Jan. 1899.] BOTANICAL SOCIETY OF EDINBURGH 161 



The sensitive salt was deposited only on the surface of the 

 mucilage, and not in the interior at all. This method, 

 however, gives better results with flax seeds, but here the 

 same difficulty of impregnating the mucilage with a 

 sensitive silver salt again presents itself. Potassium 

 dichromate readily saturates mucilaginous membranes and 

 mucilage, perhaps in virtue of its property of combining 

 with them on exposure to light. It is hardly possible to 

 impregnate the mucilage of fresh-water plants with either 

 potassium iodide or silver nitrate, without thereby causing 

 the mucilage to shrink up and almost disappear. 



The application of this process to seaweeds is interesting, 

 as it apparently indicates a method of determining the 

 distribution and the chemical nature of salts present in the 

 tissues of these plants. 



The medulla of the thallus of Fucus consists of cells 

 apparently embedded in a transparent gelatinous matrix. 

 Treatment with Schulze's fluid shows that the cell mem- 

 branes consist of two strata — an inner dense layer 

 immediately around the cell contents, which gives a blue 

 reaction with Schulze's fluid ; and an outer gelatinous 

 layer, which forms the apparently gelatinous matrix, and 

 which is unacted upon by the above treatment. 



When sections of Fucus are swelled out for a short time 

 in fresh water, and then treated with weak silver nitrate 

 solution, certain unstable silver salts are formed, and 

 impart a white coloration to the sections. They now 

 behave like sensitised gelatine plates, being, in fact, 

 specially sensitive, and may be developed by the usual 

 methods. Before developing, they should be brushed over 

 in fresh water with a camels' hair brush, to remove any 

 silver salt precipitated superficially. They may now be 

 transferred to a slide, developed with hydroquinone, and 

 fixed with sodium thiosulphate. An exposure of three 

 minutes in gaslight is quite enough to give a very deep 

 coloration on subsequent development. The gelatinous 

 matrix stains of a uniform yellowish brown, the cellulosic 

 layer does not stain at all, and there appears between it 

 and the gelatinous matrix a thin darkly-coloured ring. 

 The cell contents are blackened, and the appearance of the 

 finished preparation is rather remarkable. The dark ring 



