THE MATERIAL OF PLANTS 13 



on the outside of tlie plant as a hard covering 

 easily visible. This is especially true of the scour- 

 ing rush {Equisetum). A complete coat of silicon 

 covers the surface of the stem and gives a rough 

 feeling when handled. The mineral sheath is so 

 heavy that the tissues of the plant may be de- 

 stroyed and it will remain intact. To demonstrate 

 this, put two or three short pieces of the stem of 

 the scouring rush into a dish or test tube and cover 

 with a mixture of three parts water and one part 

 hydrochloric acid [muriatic acid). Warm the prepa- 

 ration nearly to the boiling point for two hours. 

 Pour off the acid and wash with clean water two 

 or three times. The beautiful silicon sheath may 

 now be handled and preserved in water indefinitely. 

 The silicon sheath may be separated in a still 

 more simple manner if a section of the stem is 

 held in the blaze of a fire or the flame of a lamp. 

 The water will be driven off, the dried material will 

 burn, and then the mineral sheath will reach a red 

 heat, retaining its original shape. This may be 

 preserved dry or in water. 



11. Uses of mineral coatings. — The mineral cover- 

 ing of the scouring rush prevents grazing animals 



