Photo. 4. — Primula obconica ; HCl ; m/4; two days ; shows clearly the edg-e of the 

 wave which is entirely independent of the veins. 



Photo. 5.— Primula obconica ; H^SOj; 

 m/4 ; two days. The edge of the wave 

 follows in a measure the contour of the 

 leaf. 



Photo. 6. — Nicotiana tabacum ; H^SO^; 

 m/4 ; three days ; yellow striped centre, 

 bounded by a g'reen stripe shown at G, 

 beyond which is a brown surface. Xylem 

 (m) laid bare by the acid. 



