PLANTS WITHOUT FLOWERS 127 



scum from different sources will allow the children to become 

 famili&r with some of the most common forms which will repeat- 

 edly occur. The names of many of these may be ascertained and 

 supplied in time. 



DIATOMS. 



Diatoms are almost always present in the above described 

 (preparations. They are small objects, generally brownish in 

 color, often tapering at each end like a canoe. They move across 

 the field like small boats. There are many other shapes than 

 these among the diatoms. Diatoms are plants having delicate 

 ehells of silica. When the plant dies, the shell drops to the 

 bottom of the pond. In certain places, ponds or lakes which 

 have been the homes of diatoms for ages have a deposit of fine 

 mud at the bottom made up largely of diatom shells. In ancient 

 geological times there were thus formed in some places in Cali- 

 fornia and in other countries, deposits making thick strata of 

 rock composed almost wholly of diatom shells. 



If in the first lesson neither Spirogyra nor diatoms are in the 

 material observed, equally interesting forms will be seen. 



