106 T. Lyttleton Lyon and James A. Bizzell 



Through what process the stimulation that one plant often seems to 

 exert on another is accomplished, still remains to be ascertained. This 

 apparently resembles, in some ways, the stimulus that certain plants 

 exert on nitrate formation. Thus the stimulus is exerted more strongly 

 during the earlier part than during the later part of the life of the plant. 

 This is indicated by the field experiments already described, in which 

 nearly all the primary crops harvested at bloom gave a larger yield in 

 combination than alone, while similar mixtures that were allowed to 

 mature gave opposite results. As a considerable part of the nutrients 

 absorbed by plants had been taken up when the time of blossoming came, 

 the competition of the secondary crop during the later growth could not 

 have been very severe. It was also noticed that in quartz cultures the 

 secondary crop remained almost stationary during the later groT^i^h of the 

 primary crop and made an excellent growth after the latter had ceased growth 

 entirely. Like the effect of the plant on nitrate formation, tliere appears 

 to be a depressing effect during the later stages of groTvlh. Another 

 point of resemblance is that the influence of certain kinds of plants is 

 more marked than that of other kinds, and in this respect maize and red 

 clover were found to be especially active in stimulating both processes. 



The natural growd^h of plants in associations, in which one plant follows 

 another in succession through a considerable part of the season, may be 

 to some extent a utilization of this apparent property of the plant to 

 stimulate the growth of other plants. 



Summary 



The nitrate content of soil under timothy, maize, potatoes, oats, millet, 

 and soy beans was different for each crop when on the same soil. There 

 was characteristic relationship between the crop and the nitrate content 

 of the soil at different stages of growth. During the most active growing 

 period of the maize crop, nitrates were frequently higher under maize 

 than in cultivated soil bearing no crop. Under a mixture of maize and 

 millet, nitrates at this period were higher than under millet alone, 

 although the crop yields were about the same on both plats. 



These phenomena may be accomited for on the assmnption that nitrate 

 formation is stimulated by some processes connected with the active 

 growth and absorbing functions of some higher plants, particularly of 

 maize, although there are indications that the maize plant obtains a 



