ELM STEM 91 



material for the study of such silicified walls. Strip off the super- 

 ficial tissues, together with the stiff hairs which they bear, and after 

 soaking them well in nitric acid, or in Schulze's macerating fluid 

 (see Appendix A), ignite on platinum foil : treat the ash with 

 dilute nitric acid, and it will not be completely dissolved. Mount 

 some of the residue in water, and examine under a low power : 

 it will then be seen that this residue of the ash consists of 

 skeletons showing the exact conformation of the original hairs, 

 together with a variable area of the tissue which surrounded the 

 bases of the hairs : these skeletons consist of silica, with which 

 the cell-walls must have been completely impregnated. Compare 

 the epidermis of Equisetum, which will be described later, and is 

 strongly silicified. 



2. The periderm (when present) lies immediately 

 below the epidermis : its cells are arranged, in radial 

 rows, without intercellular spaces. Select a thin part 

 of the section for special study of these radial rows, 

 and note in each the following succession of tissues, 

 passing from without inwards : 



a. A series of cork-cells as above described : walls 

 stained yellowish brown with chlor-zinc-iodine. 



I. At least one cell with very small radial diameter, 

 and with protoplasmic contents, and thin cellulose walls : 

 this is the cork-cambium, or phellogen. 



c. Cells with thick cellulose walls, and protoplasmic 

 contents with chlorophyll : no intercellular spaces : this 

 is the phelloderm, which is also derived from the 

 cork-cambium. 



Treat a thin section with concentrated sulphuric 

 acid : the walls of all the tissues will swell, and gradu- 

 ally lose their sharpness of outline, with the exception of 

 the cuticularized outer wall of the epidermis, and the 

 cork, both of which resist the action of the acid. A 



