3 



At each of the nine places employed in this investiga- 

 tion, a series of cultures was grown in the onen with no cov- 

 ering other than a screen of large-meshed wire ne-^ting to nre- 

 vent injury to the plants, "^hese have been termed the ex- 

 posed stations. In addition, at Oakland, Baltimore, and 

 Easton, a series of cultures was grown under glazed cold 

 frame sash supoortel horizontally 1 meter (3.3 feet) above 

 the surface of the soil. These have been termed the covered 

 stations. They were placel within several meters (6.5 feet) 

 of the enclosures containing the plants of the exposed stations 

 and were subjected to the same climatic conditions as the ex- 

 posed plants except in so far as these conditions were raodi- 

 fied by the glazed cold frame sash. At Baltimore a series of 

 cultures was grown in the woods near the Laboratory of Plant 

 Physiology of the Johns Hopkins University. This has been term- 

 ed the Baltimore Forest Station. These plants like those of 

 the exposed stations had no covering other than a protective 

 wire screen. Ov;ing to their locatioii, they were, of course, 

 subjected to a set of climatic conditions quite different from 

 those acting on the exposed and covered plants at Baltimore. 

 The I^'orest Station at Baltimore wa^ distant about 150 meters 

 (490 feet) from the exposed and covered stations. There are 

 thus plant data available from 13 series of cultures in all, 

 each series having been exposed to a different set of envi- 

 ronmental conditions throughout the season. 



