42 MINNESOTA BOTANICAL STUDIES. 



all intents drawing on its reserve food Plants grown in soils 

 free from nitrates have grown in such manner as to indicate a 

 capability of the absorption of free nitrogen. A series of crit- 

 ical tests on this point are now in operation. A consideration 

 of the data at hand suggests the following tentative conclus- 

 ions: 



1. The tubers are not essentially storage organs, although 

 they may function in part as such. This is demonstrated by 

 their formation before the secondary thickening of the root 

 has begun, and their behavior during active growth of the 

 stems and leaves. 



2. The tubers are, so far as known, found on all mature 

 plants, and, if pathological formations, do not act to the de- 

 triment of the plant. 



3. Pending critical culture tests, the incomplete observa- 

 tions point to an assimilation of free nitrogen by Isopyrum. 



