Relations of Higher Plants to the Formation of Nitrates 105 



in nitrate formation, maize and legumes were found to have a favorable 

 influence on nitrate formation. The other two crops did not enter into 

 these experiments. 



The much more pronounced influence of the secondary crops on the 

 growth of the primary crops in the field soil than in the nutrient solution 

 in crushed quartz leaves some question as to whether the influence of 

 the secondary crops on nitrate formation was a factor in the growth of 

 the primary crops. The result of the experiments with nutrient solution 

 opens the question as to whether there is any other way in which such 

 an influence could be exerted, if indeed the secondary crop really does 

 affect favorably the growth of the primary crop. 



It is true that there are certain combinations of plants in which the 

 primary crop made a better growth than it did in other combinations; 

 but in most such cases the primary crop gave a larger yield in the com- 

 bination than it did alone, thus indicating a favorable influence on the 

 part of the secondary crop and not merely an innocuous one. 



From other experiments results have been obtained that indicate a 

 stimulating influence of certain kinds of plants on the growth of other 

 kinds. Dandeno (1909) grew oats, barley, buckwheat, wheat, and flax 

 in soil contained in greenhouse pots, both with and without underground 

 shoots of Canada thistle, and repeated the experiment with a young elm 

 tree. All the crops except buckwheat grew better with the roots of Canada 

 thistle than alone. All grew less with the elm tree. The stimulating effect 

 of the thistle was most pronounced twenty-two days after planting. 

 Dandeno suggested as a cause the excretion of substances that might 

 stimulate the growth of the other plant or release plant food not otherwise 

 available, or, on the other hand, might injure the other plant or tie up 

 soluble nutrient material. 



It is reported by the Norsk Landmandsblad (1903) that in Jutland 

 spruce trees make a good growth on waste areas where mountain pines 

 grow, when their roots are in contact, but otherwise not. If the pine is 

 cut down while the spruce is still young the latter will die or will make 

 a sickly growth; if, on the other hand, it is not cut until after several 

 years, the spruce not only will survive but appears to grow faster than 

 would have been the case if the pine had been left standing. The Nor- 

 wegian experimenters account for this by supposing that pines utilize 

 atmospheric nitrogen and furnish spruces with a supply of nitrogen. 



