 bisiana and penicillata, displayed an anomaly I could not explain 

 in reference to the rest of the conditions. It consists of many 

 concentric layers, and its substance, with the exception of the 

 external cuticular investment, is rapidly coloured blue by iodine 

 and chloride of zinc, like cellulose; yet only a thin external 

 layer acted like cellulose in polarized light, and all the inner 

 layers in the contrary way. 



Finally, the starch-granules of all the plants I examined in this 

 respect have the optical behaviour opposite to that of cellulose. 



When we turn from the examination of cross- sections of cells 

 to the elucidation of the phsenomena which cells present when 

 viewed laterally, a far greater multiformity is met with. 



