Minnesota Plant Life. 



America and, indeed, in the world, to observe their action is at 

 the Yellowstone National Park, where great masses of traver- 





FIG. 8. Portion of a board which had been standing in the tank shown in fig. 10. It is en- 

 crusted with lime stone deposited by a colony of blue-green algae. After photograph by 

 Miss Josephine E. Tilden. From the Botanical Gazette. 



tine and sinter are formed, covering acres around the hot springs 

 in which the blue-green rock-forming algae find a congenial 

 home. Travertine is a kind of limestone; sinter is a kind of 



