THE MATERIAL OF PLANTS 15 



curious rock formations in the Yellowstone Park 

 are made up in this manner. Some of the powders 

 used to polish metals are taken from deposits in 

 the soil, and they consist of the silicon sheath of 

 minute plants. 



13. "Water cultures" loith and loithout mineral 

 salts. — Fill a quart fruit jar with clean rain water, 

 and a second from a stream or pond. Put in each 

 a freshly cut twig of willow (Salix) or a stem of 

 coleus, and place both in sunlight in a Avarm room. 

 Replace the water weekly, and clean out the jars. 

 The cuttings will soon begin to grow and form 

 leaves, even if the experiment is performed in 

 winter. Compare the amount of growth in three 

 or four weeks. Have they grown equally? If a 

 difference is shown in the number and size of the 

 leaves and roots, to what is it due ? The follow- 

 ing demonstration may throw some light on the 

 subject. 



14. Ash or mineral salts in the loater of streams. 

 — Boil a quart of rain water in a dish with a pol- 

 ished inner surface until the liquid has all disap- 

 peared. Examine the bottom of the dish. Nothing 

 has been left behind, and rain water is seen to be 



