3«4 



ECOLOG Y. 



crops (sedum), or small thick leaves which are closely overlapped 

 as in the Saxifraga oppositifolia. 



693. Few of us, unfortunately, can make the trip to the arctic 

 regions to study these interesting plants which play such an im- 

 portant role in the economy of nature. Rocky places, however, 



Kig. 4S3. 

 Rock lichen (umbilicaria) from Greenland. 



or loose stones are common nearer home. Observation of their 

 flora, and the means by which such plants derive nutriment, store 

 moisture, or protect themselves from drought, will well repay out- 

 door excursions. 



694. Filling of ponds by plants. — Not only are plants im- 

 portant agencies in the formation of soil in rocky regions, they 



