DIFFERENT LAYERS IN THE WALL. 35 



plates, viz., a thin inner one (i), a thicker middle one (z), 

 and a thin outer one (in). The latter is apparently common 

 to the two contiguous cells, and is the " primary cell- wall " 

 of some authors and the " intercellular substance" of others. 



The deportment of these layers on the application of reagents is 

 interesting. 



1. On treatment with a solution of iodine the outer and middle plates 

 turn yellow. 



2. On treatment with iodine and sulphuric acid the outer and middle 

 plates turn yellow and the inner one blue. 



3. On treatment with concentrated sulphuric acid the inner and 

 middle plates are dissolved, while the outer remains. 



4. On boiling in nitric acid with potassium chlorate the outer plate 

 is dissolved, while the middle and inner are not. By this latter process, 

 called " Schulze's Maceration," the cells may be isolated, but it must 

 be borne in mind that such isolated cells have lost by sol ution their 

 outer plate. 



41. In some cases the differentiation is of such a nature 

 that one or more plates become converted into mucilage in 

 water. In the dry state the mucilaginous portions are hard 

 and cartilaginous. Examples of the change of the outer plates 

 into mucilage are common in the Fucacese, and of a sim- 

 ilar change of the inner ones in the seeds of flax and quince.* 



42. Incombustible Substances, as silica and lime, are 

 frequently deposited between the molecules of cellulose in 

 the wall. Cell-walls which are filled with considerable quan- 

 tities of these substances, upon burning, leave ash-skeletons, 

 which retain the form and markings of the cell. The Di- 

 atoms furnish excellent examples of highly silicified walls. 

 Silica is abundant also in the epidermal cells of grasses 

 and scouring- rushes (Equisetacece). 



Lime-skeletons may be obtained by the combustion of thin slices of 

 the tissues of many plants upon glass r platinum-foil. The vessels of 

 Cucurbita Pepo yield (according to Sachs) beautiful skeletons under this 

 treatment. 



Silica-skeletons may be obtained by first soaking the tissue in nitric 

 or hydrochloric acid and then burning, or by burning (upon platinum- 

 foil) in a drop of sulphuric acid. 



* Sachs attempts to reduce the chemical differentiations of the cell- 

 wall to three categories, viz., Outicularizing, Lignification, and Conver 

 sion into Mucilage. 



